In a previous post, I talked a little bit about galaxy clusters in the context of that glorious object, the Bullet Cluster. Galaxy clusters are the biggest objects in the universe that are held together by their own gravitation. And they can really be big: the Virgo Cluster has over 2000 galaxies; the Coma Cluster (shown above) has over 1000 galaxies. As a comparison, our Milky Way galaxy is in a group of galaxies called the Local Group (someone was feeling really creative that day), which has over 40 galaxies, only three of which are big enough to be counted as full-size galaxies. Most galaxies seem to be in groups of clusters.
In visible light (the stuff most of us think of as “light”), what you see is the galaxies, but that’s not all of what’s in a cluster. In fact, the galaxies themselves come in third place after dark matter (first place) and hot hydrogen gas (a distant second). Dark matter is the most elusive bit, of course, since the only way so far to “see” it is through its gravitational effects; I’ll probably revisit that question in a later post, for those of you who are interested. Today I want to focus on the hot gas—so insert your favorite joke about gasbag scientists here.
The best way to see the hot gas directly is through the light it emits. In contrast to thermal emission like what we see from our Sun, the gas between galaxies in a cluster produces something known as breaking radiation or (if you’re feeling German and jargon-y) bremsstrahlung. This kind of radiation is a lot more energetic than visible light—the picture on the right shows the Coma Cluster in X-rays, in which the galaxies are barely a blip compared to the huge amount of light coming from the gas. In fact, the hydrogen gas is so hot it’s actually a plasma: the electrons are separated from the protons and are free to move around, and they are moving fast.
There’s a second way we can see the gas, which can be especially useful for extremely distant clusters and for understanding the evolution of clusters over the history of the universe. The cosmic microwave background (CMB) is radiation left over from when the universe was about 400,000 years old. Microwave light is a lot lower energy than X-rays, but when a photon from the CMB collides with one of those fast-moving electrons in the cluster gas, it gets a boost of energy. This scattering process is called the Sunyaev-Zel’dovich effect for the two astrophysicists who discovered it, and despite its fairly cumbersome name it’s very useful.First of all, because the CMB photons get boosted in energy, they get taken out of the cosmic microwave background—the presence of the galaxy cluster slightly changes the appearance. Since the CMB has a thermal spectrum, these missing photons make the galaxy cluster look like a cold spot in the CMB, despite the gas in the cluster being very hot! Sometimes called Sunyaev-Zel’dovich shadows, these cold spots are distinctive signatures. Studying this effect is really important for cosmology for a simple reason: it’s independent of how fast the galaxy cluster is moving with respect to us, so we’ll see it at the same level whether a cluster is close or far away. By contrast, the light from the cluster itself will experience the cosmic redshift, where the wavelength of the light gets stretched out as the universe expands. The Sunyaev-Zel’dovich shadow, though, will be unaffected by the expansion of the universe.
So what’s the big deal? Well, we can use the Sunyaev-Zel-dovich effect to perform a cosmic census of sorts: by knowing how many clusters there are over time, we can put constraints on various scenarios for cosmic evolution. This includes the properties of dark energy, the name we’ve given to the unknown property that makes the expansion of the universe accelerate. By understanding how dark energy behaves over the history of the cosmos, we might be able to get a better handle on what it actually is, a problem that has stumped scientists for the last 13 years.
(Note: I originally meant to write and post this on my birthday, which I share with Yakov Borisovich Zel’dovich.)