Is it possible for galaxy clusters to interact?

Is it possible for galaxy clusters to interact?

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Galaxies commonly interact and collide with other galaxies. Is it possible that clusters of galaxies similarly interact and collide? Have people tried to study this phenomenon previously? I couldn't find anything specific in the literature.


Theoretically, structure is expected to form first on small scales (stars and stellar clusters), and later on increasingly larger scales - galaxies, groups, and eventually galaxy clusters (see e.g. Longair 2006). This is confirmed, at least to some extent, observationally. For instance, galaxies have been detected out to a redshift of $z=11.2$ (400 million years after Big Bang; Oesch et al. 2016), while cluster have only been detected out to $z=2.5$ (2.6 billion years after Big Bang; Wang et al. 2016).

Thus, when clusters formed, the Universe had already expanded so much that interaction between them is rather rare.

It does happen, however. One of the most important examples is the Bullet Cluster, which consists of two colliding galaxy clusters. The reason I mention this is that it really beautifully confirmed the existence of dark matter. The image below (from NASA's Astronomy Picture of the Day) shows the two clusters after the collision. The stars and galaxies are so far apart that collisions between galaxies are rare, and collisions between stars virtually never happen. Thus, they have just passed right through each other, as seen in the image. The gas between the galaxies, however, collides, slows down, and is separated from the galaxies. This heats the gas to millions of degrees, emitting X-rays (seen in red). The blue stuff is a map of the mass distribution, made using gravitational lensing. This mass is clearly separate from the gas, but coincides with the galaxies, and gives a much higher mass than the visible mass, roughly 5.5 times more, which is exactly what is found for the ratio between dark and normal matter using other methods.

Watch this video, I think it will give you an answer, sometimes it is better at once to see , then 10 times to read: Laniakea: Our home superclaster

Astronomers see the Milky Way eating one of its own

The globular cluster M92 is a gorgeous example of its kind. A roughly spherical ball of hundreds of thousands of stars held together by their mutual gravity, it's one of about 160 such that orbit the Milky Way.

Well, for now. It turns out our galaxy is eating it.

More Bad Astronomy

Astronomers have discovered a stream of stars ahead and behind the cluster as it orbits, stellar citizens stripped away by the gravity of the Milky Way. This is surprising, given the great age of M92 — about 11 billion years old — which means something happened recently* to change things.

A lot of these stellar streams have been found over the past couple of decades. Some are from small galaxies that have passed too close to the Milky Way (which is a big galaxy), and some from globular clusters. Some we're not sure. A half dozen or so have globulars associated with them, and more from dwarf galaxies, while some don't seem to have any particular source it's likely they came from objects that the Milky Way has completely torn apart.

Basically, as a small object like a cluster or dwarf galaxy approaches, stars on their outer edges feel more gravity from the Milky Way than from their parent object, and get pulled off. It's a little bit like a cloud of dust stripped off a truckbed full of dirt, with the cloud streaming behind the truck. In this case though, because it's due to tides, the galaxy's gravity pulls these stars both ahead and behind the victim. The result is a thin noodle of stars spread out across the sky.

Stars in some of the stellar streams recently discovered using Gaia data superposed on a map of the galaxy. Credit: ESA/Gaia/DPAC

These streams have escaped attention for centuries because they're extremely difficult to spot against the billions of other stars in the galaxy. But big sky surveys looking at millions or billions of stars make it possible to spot them. One way is to look at just the motions of stars they all appear to be moving together in one direction. If distances can be found, they all fall along a single arc, the trajectory orbit of the parent object.

In the case of M92, astronomers first looked at a pair of huge surveys of stars done using the Canada-France-Hawaii Telescope and Pan-STARRS. They took an area of the sky around M92 and looked for stars with colors that matched that of the cluster — knowing the age of the cluster, and that the color of a star depends on its mass and age, they could make a cut filtering out stars that didn't match the cluster. They also looked for stars at about the same distance of the cluster as well, since the stream would have about that distance.

When astronomers plotted the positions of stars that matched the colors of M92, the stream could be seen in their data (arrowed). Credit: Thomas et al.

When they did, the stream popped right out of their data. The total number of stars implies the stream has about 30,000 times the mass of the Sun in it, which is about 10% of the stars in the cluster now! So this cluster is just shedding stars.

Now that they had a list of stars in the stream, they turned to Gaia, a satellite that for the past few years has measured the positions, color, distance, movement and more for over a billion stars (yes, a billion). That allowed them to trace the orbit of the cluster backwards in time, and they found it passes through the galactic bulge, the flattened sphere of stars surrounding the galactic center. It also passes right through the galaxy's bar, a more complicated elongated structure in the galactic center.

The structure of the Milky Way: A flattened disk with spiral arms (seen face-on, left, and edge-on, right), with a central bulge, a halo, and more than 150 globular clusters. The location of the Sun about halfway out is indicated. Credit: Left: NASA/JPL-Caltech right: ESA layout: ESA/ATG medialab

That's interesting, because they could also measure the motion of these stars away from the cluster, which gave them a handle on how the stream has changed over time.

Given the distance and velocity of the stars in the stream, they found that these stars were stripped from M92 only about 500 million years ago, and the majority of them less only in the past 300 million years. That's very recent, compared to the 11 billion year age of the cluster.

If it passes through the galactic center every orbit, then, given the rate of star loss, it should be long gone. That in turn implies the orbit recently changed. Otherwise the cluster shouldn't exist.

M92, a globular cluster about 27,000 light years from Earth. Credit: ESA/Hubble & NASA Acknowledgement: Gilles Chapdelaine

It's possible this last pass through the galaxy changed the orbit, since the gravitational field in the galactic center is complicated. If something else can make that big an orbital change, I'm at a loss as to what it is. It could have passed another cluster, and their gravitational interaction affect them both, but the space surrounding the galaxy is very roomy, and clusters very few. The odds of a near collision are extremely small.

The astronomers plan on looking for more stars from the cluster, perhaps ones that got farther away, creating a halo around the stream. That could help them figure out what's going on. The work is painstaking, though, so it may be a while before they have any further information.

Plotting the number of stars in a given area of sky (darker is more stars), the M92 stream is apparent, with stars leading the cluster in its orbit (right) and those trailing behind (left). X and Y axes are degrees on the sky. Credit: Thomas et al.

These streams are fascinating to me. They tell us about the distant past of the galaxy, and also about the construction of it now. As these streams flow, the density of stars in any given part may change due to varying gravitational field sin the galaxy for example, if they pass near a massive giant molecular cloud of gas and dust, or if the halo of dark matter around our galaxy isn't smooth but instead lumpy. They also tell us about the way globular clusters behave now, how they're losing stars, and how that affects their structure to.

Pretty amazing, considering we didn't even know about these streams a couple of decades ago. Advances in technology have given us this chance to peer into our galaxy's past, and — as usual — things are a lot more complicated and a lot more interesting than we used to think.

* Of course, "recently" for an astronomer is different than to you, probably. I mean sometime in the past 500 million years, not long after animals invented hard body pieces instead of being goo swimming in the oceans.

Is it possible for galaxy clusters to interact? - Astronomy

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Ongoing research by an international team of astronomers is providing new insights into cataclysmic cosmic collisions between galaxy clusters.

Using the world's most powerful X-ray space observatory, the team is unraveling the complex interactions that take place in the "traffic pile-ups" that occur as clusters containing hundreds of galaxies and trillions of solar masses of gas and dark matter interact and merge.

Speaking on Friday at the RAS National Astronomy Meeting in Birmingham, Dr. Elena Belsole (University of Bristol) will present new results obtained with ESA's orbiting XMM-Newton observatory. The images and other data reveal an environment racked by violent shock waves that squeeze and compress the intra-cluster gas, raising its temperature to many millions of degrees.

Galaxy clusters, measuring up to 6 million light years across, are the largest objects whose mass can be measured by astronomers. From observations of many clusters, it is possible to estimate the distribution of mass in the Universe as a whole. This provides important information about what the Universe is made of, how it began, and how will it end.

