Giants lurk the heart of the Westerlund 1 cluster. It’s an open cluster that contains some of the most massive stars in our galaxy. It contains yellow hypergiants, red supergiants, Wolf Rayet stars and supergiant stars. Given its size and density, Westerlund 1 will likely evolve into a globular cluster.

Because these different types of massive stars have particular lifetimes, we can actually get a pretty good handle on the age of the Westerlund 1 cluster. Red supergiants, for example, generally don’t form until the star is about 4 million years old. On the other hand, Wolf Rayet stars tend to die off after about 5 million years. So the cluster should be around 4 – 5 million years old.

Since Westerlund 1 is only about 12,000 light years away, the cluster provides an excellent opportunity to study the dynamics of large stars. Since large stars create strong radiation fields, and eventually explode to release the gas and dust of heavier elements, they play a central role in the evolution of galaxies.

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There’s a region of space about 150 million light years away that is gravitationally attracting the galaxies in its region, including ours.  It is known as the great attractor, and we’re not entirely sure what’s there.  The problem is it happens to lie in the direction of the zone of avoidance, so our own galaxy is blocking our view.

When we measure the motion of our galaxy relative to other nearby galaxies, we find that the Milky Way is moving in the general direction of the great attractor at a speed of about 2.2 million kilometers per hour.  Observations of other galaxies also indicated motion in that direction, which gave rise to idea that a large concentration of galaxies must exist within that region.  But since the great attractor is within the zone of avoidance, it was difficult to determine what is there.

As I mentioned last time, the zone of avoidance is the region of sky where the plane of our galaxy is located.  Because of the gas and dust in the plane it is difficult to see beyond our galaxy in that direction.  But gas and dust obscure some wavelengths more than others.  Radio, infrared and x-rays, for example, can penetrate the region more readily, so with the rise of astronomy at these wavelengths (particularly x-rays) we began map the region of the great attractor.

A map of the great attractor and Shapley supercluster. Credit: IfA

By the 1980s we were able to pin down the location of the great attractor, and found that it was centered at a large cluster of galaxies known as the Norma cluster.  Later observations found that this region doesn’t have enough mass to be the sole source of the attraction.  Instead a larger supercluster known as the Shapley supercluster.  It contains about 10,000 Milky Way sized galaxies, and is the largest known supercluster in the visible universe.  So now we have an understanding of the clusters pulling our galaxy.

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Yesterday I wrote about a galaxy cluster formed by the collision of four clusters.  This raised a question by a reader who wondered how galaxies can collide if they are all moving away from each other.  The short answer is that they aren’t all moving away from each other, but if the universe is expanding, shouldn’t all the galaxies be racing away from each other?  It all has to do with the way cosmic expansion and gravity work, and it tells us something about the nature of dark matter and dark energy.If the universe were not expanding, then you would expect some galaxies to collide with each other.  The gravitational attraction between galaxies would cause them to accelerate slowly toward each other, eventually leading to their collision.  Some galaxies might be moving to fast to be caught in the gravity of another galaxy, but even then there could be collisions.  In a static universe we would expect galaxies to be moving in all directions, with galactic collisions scattered throughout the visible universe.

Diagram from Hubble’s 1929 paper. Credit: Edwin Hubble.

But in 1929 Edwin Hubble published a paper showing the universe was not static.  He compared the distances of several galaxies with their redshift, and found that their motion followed a relation where the more distant the galaxy, the greater its redshift.  This meant galaxies were not simply moving at random, as you would expect in a stable, uniform universe. Instead, the more distant the galaxy the faster it is moving away from us (thus the greater its redshift). This relation between distance and speed is the same in all directions, which means the universe seems to be expanding in all directions.

If these galaxies were all expanding away from a single point (a common misconception about the big bang), then you wouldn’t expect to see many galaxies colliding with each other.  In fact, if they all radiated away from a single point, then you would expect the more distant galaxies to spread more thinly than closer galaxies.  This isn’t what we see at all.

Galaxy clusters in the universe.
Credit: Sloan Digital Sky Survey.

What we see is that galaxies tend to clump together in clusters, and those clusters tend to clump into superclusters.  Within a cluster or supercluster the galaxies tend to gravitate toward each other, leading to collisions between galaxies here and there.  Between the superclusters are large voids where there is very little.  What this tells us is that the galaxies aren’t just racing from a single point in space.  Instead the entire universe is expanding.  It is cosmic expansion that gives the relation between distance and redshift.  But even distant galaxies can be moving slowly relative to each other, so they can clump together just as easily as nearby galaxies.  So galaxies throughout the universe tend to clump into clusters and superclusters.

What’s particularly interesting is that if you look at the distribution of galaxies in the universe, you find that they clump in a particular way.  If the galaxies were just made of matter, then they wouldn’t clump as much as they do.  So the level of clumping means there must also be dark matter.  The gaps between the galaxies are larger than what you would expect for matter and dark matter alone, so there must be something accelerating the expansion of the universe, which is dark energy.

So rather than galaxy collisions being a mystery, they are actually a part of the obsevational evidence telling us that dark matter and dark energy exist.

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The Perseus cluster is a cluster of nearly 200 galaxies about 240 million light years away. It is the advantage of being a large cluster of galaxies at a relatively close distance, so it is often the focus of investigations on the behavior and evolution of galactic clusters. Yesterday in Nature (paywalled, arxiv version here) a new study on the Perseus cluster has been released, and the results are very interesting.

The team looked at x-ray scattering off of iron atoms, which they could detect by looking at a specific emission line of iron. Since only iron gives off this particular x-ray emission, a map of the cluster at that particular wavelength gives us a map of the distribution of iron atoms in the cluster. The results can be seen in the figure above.

The dashed circle marks the rough edge of the cluster, and the spokes show the concentrations of iron along different directions, with red being high, green middling, and blew low. As you can see, the central region has the highest concentration, but the green region extends pretty far. There are even pockets of green near the outer regions of the cluster. What this means is that iron is not simply clustered in the galaxies, but is much more uniformly distributed.

The team looked for iron because it is easier to observe than other elements, but iron can only be produced in the cores of stars, just like all “metals” (elements other than hydrogen and helium). So the distribution of iron is also a good measure of the distribution of all metals. And that tells us something about how the universe was seeded with heavier elements.

We’ve long known that “metals” are formed within the cores of stars. When large stars or white dwarfs explode a supernovae, those metals are thrown out into the universe, where they can be used to form other stars and planets. This is sometimes called the seeding process. What we haven’t been clear on is whether this seeding happened earlier or later in the history of the universe. Did the seeding process occur before clusters like Perseus formed? Or did the clusters form first, and seeding largely occur afterward?

The team found that about 60% of the iron distribution is in the outer half of the cluster. If the iron had been seeded after the cluster formed, it wouldn’t be so widely distributed. That means the seeding occurred before the cluster formed. This agrees with other evidence that points to the seeding process reaching a peak around 10-12 billion years ago, with galaxy clusters forming later.

Of course this is an observation of only one cluster. If the early seeding model is correct, one would expect similar clusters will also have a wide iron distribution. It will take further observation to determine if the early seeding model holds, or if the Perseus cluster is an unusual exception.

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