Finding Galaxy Clusters with the Sunyaev-Zel'dovich Effect

Finding galaxy clusters is an important task in cosmology. By finding them and mapping them, we've determined the filamentary structure on the largest of scales in our universe. Their properties constrain cosmological models.

But finding them isn't an easy proposition. We can do it fairly easily in the local universe, but since light follows the inverse square law, they become very faint very quickly. As such, finding them by looking for their light isn't terribly easy. Often distant galaxies are found when something special happens: like a GRB or a supernova.

The most recent issue of the ApJ has a cool article that adds a powerful new trick to the mix; It employs the Sunyaev-Zel'dovich (SZ) effect.

First, a science lesson:

The SZ effect is essentially reverse Compton scattering. Compton scattering is what happens all the time in our atmosphere: (relatively) high energy photons come in, bounce off (relatively) low energy electrons and atoms and molecules and lose a bit of that energy (which is why the sky is blue) as well as bouncing off in a new direction (which is why we see light coming from the entire sky and not just directly from the Sun).

Of course, as with most physics, if it can go one way, why not posit the reverse and slap your name on it.

The SZ effect occurs when a relatively low energy photon passes through a hot gas, and picks up some of that energy. In other terms you could say the photon gets blueshifted or gains a shorter wavelength.

Now if you're galaxy savvy, you probably realize that clusters of galaxies are (usually) full of hot, ionized gas. (I say usually since galactic collisions can strip them of their gas, but this is more on a individual galactic scale as opposed to a cluster scale.) Propagating though this gas is low energy microwave photons from the Cosmic Microwave Background.

There's been several observations of known clusters showing the SZ effect, but this paper is the first to go the other way around: Using the SZ effect to find the clusters.

With this technique, the group found 4 clusters of galaxies in a ~40º section of the sky, three of which were previously unknown. And looking at the optical images of them, it's not hard to see why they were missed! The green circles show the cluster and the blue ones note galaxies that are gravitationally lensed.

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Staniszewski, Z., Ade, P., Aird, K., Benson, B., Bleem, L., Carlstrom, J., Chang, C., Cho, H., Crawford, T., Crites, A., de Haan, T., Dobbs, M., Halverson, N., Holder, G., Holzapfel, W., Hrubes, J., Joy, M., Keisler, R., Lanting, T., Lee, A., Leitch, E., Loehr, A., Lueker, M., McMahon, J., Mehl, J., Meyer, S., Mohr, J., Montroy, T., Ngeow, C., Padin, S., Plagge, T., Pryke, C., Reichardt, C., Ruhl, J., Schaffer, K., Shaw, L., Shirokoff, E., Spieler, H., Stalder, B., Stark, A., Vanderlinde, K., Vieira, J., Zahn, O., & Zenteno, A. (2009). GALAXY CLUSTERS DISCOVERED WITH A SUNYAEV-ZEL'DOVICH EFFECT SURVEY The Astrophysical Journal, 701 (1), 32-41 DOI: 10.1088/0004-637X/701/1/32