However, only 5% of the mass of galaxy clusters lies in stars and galaxies. The space between the galaxies is filled with gas which is so hot (10 - 100 million degrees Celsius) that it can only be seen at X-ray wavelengths.

How did the gas between the galaxies get so hot? Galaxy clusters grow through the action of gravity, continuously pulling in smaller galaxy systems and undergoing the occasional violent collision with an object of comparable size.

In such events, the clusters start to feel each other's gravitational pull: they interact and, after a prolonged period, they finally merge. These mergers are the most energetic events to have taken place in the Universe since the Big Bang. The energy released in cluster collisions irreversibly modifies the physical conditions within a cluster through compression waves and shocks which heat the gas to temperatures 10,000 times those on the surface of the Sun.

By using space-based instruments able to see at X-ray wavelengths, Belsole's team has been able to measure the origins and energy of X-rays from galaxy clusters. From the positional information, they were able to map the distribution of the gas in the clusters. From the X-ray energy, they were able to measure the gas temperature. By combining the two, they could map the temperature structure of the cluster gas.

The temperature is the key quantity which allows the scientists to discriminate between clusters which are undergoing dramatic collisions and those which are not. The temperature shows directly the conversion of enormous amounts of kinetic energy into the thermal energy which heats the gas.

"Thanks to observations obtained with XMM-Newton, the most powerful X-ray detector ever built, we are now able to describe fully the gas in galaxy clusters," said Belsole.

"From the temperature, we calculate that clusters can collide at velocities greater than 2,000 km/s. We observe that clusters are unique in their morphology and temperature distribution, and it is through these differences that we can say whether a cluster is young or old."

Belsole's team has recently investigated three different merging clusters each made up of hundreds of galaxies. One of these, known as Abell 1750 (A1750), is a young merger located 1.1 billion light years from Earth. It involves two clusters, separated by more than 3 million light years, which are just starting to interact.

Each of these colliding clusters has a total mass about 500 trillion times that of the Sun and is moving at a speed of around 1,400 km/s. The violent interaction between them causes shocks and compression of the intra-cluster gas, producing an arc-like region of gas between the two with a temperature of 70 million degrees Celsius. The collision will reach its climax in 1 - 2 billion years, when the cores collide and the energy release is at its maximum.

A more complicated example is A3266, located 800 million light years from Earth. Two clusters, of unequal mass, are seen just after the point of closest encounter. This creates a hot, boomerang-shaped region where a shock wave is propagating in the direction of motion of the smaller, infalling cluster. A1750 will look like this in 1 - 2 billion years.

An older example is A3921, located 1.2 billion light years from Earth. In this case, the very asymmetric morphology and temperature distribution reveal that two clusters, again of unequal mass, have already had their first encounter. The smaller cluster, about three times less massive than the main cluster, was almost totally destroyed by the encounter. The collision shredded the smaller cluster, at the same time producing a hot region of shocked gas stretching from the centre of the main cluster.

"This research shows the violent manner by which the largest structures in the Universe form, and that the formation has happened in the recent past," said Belsole. "The process is still taking place today. In several billion years, the group of which our galaxy, the Milky Way, is a member, will be torn apart as it merges with the nearby Virgo cluster."

The ongoing research is described in several papers. Studies of A3266 will be published in a forthcoming issue of Astronomy and Astrophysics.


The past decade has witnessed spectacular progress in our empirical exploration of the Universe. WMAP observations and the study of distant supernovae have ushered in a new era of precision cosmology, including discoveries into the geometry of the universe, the kinematics of the Hubble expansion, and the cosmic mass-energy content. However, structure formation continues to be elusive due to inability of measuring key properties of galaxies without large systematic errors and inaccuracies in predicting details about star formation. Cosmology surveys have pushed the most powerful ground- and space-based facilities to their limits to partially reveal the evolution of some types of galaxies however, a number of fundamental limitations are becoming increasingly clear. due to biases in optical and near-IR based galaxy selection methods and the inability to measure results in spectroscopic redshift for galaxies at redshift z > 6.5. Consequently, a different, complementary approach is needed to obtain the complete picture of cosmic star formation history and galaxy evolution.

Galaxy Clusters

Clusters of galaxies, having nearly reached dynamical equilibrium, offer an impressive laboratory to test models of large scale structure formation and the dependence on environment of galaxy formation and evolution. Historically, mm-wavelength observations of clusters have focused on the redshift-independent brightness of the Sunyaev-Zel'dovich Effect (SZE), but with its high resolution and exquisite surface brightness sensitivity, the LMT offers a fundamentally new observational window into the study of galaxy clusters, groups, and other mass-biased environments. LMT users will map the distribution of the intracluster medium (ICM) with 6-10 times higher angular resolution than previous studies. This will in turn enable us to probe the formation process of clusters. Using the Redshift Search Receiver, LMT users will study starburst galaxies in order to better understand the rates of star formation within galaxy clusters.

Left: Preliminary map of the Sunyaev-Zel'dovich effect and sub-millimeter galaxy background in the Bullet Cluster by AzTEC at 1.1 mm wavelength. The bright point source in the East is a background luminous infrared galaxy at z

2.7 being lensed by the cluster potential. Right: AzTEC contours overlaid on the X-ray image of the Bullet Cluster.

The LMT also offers an opportunity to study cooling flows in clusters. It has long been understood that the high density and short cooling time in cluster centers should lead to a cooling flow, unless the cooling flow is shut off by an additional source of energy. LMT users will be able to investigate the nature of cooling flows and possible re-heating mechanisms through detailed mapping of the gas and dust distribution in nearby cooling clusters.

Dark Matter and the Structure of Galaxies

According to current theory of structure formation, the matter content of the universe is dominated by cold dark matter (CDM). Because of gravitational instability, perturbations in the CDM density distribution grow with time and form quasi-static clumps called dark matter halos. Luminous objects, such as galaxies and galaxy clusters, are assumed to form in the gravitational potential wells of CDM halos. Thus, a first step in understanding galaxy distribution in the universe is to understand how CDM halos are distributed in space and how galaxies interact with them.

The properties of the dark halo population can be studied in great detail through numerical simulaions and analytical modeling. One method of exploring CDM halo reaction with galaxies is based the conditional luminosity function model, which links galaxies and dark matter halos by matching the number density and clustering properties of galaxies with those of dark matter halos in the current CDM model. Another method uses galaxy systems identified from large redshift surveys of galaxies.

A globular cluster where stars collide

Gemini Observatory near-infrared image of the globular cluster Liller 1 obtained with the GeMS adaptive optics system on the Gemini South telescope in Chile. Image credit: Gemini Observatory/AURA. Scientists have imaged a cluster of stars, heavily obscured by material in our galaxy, where stars are so densely packed that it is likely a rare environment where stars can collide. “It’s a bit like a stellar billiards table where the probability of collisions depends on the size of the table and on the number of billiard balls on it,” said Francesco R. Ferraro of the University of Bologna (Italy), one of the team members who used the Gemini Observatory to make the observations.

The cluster of stars, known as Liller 1, is a difficult target to study due to its distance and also because it is located close to the centre of the Milky Way (about 3,200 light-years away from it), where the obscuration by dust is very high. The unprecedented ultra-sharp view of the cluster reveals a vast city of stars estimated by the team to contain a total mass of at least 1.5 million suns, very similar to the most massive globular clusters in our galaxy: Omega Centauri and Terzan 5.

“Although our galaxy has upwards of 200 billion stars, there is so much vacancy between stars that there are very few places where suns actually collide,” said Douglas Geisler, principal investigator of the original observing proposal, from University of Concepcion (Chile). “The congested overcrowded central regions of globular clusters are one of these places. Our observations confirmed that, among globular clusters, Liller 1 is one of the best environments in our galaxy for stellar collisions.”

Geisler’s team specialises in the study of globular clusters near the centre of the Milky Way, while Ferraro’s team is adept at the reduction of infrared data on globular clusters. Both groups worked together to obtain the beautiful and detailed observations of Liller 1 with Gemini.

Liller 1 is a tight sphere of stars known as a globular cluster. Globular clusters orbit in a large halo around the centre, or nucleus, of our galaxy and many of the closer globular clusters are spectacular showpieces, even in small telescopes or binoculars. “This isn’t one of these showpieces, it is so obscured by material in the central bulge of our galaxy that is almost completely invisible in visual light,” observed Sara Saracino, lead author on the paper, from the University of Bologna. Indeed, Liller 1 is located at almost 30,000 light-years from Earth, in one of the most inaccessible regions of our galaxy, where thick clouds of dust prevent the optical light from emerging. “Only infrared radiation can travel across these clouds and bring us direct information on its stars,” commented Emanuele Dalessandro of University of Bologna.

The observations of the tightly packed cluster used Gemini Observatory’s powerful adaptive optics system at the Gemini South telescope in Chile.

A technical jewel named GeMS (derived from “Gemini Multi-conjugate adaptive optics System”), in combination with the powerful infrared camera Gemini South Adaptive Optics Imager (GSAOI), was able to penetrate the dense fog surrounding Liller 1 and to provide astronomers with this unprecedented view of its stars. This has been made possible thanks to the combination of two specific characteristics of GeMS: first, the capability of operating at near-infrared wavelengths (especially in the K pass-band) second, an innovative and revolutionary way to remove the distortions (blurriness) that the Earth’s turbulent atmosphere inflicts on astronomical images. To compensate for the degradation effects of the Earth’s atmosphere, the GeMS system uses three natural guide stars, a constellation of five laser guide stars, and multiple deformable mirrors. The correction is so fine that astronomers are provided with images of unprecedented sharpness. In the best K-band exposures of Liller 1, stellar images have an angular resolution of only 75 milliarcseconds, just slightly larger than the theoretical limit of Gemini’s 8-metre mirror (known as the diffraction limit). This means that GeMS performed with almost perfect corrections of atmospheric distortions.

These images are comparable in sharpness with those of the Hubble Space Telescope (HST) at infrared wavelengths, with one big additional advantage: a much larger collecting area (a mirror of 8-metre diameter at the Gemini South telescope in Chile, compared with a 2.4-metre mirror on the Hubble Space Telescope).

The observations for this project also included several other globular clusters. The results achieved on their first target, Liller 1, have encouraged the team to expand their collaboration and are now working on the other clusters that promise to deliver even more exciting science.

Background: Stellar Collisions
Stellar collisions are important because they can provide the key to understand the origin of exotic objects that cannot be interpreted in terms of the passive evolution of single stars. Nearly head-on collisions in which the stars actually merge, mixing their nuclear fuel and re-stoking the fire of the nuclear fusion are suggested to be the origin of (at least part) of the so-called Blue Straggler Stars. But collisions can also involve binary systems, with the effect of shrinking the initial size of the system and thus promoting the two components to interact and producing a variety of objects like low mass X-ray binaries, millisecond pulsars, etc. In particular millisecond pulsars are old neutron stars reaccelerated to millisecond rotation period by mass accretion from a companion in a binary system. Indeed, Liller 1 is suspected to have a large population of such exotic objects. Although no millisecond pulsar has been directly observed up to now, a large hidden population has been suggested because of the detection of an intense gamma-ray emission (the most intense detected so far from a globular cluster). The Gemini observations indeed confirm that this is possible.

“Indeed our observations confirm Liller 1 as one of the best ‘laboratories’ where the impact of star cluster dynamics on stellar evolution can be studied: it opens the window to a sort of stellar sociology study, aimed at measuring the impact of the reciprocal influence of stars when they are forced to live in conditions of extreme crowding and stress,” concludes Ferraro.

A cluster of clusters: The globulars of Coma

Big fleas have little fleas upon their backs to bite 'em,
And little fleas have lesser fleas, and so.
ad infinitum.
And the great fleas, themselves, in turn, have greater fleas to go on
While these again have greater still, and greater still, and so on.

One of my favorite things to know in astronomy is that some of the biggest structures in the Universe are made of some of the littlest.

The big structures I’m talking about here are galaxy clusters, vast collections of hundreds of galaxies or more, where each galaxy is a collection of billions of stars, huge volumes of gas and dust, and quite a bit of dark matter.

The structure I’m thinking of specifically is the Coma Cluster, so named because it resides in a part of the sky designated by the constellation Coma Berenices (which means “Berenice’s hair” in Latin). It’s a mammoth thing, a vast sprawling collection of well over a thousand galaxies all moving about, held together in the cluster by their mutual gravity. It has a total mass of a staggering 700 trillion times the mass of the Sun, and our view of it is pretty good even from over 300 million light years away.

The inner part of the massive Coma galaxy cluster, where thousands of galaxies swarm. Credit: NASA, ESA, J. Mack (STScI), and J. Madrid (Australian Telescope National Facility)

Yowza! Look at all those galaxies! And this isn’t the full extent of the cluster it’s really just the central region. The detail is overwhelming, especially if you grab the full resolution 28,750 x 16,550 pixel 620Mb PNG of it. It’s total worth destroying your bandwidth for a while to get it.

Nearly everything you see in that image is a galaxy the cluster happens to lie nearly straight up in a galactic sense in terms of the disk of our galaxy, so the number of stars you see in the Milky Way here is minimized.

Here’s the thing, though: Hidden in that image are lots of very tiny dots. You can barely see them, but they’re there. These are globular clusters. These are like galaxy clusters but instead of being collections of galaxies they’re far smaller collections of stars, from tens of thousands up to a million or two. They’re highly concentrated and roughly spherical (hence the name), looking like sparkly beehives in space.

The mighty Omega Centauri, the largest globular cluster orbiting the Milky Way. Credit: ESO/INAF-VST/OmegaCAM. Acknowledgement: A. Grado, L. Limatola/INAF-Capodimonte Observatory

Yeah. They’re one of my favorite targets when I’m out with my own ‘scope. The Milky Way has about 150 of these orbiting it, generally out a few tens of thousands of light years, roughly the same distance as the size of the galaxy itself.

We see them around lots of galaxies, so it makes sense the Coma Cluster galaxies have them too. They’re tough to spot from 300 million light years, but Hubble is up to the task.

It’s worthwhile doing this. When we look at that cluster we can see the galaxies and get an idea of how they’re interacting, but the view is limited. In other clusters (like Virgo and Fornax) deep surveys have shown that when you look at the faintest light from a cluster it gives you a better idea of how those galaxies are behaving. When two pass each other they interact gravitationally, creating all kinds of havoc, which can be seen in the very faint gas in the cluster, or stars ejected from the galaxies. It’s possible that the globular clusters may give us clues on how that happened.

A team of astronomers decided to take a look. They started with the Advanced Camera for Surveys (ACS) Coma Cluster Treasury Survey, which was set up specifically to map the cluster. Unfortunately ACS shorted out before the survey was complete (though it was later fixed by astronauts in orbit), so the astronomers used archived images taken for other projects to fill in the gaps.

They then wrote software to pick out just the globulars. There are roughly 100,000 individual objects in the image (. ), and they taught the computer to look for things that were small (but not as small as stars), about the right brightness, and the right color to be globular clusters.

They found 22,426 candidate globulars.

Holy cow. OK, fair enough I’ve written software like that before, and it’s difficult but you can tweak it to work pretty well. The problem is knowing how well. Is it finding extremely distant background galaxies and thinking they’re globulars?

Well, I was wondering about that when I came across this in the research paper:

Through detailed visual analysis of the properties of the candidates we produced a final list of globular cluster that is virtually free of contaminants such as background galaxies and artifacts. We stress that all globular clusters in the final list of candidates were validated through visual inspection by displaying the detections on the screen and scanning them on each image, and in both filters.

Um. If I’m reading that correctly, they checked every single candidate by eye. Whoa.

At this point I’ll also note that the team consisted of quite a few undergraduate students who didn’t necessarily have much experience in astronomical research. But it’s easy, even for a beginner, to be taught how to recognize certain things in data — humans are eerily good at pattern recognition — and are in general better at it even than sophisticated software. And they get their names on a paper! Pretty good deal. Even if it means examining over 22,000 points of light.

So what did they find? Ah, yes, this gets really interesting.

A “heat map” showing the density of globular cluster locations in the Coma Cluster. You can see three major clumps, while some galaxies have very few globulars nearby. Credit: Madrid et al.

When they mapped out the location of the globulars, they found three major concentrations, around three of the biggest, brightest galaxies in the cluster: NGC 4874, 4889, and IC 4051. You might expect that, but when you look closer things aren’t quite as expected.

For one thing, the halo of globulars around each galaxy goes out much farther than it does for the Milky Way, as much as 5-6 times the physical size of the galaxy itself. Also, the number of globulars around those three galaxies is 10–30 times denser than around other galaxies in the cluster! Clearly these galaxies are hogging them.

Well, kinda. Some of the galaxies are clearly sharing globulars between them. And look at the two biggest concentrations: There’s a little bridge between them, too. It looks like there have been multiple interactions between galaxies, where bigger ones have stolen the globulars from others. You can also see regions in the cluster where there are very few globulars, likely the scene of some of those crimes.

There’s more. Some globulars have slightly bluer stars in them then others, which are redder. It turns out that redder globulars are more highly concentrated around galaxies, while blue ones are more spread out.

The thinking is that bluer ones are likely made in smaller, dwarf galaxies, while redder ones are made as part of the bigger galaxies themselves. The latter steals them from the former, so they tend to lie in the outskirts of these galaxies, while the homegrown redder clusters huddle closer in.

So just by looking at the location and colors of those globulars, we can learn about the history of this ridiculously huge city of galaxies.

And speaking of history, it occurs to me: When the light you see here left those galaxies, dinosaurs had yet to evolve on Earth, and untold countless numbers of plants died and later formed a layer in the Earth that we now (though hopefully for not too much longer) mine for coal.

Frontiers and Controversies in Astrophysics

Chapter 1. Review of Issues in Cosmology [00:00:00]

Professor Charles Bailyn: Okay, we’re talking about the origin and fate of the Universe. And let me remind you of the story so far. There are basically two sets of observations that are important here. One is the existence of the Hubble Diagram and Hubble’s Law, which is the observational relationship between distance and velocity for galaxies. And this leads you to the idea of a universal expansion. And the other is what we discussed last time: that if you look back into the past, if you observe at a large distance–that is to say, a large lookback time–what you discover is that things were different in the past. That the Universe, as a whole, looked somewhat different and, in particular, was significantly denser, which is exactly what you would predict if the Universe was expanding.

And these two things–these two observational facts put together are really what lead to the idea of a Universe with a Big Bang cosmology. And this is great because you can then use this assumption that everything is governed by the scale factor of the Universe. And the scale factor starts either at zero, or very close to zero, and gets bigger with time.

And you can use that concept to do all sorts of wonderful things. You can describe the past. And in particular, one of the things we did last time was to calculate the age of the Universe from the observations of the Hubble Constant. And you can predict the future. And the future depends on how the expansion of the scale factor changes. If the scale factor just continues to expand at its current rate, the Universe will continue to expand and gradually get sparser and sparser, and colder and colder, and more and more boring.

But it’s not expected that the expansion rate stays the same. It’s expected that the expansion rate will change. And, in particular, it’s expected that the expansion rate will slow down. Why? Because there’s matter in the Universe, and matter exerts gravity, and gravity tends to pull things back together again.

And so, this is where we ended up last time. If you assume that gravity is the dominant force–that is to say that any changes in the expansion rate of the Universe will be due to gravity, then, you can derive this critical density, which we did last time, which is a quantity equal to 3H 2 / 8 π G. H, you measure. The other things are just constants, and you can calculate what this quantity is. Now, at this point, let me write down a piece of astronomical jargon, which I didn’t do last time.

The actual density of the Universe, divided by this critical density, is given a letter of its own. This is written down as a capital Omega. So Ω is the true–the actual density of the Universe, whatever that turns out to be, divided by the critical density. And then, you can describe the future of the Universe, depending on what Ω is. If Ω is greater than 1, that means that the density’s greater than the critical density. And this leads to re-collapse and the “Big Crunch”–whereas, if Ω is less than 1, the Universe expands forever.

Somebody asked, what happens if Ω is exactly equal to 1? In that case, there is no Big Crunch. The Universe expands forever, but the expansion rate asymptotically approaches zero. But, of course, in real life, it’s very hard to get something that’s exactly some–any physical quantity to be precisely equal to any theoretical value.

And so, with this in mind, it then becomes very important to actually go out and measure the average density of the Universe because, then, you could divide it by this critical density. We’ve already measured H, so we know what this quantity is. And then, you could figure out what’s going to happen. So, the goal here is to determine the density of the Universe.

And conceptually, this isn’t such a hard thing to do. You go out and measure the mass of everything you can see. You try and do it over a large volume, because what you want to avoid–the mistake you want to avoid is to measure the density of a piece of the Universe that doesn’t represent the overall average. If we measured the density of material in this room, it would be something like 27 orders of magnitude bigger than the critical density. And if we assume that the Universe were just like this room, obviously, it would re-collapse. In fact, it would have re-collapsed long ago. But, we don’t do that because, of course, most of the Universe is not like this room. Most of the Universe is empty.

So, you say, well, we better include a lot of stars and the empty spaces between them. But even that’s a mistake, because you’re measuring stars in our galaxy. So, you say, well, we better include lots of galaxies and the empty spaces between them. That still doesn’t work for a while, because there are clusters of galaxies. There are clusters of clusters of galaxies. And so, you have to go really, quite far out, before you have a fair sample of what the average conditions in the Universe are like. But, in principle, that’s certainly possible to do. You just keep measuring things further and further and further away, until you get to a point where, if you increase the distance–where, as you increase the distance, that density doesn’t change anymore. So, you’re out to the part where you’ve really achieved the average. How do you know you’ve achieved the average? Well, you look out twice as far and you get the same answer.

And so, in principle, the way you do this is, you add up all the mass in some sizeable chunk of the Universe–in a sufficiently large chunk of the Universe, where sufficiently large is sufficiently large to average over any local perturbations. So, you add up all the mass and you divide by the volume. You divide by the volume that that mass occupies. And so, obviously, you have to identify all the different kinds of mass. And you have to make sure that whatever volume you’ve taken, you’ve found all the mass in it. You add it all up. You divide by volume. You determine–that gives you a value for density. You divide by the critical density and you know what’s going to happen to the Universe.

Chapter 2. Determining Mass [00:08:28]

Okay. Now, how do you find the mass of things? Determining mass. Well, one way you can do it is you can just go out and measure how bright ‒ yes, go ahead.

Student: Can you put the other slide up?

Professor Charles Bailyn: Oh, put this back for a second. Top part? Bottom part? What do you-

Student: [Inaudible] if you don’t mind putting it on.

Professor Charles Bailyn: Yeah, yeah. So, you’ve determined the density of the Universe by adding up the mass. Divide it by volume. And then, the question becomes, “How do you determine the mass?”

And one way you can do it is, you look at how bright things are. Add up the light you see. And then, you assume some value for the amount of mass it takes to create a certain amount of light. So, that’s assuming what’s called a mass-to-light ratio. And so, you can do that, you know. If it’s the Sun, then one solar mass produces one solar luminosity. If all stars–if all objects are exactly like the Sun, then everything would be like that. It turns out that isn’t the case, but you can take local samples of stars and figure out what the average mass-to-light ratio is. And if you have some value that you’re happy with, of mass-to-light ratio, then you multiply the amount of light by the mass-to-light ratio, and this gives you a mass.

Student: Do you need to adjust for distance?

Professor Charles Bailyn: Sorry.

Student: Do you need to adjust for distance?

Professor Charles Bailyn: Well, what you mean by light is the–do you need to adjust for distance? What you mean by light is the intrinsic light. You mean the equivalent of the absolute magnitude, which takes the distance into account. So, what you need to ask is not how bright it looks, but its intrinsic brightness in this particular case. Yes. So, you do need to account for the distance, and so, you need to be thinking about absolute magnitude rather than apparent magnitude, yes.

And that’s one of the problems. That’s hard to do. The other problem, of course, is this awkward word, here [“assume”], which is the kind of thing that makes people nervous, because you could get that wrong. If you’re looking at one kind of star and it’s actually some other kind of star, which happens to be much more massive but dimmer, like white dwarfs or something like that, then you’re going to make a mess of this.

So, there’s an alternative method, which you may already have considered, because we’ve done it in both of the previous parts of this class, which is, you measure orbits. And you do the same thing we did with–in part one and part two of the class. You find some star in the distant portion of the galaxy, orbiting around the galaxy. You figure out how fast the thing is going. You figure out how far the thing is going. You use Kepler’s Laws. And you determine the mass from orbital theory, from Kepler’s Laws, basically.

And, in particular, you know, V 2 = GM/a. And so, you can measure this from the Doppler shift. You can determine this basically, in the case of galaxies, galaxies are big objects. You can physically measure the angular separation on the sky. Use the small angle formula, if you know the distance, to determine this. So, this can also be measured, and therefore, this can be calculated.

And so, you go and do that for a whole bunch of galaxies. And this has been done. And let me give you some examples, here. Let me actually write down some numbers and do some calculations. Supposing you have a galaxy at a distance of 20 megaparsecs [Mpc]. And supposing it has an apparent magnitude of, something like, 14. These are kind of typical numbers for nearby galaxy clusters. There’s a particular–the nearest big galaxy cluster to us is a cluster in the constellation of Virgo, known as the Virgo cluster. If you want to know about the Virgo cluster, ask Hugh Crowll [ a graduate teaching assistant for the course] who is devoting his life to the study of this object and the galaxies within it. But these are sort of quasi-typical numbers, adjusted slightly because it’s actually 17 Mpc, which is kind of a pain.

All right. So, what do you know about the mass? What can you determine about the mass of such a galaxy? Well–oh, and let me warn you before we even begin that, of course, astronomers have played you a dirty trick–namely, that the symbol we use for magnitude is M. The symbol we use for mass is also M. So, you’ve got to keep those clear in your mind.

All right. So, what do we know about this? We know the relationship between apparent and absolute magnitude. And, as I said just a minute ago, it’s the absolute magnitude that we need to know in order to actually determine anything.

m - M = 5 log (D / 10 parsecs). So, let’s figure out the right-hand side first. That’s 5 log (2 x 10 7 ). That’s 20 Mpc. 1 Mpc is 10 6 .

Over 10. That’s 5 log (2 x 10 6 ). Now, what do I do about that? Let’s see. That’s 5 times log of 10 6 , that’s pretty straightforward, plus the log of 2. Because, if you add logs, then you multiply the thing inside the parentheses. So, log (2) + log (10 6 ) = log (2 x 10 6 ).

log (2) = .3. It’s just a useful number to know. The log of 2 is around .3. The log of 3 is around .5. The log of 5 is around .7. You could look it up.

And so, this is equal to 5 x 6.3.

Let me caution you at this point. So, let me give you a little side note, here. Do not approximate magnitudes. Why not? I mean, we approximate everything else in this course. Magnitudes are a logarithmic quantity, right? And so, you don’t approximate magnitudes for the same reason that you don’t approximate the exponents. You can’t say, 10 7 is equal to 10 6 . You can say 7 equals 6, but you can’t say 10 7 is equal to 10 6 , because that’s a factor of 10 difference, whereas the difference between 7 and 6 is just a little more than 10%. Similarly, this .3. You would have been tempted to get rid of it, right? Because who cares about the difference between 6 and 6.3? But, in fact, it comes out of this log of 2. And so, .3 in the log is actually a factor of 2. And so, you got to not approximate the exponents. This is important. Yes?

Student: Does this mean we should also try to be more precise when we’re dealing with magnitudes?

Professor Charles Bailyn: Well, yes. That’s saying–I guess that’s saying the same thing. You should be more precise. That means you shouldn’t approximate. Yeah, so, I guess. But, it’s for the same reason that you don’t approximate the exponents. And it’s also true that the numbers are easier to work with, because it turns out that you add them rather than multiplying them most of the time, so, it’s not such a bad thing. Anyway, here we are at 31.5, so what have we got? We’vem - M = 31.5. This M was stated in the problem to be 14. So, 14 - 31.5 = M.

So, M = -17.5. Okay. That’s not such a bad number. We can work with that.

So, now we know the absolute magnitude. We know how bright the thing is. So, now we can figure out how many times brighter than the Sun it is. Why is that a useful thing? Because if you then make the assumption that the mass-to-light ratio is the same as the Sun, that this galaxy consists entirely of Sun-like stars, then you can determine how massive it is. So, let’s do that.

How many Suns–and this is the other magnitude equation. This is, you know, M1M2 is equal to–for two different objects, is equal to - 5 ⁄2 log of the brightness of 1 over the brightness of the other.

But I think I want it in the other form. I think I want it in the form of 10 -0.4 , or 10 -2/5 (M1-M2) = b1 / b2. This is the exact same equation, as you’ll recall, just having been–getting rid of the log, taking everything, putting it into 10 to the something power.

The reason I want it in this form is that this is the answer I want. I want b1 / b2. I want one to be the galaxy. I want two to be the Sun. So, then, I’ve got 10 -2/5 , and then, the galaxy is -17.5, that’s the absolute magnitude. The Sun is 5, has an absolute magnitude of 5. And that’s going to give me the brightness of the galaxy over the brightness of the Sun. That’s 10 -2/5 (22.5) . Let’s see. The minuses cancel out, so that’s a plus, actually.

2 ⁄5 x 22.5 ‒ well, let’s see. 2 x 22.5 = 45. A fifth of 45 is 9. So, this is equal to 10 9 .

So, this galaxy is a billion times brighter than the Sun, 10 9 times brighter than the Sun. So, if it were made out of Sun-like stars, it would have a mass of a billion solar masses. So, mass would equal 10 9 times the mass of the Sun, if all Sun-like stars.

But, it turns out that galaxies tend to be somewhat dimmer than the Sun, per unit mass. Most stars are a little bit less massive than the Sun, but a lot less bright. This is just the way stars turn out to be. And so, typical mass-to-light ratios of populations of stars tend to be on the order of 10, or something like that, times the Sun. So, probably it needs to be more massive, because typical stars are fainter than the Sun. Typically, stars are fainter. So, you could guess and say, mass, maybe, should be, I don’t know, 10 times greater than that, 10 10 solar masses.

And you can see why this particular line of reasoning starts to get pretty dubious, because I picked this number completely out of the air. There’s actually some modest basis for it, but you could pick other numbers. You could argue about this endlessly and you wouldn’t get very far. Why should it be 10 times the Sun? Maybe it’s 100. Maybe it’s 1,000. Maybe it’s less than the Sun. How would you really know?

And so, let’s go back and do the other approach–namely, figure out its mass from orbits of things around it. So, let’s look at–supposing it’s an edge-on galaxy. Here’s the center of the galaxy, or–and, actually, let’s look at it from the top. So, here’s a nice spiral galaxy of some kind. Here’s the center of the spiral galaxy. Here’s some star way out on the edge. That star is moving around the center of the galaxy. It has to be, or it’s going to fall in. So, it’s orbiting around the center of the galaxy, presumably in some circular orbit. You’re down here, looking at this thing.

And, of course, you can measure the velocity of that star by the Doppler shift, because it’s moving away from you. And so, one can measure this velocity. You can measure this distance. That would be the equivalent of a in our formulas, because it’s the distance between the orbiting object and the center. Stars are much less massive than galaxies so we don’t have to worry about the motion of the galaxy. And you can use a familiar equation–namely, V 2 = GM / a.

So, now, let’s give this some numbers. Typical velocities of things orbiting around the galaxy turn out to be something like 200 kilometers a second, or 2 x 10 5 meters per second. And the size of a typical galaxy, you know, out to where it stops being easy to see stars is, oh, I don’t know, what number did I take here? Yeah. Let’s call it 20 kiloparsecs, which is 2 x 10 4 parsecs. And a parsec is 3 x 10 16 meters. So, this is 6 x 10 20 meters. So, now, let’s calculate M.

[(2 x 10 5 ) 2 (6 x 10 20 )] / (7 x 10 -11 ). Get rid of those–let’s see, that’s (4 x 10 30 ) / 10 -11 .

4 x 10 41 , this is in kilograms.

One solar mass, you recall, is 2 x 10 30 . So, this mass, in units of the Sun, (4 x 10 41 ) / (2 x 10 30 ), which is something like 2 x 10 11 solar masses.

And now, we have a problem, right? You probably don’t remember what the answer to the previous version of this problem was, where we did it with light. That came out to a magnitude of–the brightness was about 10 9 times the Sun. Maybe the mass is 10 10 times the Sun. But now we’ve just calculated it in this other, more reliable way, and it’s 2 x 10 11 . It’s 20 times more massive than you thought it was going to be, given how bright the light from this thing was. Yes, question?

Student: [Inaudible] mass of the galaxy?

Professor Charles Bailyn: This is the mass of the galaxy, yes.

Now, before I go on let me just point out–those of you who have taken a look at the problem set–what I’ve just done here, this calculation I’ve just done, is problem one of the problem set, except done backwards. On the problem set, what I did is, I told you what the density was, what the critical density was, and then, you had to derive characteristics of the galaxies from that.

Here I’ve told you what the galaxies are like. We figured out how big–how massive they are. If we divide by the volume, we’ll get a density. So, we’re doing the same problem backwards. I should say, the numbers I’ve chosen here are different, so, you can’t know the answer to the problem set by looking at the premises of these particular things. But, what I’m doing is the exact same set of calculations, only done backwards. So, that may or may not be helpful.

Chapter 3. Dark Matter: WIMPs? [00:26:39]

But let’s pause here for a moment, because this is now–we’re now up to–we’re making progress. We’re now up to Frontiers and Controversies circa 1985. You’ll remember, in 1920, they were worried about whether the spiral nebulae were actually galaxies. In 1950 they were worried about, maybe the “steady state” was the correct response. And by the time 1985 rolls around, the big issue is mass is determined by orbital rotation. So, what you might call dynamical masses–that is to say, determined by orbits of things around galaxies. Orbits around galaxies. And also, I should say, galaxy clusters. You can have galaxies orbiting around each other and galaxies orbiting around whole clusters of galaxies, and the same thing is true. And so, around galaxies and galaxy clusters–are much bigger than you expect from the light they give off. And therefore–by about a factor of 10. By approximately a factor of 10.

So, there’s 10 times more mass than you can account for by adding up all the stars. Now, there’s mass in other forms than stars. There’s also dust. There’s also gas. These are things you can detect in other ways. You add them all up and you’re still off by about a factor of 10. So, there’s 10 times more mass than you have any way of accounting for. This is the so-called dark matter problem. So, this is Frontiers and Controversies in 1985. There’s all this dark matter. Most of the matter in galaxies is in some form that we can’t detect. It’s dark matter, and what is it?

Now, unlike Frontiers and Controversies in 1920 and 1950, this is one that we haven’t solved yet, so I don’t know the answer. For a quarter of a century, people have been busy trying to figure this out. There’s still no good answer. And ten years ago, when I taught this course, this question of what is the dark matter was a big focus of this part of the course. Now, I’m going to talk about it only in this class, only in one lecture, because we got way bigger problems, even, than this. That’s saying a lot. I’ve just told you that we don’t know what 90% of the mass in the Universe is, and then, we’ve got bigger problems than that. So, things are getting a little murky, here, and not just because the matter is dark.

Okay. But, let me pause a little bit on dark matter, because it’s an interesting problem. And, as I say, we have no idea what this stuff is. What are the possibilities? So, here’s a hypothesis. Hypothesis #1 is that what this stuff is, is some kind of unknown sub-atomic particle. And it has to have two characteristics, this sub-atomic particle, for it to work out. It has to have mass. That’s pretty basic. If you’re using it to explain mass, you can’t have photons, right? Photons don’t carry any mass.

It has to have mass, but it has to not interact with light. No interaction with light. If it absorbs light, it would be opaque, and we would know it was there, because galaxies behind this stuff would look dim. Alternatively, if it gives off light, then we’d see it. And so, it has to not interact with light, or interact with light only very weakly. And so, these are given the name, generically, Weakly Interactive Massive Particles, or WIMPs.

So, here’s the hypothesis: the Universe is 90% WIMPs. This is not such a crazy idea as it might, at first, seem. There are known sub-atomic particles that have these properties. There’s something called the neutrino. There are trillions of them going through this room every second. They have mass and they don’t interact very much with anything. They’re known to exist from particle accelerator experiments, and they have been detected from celestial sources.

Now, we know that–for various reasons, that the dark matter doesn’t consist of neutrinos. But, there could be many other kinds of particles with these kinds of characteristics, and indeed, some are predicted by current particle theories. As I say, WIMPs have been detected–I’m sorry, WIMPs have not been detected, but neutrinos have been detected.

Here’s how they do it. It’s kind of an amazing experiment. They took a mineshaft in South Dakota and filled it with cleaning fluid. And the reason they did that was that every so often–neutrinos don’t interact with light, but they do interact, occasionally, with chlorine atoms. And the effect of a neutrino banging into a chlorine atom is to turn it into argon. And so, this happens–there are–as I say, trillions of neutrinos flow this mine every second. Once a day or so, one of them will hit a chlorine atom just right, create an argon atom.

So, here’s what you do. You fill your mineshaft with cleaning fluid, a large fraction of which is chlorine, and you count the argon atoms that bubble off the top. And this has been successful. They detected neutrinos emitted from the Sun. The Sun is–all stars that have nuclear reactions going on in them, emit neutrinos as part of the output of these nuclear reactions. And then, they had a problem, because they had predicted how many neutrinos you ought to see from the Sun in an experiment of this kind, and they didn’t see enough of them. They only saw a third of them.

And it turns out–and then, there was a big debate for a long time. This is Frontiers and Controversies circa about 1975. There was a big debate for a while. Where are all the solar neutrinos? Is it possible that we don’t understand nuclear reactions in the Sun? Is it possible that we don’t understand the chemistry of chlorine or argon? After all, you’re counting individual argon atoms, so that’s kind of a difficult task.

No, it turned out that what was going on was, we didn’t understand neutrinos. And it turns out there are three kinds of neutrinos. And neutrinos switch back and forth between these different kinds, and you could only detect one kind by the chlorine. And so, they were all emitted from the Sun as if they were in the form that you would have been able to detect them. But as they traveled from the Sun to us, some fraction of them flipped back and forth between all these other kinds, and you ended up only with about a third of them. So, it was a big piece of particle physics that was discovered.

We have also detected, by now, neutrinos coming from supernova explosions. So, there are󈝷 of them, I think, were detected, all at once. And if you’re detecting things, sort of, once per day, and then you suddenly detect 11 of them over the course of a few minutes, you’ve seen something exciting occur. And that is now known to be this supernova explosion that occurred in a neighboring galaxy.

And so, there are a bunch of–so, by analogy with that, people are looking for the WIMPs that make up the dark matter. If all this dark matter is in WIMPs, there are lots, and lots, and lots of these things, and they’re going through us every second.

So, there are a whole bunch of experiments with the same basic characteristics. You have a huge vat of something, and something is supposed to happen, occasionally, when one of these WIMPs hits whatever’s in the vat. So, the Japanese have, sort of, a cubic mile of distilled water, and they’re looking for little light flashes when the neutrino runs into the water molecule. They busted all their detectors recently, and they had a sort of earthquake, and it was bad for the little light detectors they had put on the inside of these things.

But there are a lot of such experiments. Dan McKinsey, here in the Physics Department, is a big player in one of them. And the hope is that you will see the interaction between one of these WIMPs, of which there must be an incredibly large number, with something. This has, so far, failed. So, there is no direct evidence from WIMPs.

The other hope, I should say, is that every time you build a bigger collider, you make new kinds of sub-atomic particles, and that they’ll eventually make something that looks like it could be a WIMP. And that hasn’t been–happened either. So, no detections yet. No direct detections. With considerable effort, you know, this is going to turn out to be 90% of the mass in the Universe. So, you would like to detect it because if you do, they’ll give you a Nobel Prize.

All right, that’s one hypothesis. There’s another hypothesis. So, here’s Hypothesis #2. It’s just, you know, dark chunks of something that doesn’t glow. Ordinary matter–chunks.

Student: Do these hypotheses exist today or [inaudible]

Professor Charles Bailyn: Yes, yes, yes, all of the–we don’t know what it is, and so, nothing has yet been ruled out. What happens is that they–you know, they continue to conduct these experiments, so, you can rule out WIMPs with certain kinds of characteristics, because you would have detected them. Similarly, you can rule out some of these other things with certain characteristics, because you would have noticed they were there. But both of these hypotheses are still more or less viable.

Chapter 4. Dark Matter: MACHOs? [00:37:30]

Chunks of ordinary matter that just don’t glow, that don’t emit light. Now, there’s some limitations. These chunks can’t be too small, because if what you’ve got are tiny, you know, micron-sized particles, we call that dust. And, basically, that’s what it is. It would just be dust.

The problem with dust is, dust in large quantities is opaque, and you can’t see through it. And therefore, you would know it was there, because it obscures the light of things behind it. And, indeed, we see cosmic dust this way all the time. It’s just, there isn’t nearly enough of it to account for any substantial fraction of the dark matter. So, dust would be observed because it–by obscuring light. And it also tends to glow in the infrared. And so, we know that dusts exists but we can count how much of it there is, because it obscures light and it makes its presence known in other ways.

It’s also true that these chunks of ordinary matter can’t be too big. They can’t be the size of whole galaxies, or even a substantial fraction of a galaxy. You can’t take all your dark matter and put it into one lump per galaxy, or even 100 lumps per galaxy because if they were very large masses, you’d see it, because it would disrupt the orbits of stars around the galaxy. So, if there was some huge unknown mass, you’d see things orbiting around it. And, in fact, we do. We see these supermassive black holes in the centers of galaxies and we know they’re there, because we see stars orbiting around them, just like the problem on the last Midterm.

And so, it can’t be too small. It can’t be too big. But, you could, perhaps, have, sort of, a bunch of star massed so, you could have sort of a bunch of star massed, or planet massed dark things in–it would have to be, for various technical reasons that I won’t go into, it has to be in the outer parts of galaxies, in the halos of galaxies. So, that, in principle is possible. We wouldn’t have any direct way of detecting them. These things are called Massive Astrophysical Compact Halo Objects. [Laughter] Some people get it. Massive, because they have to carry mass. Astrophysical, because they’re not particles. Compact, because if they were big you’d–you know, they’d block light and you’d see them. Halo, because that’s the part of the galaxy they’re in. These are MACHOs, right?

And so, the alternative to WIMPs is MACHOs. And so, the alternative explanation is that 90% of the Universe is MACHOs. There’s been a very clever experiment carried out to try and find these things. Here’s how you do it. You do it with gravitational lensing.

Lensing MACHO searches remember gravitational lensing? This is this business that mass bends light. So, here you are. You’re looking at some star. And, in between you and the star is a MACHO of some kind. So, here’s the MACHO. You can’t see the MACHO, but the presence of the MACHO changes the direction of the light. So, it comes into you like this, and it basically acts like a lens. And, in particular, the way it acts like a lens, in the case of MACHOs lensing stars, is it makes it brighter–makes the star brighter.

Now, in order for this to work, the alignment has to be essentially perfect. All of these objects are moving around. They’re orbiting the galaxy and stuff. So, the alignment holds for a few weeks, typically. So, what you’ll see is, you’ll see this star become much brighter. And it can really become much brighter–we’re talking tens to hundreds of times brighter than it ordinarily was. This lasts for a few weeks, and then, it goes away. These have been observed. These lensing events have been observed. Lensing events observed. But there are too few of them to explain the dark matter.

Now, there are still ways out. Let’s see. If you have particularly low mass MACHOs, so–supposing the whole Universe is filled with things about the mass of Earth, those cause lensing events that might be too small to see. Alternatively, supposing you have things that are many thousands of times the mass of a star, but not big enough to totally disrupt galactic orbits, then, there are many fewer of them for a given amount of mass, and there aren’t enough MACHO events that you would have expected to see any substantial number of them.

So, there’s still a way around the result of these experiments, if you want to believe in MACHOs. But it’s getting very tough. So, no WIMPs detected so far. No MACHOs. You could still postulate kinds of WIMPs and kinds of MACHOs that might explain the dark matter, but it’s getting kind of tough.

Most people, I think, believe in WIMPs. Most people tend to believe in this. But, and as far as I can tell, that’s because the particle physicists keep coming up with new candidate WIMPs that might exist, but that we haven’t quite been able to see, so far. And so, there’s a theoretical basis for the existence of these things, whereas, with these MACHOs, if you ask the astronomers ‒ well, fine. So, you want to have 90% of the Universe be in little Earth-like things just floating around with no star, how did they–how did that happen? How did these come into being? We really have no answer at all for that. So, there’s no theoretical basis for any of the still-allowed categories of MACHOs. And so, at the moment, people tend to believe WIMPs over MACHOs, although there’s no direct evidence for either. Yes?

Student: If 90% of the matter of the Universe is made of little Earth-like objects, then wouldn’t that be 90% of the Universe is made of metal?

Professor Charles Bailyn: Oh, Earth. Earth-mass objects is what I meant. I don’t care what it’s made out of. Yeah, maybe there are little Earth-sized balls of hydrogen. That would be fine too. Except how do you get them? We know something about how balls of hydrogen form and what they become. They turn into stars. This is well known. And one of the popular kinds of MACHOs was just very, very dim stars. And this is one of the things that the space telescope helped to rule out, because it can see really faint objects, and they weren’t there.

And so, no WIMPs. No MACHOs. And so, we don’t know what’s going on.

That was a digression. And what I digressed from was the fact that this galaxy that we had measured the mass of turned out to be 2 x 10 11 solar masses, or around 4 x 10 41 kilograms. If you have these things, one such galaxy every–I don’t know, 2 Mpc, or so, what’s the density of the Universe? Remember, that’s where we started–of the Universe. So, now, let’s finish this calculation. Let’s see the density is equal to M/V.

4 x 10 41 , from observing these orbits. And the volume, down here, is going to 2 Mpc cubed. That’s 2 x 10 6 , times 2–sorry, times 3 x 10 16 . That’s 1 parsec. So, this is 6 x 10 22 . I want to cube it.

6 x 6 = 36, times another 6, is 200.

So, that’s 200 x 10 66 , or 2 x 10 68 .

So, then, the density of the Universe.

(4 x 10 41 ) / (2 x 10 68 ), that’s equal to 2 x 10 -27 kilograms per meter cubed.

And real critical can be calculated–turns out to be, as you’ll discover on the problem set, 6 x 10 -27 in these units.

ρ over ρcritical is equal to about ⅓.

So, if you buy that, the Universe is going to keep expanding, because Ω, the ratio of the density to the critical density is only about ⅓.

But the problem is, we’ve got all this dark matter around and what we’re doing is, we’re adding up galaxies. How do you know that there isn’t a whole bunch of dark matter where there aren’t galaxies? And where there’s nothing to see orbiting around, you have no idea what this stuff is. And, indeed, most of the WIMP kinds of ideas, sort of, postulate some kind of dark matter that, kind of, pervades the Universe. And so, you’d expect there to be somewhat more of it than you can see in any given galaxy.

Well, somewhat more than 1/3 gets you into dangerous territory namely near one, which is the thing we’re trying to distinguish–whether this number is greater than 1 or not. And so, you need a new approach. This isn’t going to get you the answer. And so, there is a different approach. And that’s what we’ll talk about next time. And that will finally bring us up to Frontiers and Controversies in the twenty-first century.

Interacting galaxies produce eye-shaped “tsunami” of stars

Galaxies IC 2163 (left) and NGC 2207 (right) recently grazed past each other, triggering a tsunami of stars and gas in IC 2163 and producing the dazzling eyelid-like features there. ALMA image of carbon monoxide (orange), which revealed motion of the gas in these features, is shown on top of Hubble image (blue) of the galaxy pair. Image credit: M. Kaufman B. Saxton (NRAO/AUI/NSF) ALMA (ESO/NAOJ/NRAO) NASA/ESA Hubble Space Telescope. Astronomers using the Atacama Large Millimetre/submillimetre Array (ALMA) have discovered a tsunami of stars and gas that is crashing midway through the disc of a spiral galaxy known as IC 2163. This colossal wave of material &mdash which was triggered when IC 2163 recently sideswiped another spiral galaxy dubbed NGC 2207 &mdash produced dazzling arcs of intense star formation that resemble a pair of eyelids.

“Although galaxy collisions of this type are not uncommon, only a few galaxies with eye-like, or ocular, structures are known to exist,” said Michele Kaufman, an astronomer formerly with The Ohio State University in Columbus and lead author on a paper just published in the Astrophysical Journal.

Kaufman and her colleagues note that the paucity of similar features in the observable universe is likely due to their ephemeral nature. “Galactic eyelids last only a few tens of millions of years, which is incredibly brief in the lifespan of a galaxy. Finding one in such a newly formed state gives us an exceptional opportunity to study what happens when one galaxy grazes another,” said Kaufman.

The interacting pair of galaxies resides approximately 114 million light-years from Earth in the direction of the constellation Canis Major. These galaxies brushed past each other &mdash scraping the edges of their outer spiral arms &mdash in what is likely the first encounter of an eventual merger.

Using ALMA’s remarkable sensitivity and resolution, the astronomers made the most detailed measurements ever of the motion of carbon monoxide gas in the galaxy’s narrow eyelid features. Carbon monoxide is a tracer of molecular gas, which is the fuel for star formation.

The data reveal that the gas in the outer portion of IC 2163’s eyelids is racing inward at speeds in excess of 100 kilometres a second. This gas, however, quickly decelerates and its motion becomes more chaotic, eventually changing trajectory and aligning itself with the rotation of the galaxy rather than continuing its pell-mell rush toward the centre. Dazzling eyelid-like features bursting with stars in galaxy IC 2163 formed from a tsunami of stars and gas triggered by a glancing collision with galaxy NGC 2207 (a portion of its spiral arm is shown on right side of image). ALMA image of carbon monoxide (orange), which revealed motion of the gas in these features, is shown on top of Hubble image (blue) of the galaxy. Image credit: M. Kaufman B. Saxton (NRAO/AUI/NSF) ALMA (ESO/NAOJ/NRAO) NASA/ESA Hubble Space Telescope. “What we observe in this galaxy is very much like a massive ocean wave barreling toward shore until it interacts with the shallows, causing it to lose momentum and dump all of its water and sand on the beach,” said Bruce Elmegreen, a scientist with IBM’s T.J. Watson Research Center in Yorktown Heights, New York, and co-author on the paper.

“Not only do we find a rapid deceleration of the gas as it moves from the outer to the inner edge of the eyelids, but we also measure that the more rapidly it decelerates, the denser the molecular gas becomes,” said Kaufman. “This direct measurement of compression shows how the encounter between the two galaxies drives gas to pile up, spawn new star clusters and form these dazzling eyelid features.”

Computer models predict that such eyelid-like features could evolve if galaxies interacted in a very specific manner. “This evidence for a strong shock in the eyelids is terrific. It’s all very well to have a theory and simulations suggesting it should be true, but real observational evidence is great,” said Curtis Struck, a professor of astrophysics at Iowa State University in Ames and co-author on the paper.

“ALMA showed us that the velocities of the molecular gas in the eyelids are on the right track with the predictions we get from computer models,” said Kaufman. “This critical test of encounter simulations was not possible before.”

Astronomers believe that such collisions between galaxies were common in the early universe when galaxies were closer together. At that time, however, galactic discs were generally clumpy and irregular, so other processes likely overwhelmed the formation of similar eyelid features.

Is it possible for galaxy clusters to interact? - Astronomy

Finally, a superposition of all the data allows us to glimpse at what a crisis this merging cluster is in. Note that the optical image remains in its original color, the gas is in pink, and the mass is in blue. The image below is known as the Musket Ball Cluster. The actual collision of galaxies occurred about 700 million years ago. We can rewind the collisions in our heads and envision that blue/optical cluster on the right of the image was once on the left and so the blue/optical cluster on the left of the image was once on the right the clusters collided head on and the gas stopped dead at the center, but the galaxies and dark matter hardly stopped. There are several other images below of other dissociative cluster mergers with the same color scheme. Notice the different morphologies and distributions of mass, stars, and gas. The collisions are not always so straight forward.

The awesome thing about these cosmic mergers is how they can constrain the dark matter self-interaction cross-section. That is, exactly who much does dark matter interact with itself? The interpretation of these collisions is not always simple such as in the Train Wreck Cluster (seen above) where there seems to be an extra dark matter core not associated with any bright galaxy at the center of the image, but nonetheless these mergers can be thought of as astrophysical laboratories of dark matter. It would be very interesting to discover that dark matter self-interacts at all, however dissociate clusters will only be one piece of the extraordinary evidence necessary to make that claim.

Dawson, W., Wittman, D., Jee, M., Gee, P., Hughes, J., Tyson, J., Schmidt, S., Thorman, P., Bradač, M., Miyazaki, S., Lemaux, B., Utsumi, Y., & Margoniner, V. (2012). DISCOVERY OF A DISSOCIATIVE GALAXY CLUSTER MERGER WITH LARGE PHYSICAL SEPARATION The Astrophysical Journal, 747 (2) DOI: 10.1088/2041-8205/747/2/L42

Jee, M., Mahdavi, A., Hoekstra, H., Babul, A., Dalcanton, J., Carroll, P., & Capak, P. (2012). A STUDY OF THE DARK CORE IN A520: THE MYSTERY DEEPENS The Astrophysical Journal, 747 (2) DOI: 10.1088/0004-637X/747/2/96

Markevitch, M., Gonzalez, A., Clowe, D., Vikhlinin, A., Forman, W., Jones, C., Murray, S., & Tucker, W. (2004). Direct Constraints on the Dark Matter Self‐Interaction Cross Section from the Merging Galaxy Cluster 1E 0657󔽀 The Astrophysical Journal, 606 (2), 819-824 DOI: 10.1086/383178

Is it correct to say. (separation of solar systems and the expansion of the Universe)

No. The distances between distant galaxies are growing as you say, but nearby galaxies are bound by their gravity and are not separating. Nor are solar systems moving apart, since they are similarly bound together.

As I recall, the nearest system is actually moving slightly towards us, but not at any significant rate compared to the distance between us.

These timescales are nothing whe compared to cosmic timescale. The universe was approximately the same.

Other factors affect viewing dim stellar objects from Earth. Within my own lifetime under clear conditions on dark nights the Milky Way galaxy made a bright swath across the sky above our valley. Air and light pollution now mask all but the brightest stellar objects near cities.

While astronomical distances have not changed appreciably in a few thousand years, civilization has altered viewing.

The expansion rate is derived from the Friedmann equation, and uses an average universal mass/energy density. The Friedmann equation cannot be applied to our galaxy, so in no sense is the space in our galaxy expanding.

For example, the behaviour of the galaxy does not change over time. It doesn't matter how large the universal expansion becomes, it will never affect the dynamics of the galaxy itself. Eventually, the universal expansion rate may be huge, but that wouldn't affect bound systems like a galaxy. The equation that governs the overall expansion, based on the average mass/energy density of the universe, simply does not apply to the galaxy.