searching for clusters of galaxies

Contrary to my expectations, things have been pleasantly busy down south. It all started when the Planck Telescope collaboration published a suite of 25 papers two weeks ago. And it looks like it is going to end with me writing a paper.

The SPT, scanning the sky for clusters of galaxies.

The Planck Telescope is designed to study the Cosmic Microwave Background (CMB - look it up on Wikipedia). Their main objective is to measure the large scale primary anisotropies of the CMB to the fundamental accuracy limit set my nature. The primary anisotropies in the CMB are those fluctuations that arise from small differences is the density of matter at the time when the CMB was emitted, 370,000 years after the big bang. The CMB is the focus of my graduate research.

The Planck team recently released a paper with a catalog of clusters of galaxies that they have found through the Sunyaev Zel'dovich Effect (SZE). One of the main science objectives of the South Pole Telescope SPT (my graduate research project) is to use the SZE to count the number of clusters of galaxies in the universe. Scientists are interested in counting clusters of galaxies, because the number and mass of the clusters that we see is strongly dependent on how much dark energy is in the universe. Dark energy is the name for the fact that we observe the universe to be expanding at an accelerating rate, and we don't know why. Clusters of galaxies are pushed away from each other by dark energy, so (roughly) more dark energy means that we see fewer clusters of galaxies.

Using the SZE to detect clusters of galaxies has proven to be very effective. The basic idea is that gas in clusters of galaxies scatter light from the CMB in a predictable way. Because of this, clusters of galaxies show up as holes in the background pattern of the CMB. We call these holes "decrements" in a sky map, and we characterize how robust the detection of a cluster is by its "signal-to-noise" ratio, i.e. the ratio of the signal (the decrement caused by the potential cluster) to the noise from the instrument, the CMB, and other astrophysical sources.

In any case, the Planck paper reported 20 previously undiscovered clusters of galaxies, 9 of which had not been confirmed by other methods of observation (X-ray, optical telescopes, etc). Five of these unconfirmed clusters are in the southern hemisphere of the sky, and are thus visible to the SPT. We had just included one of these clusters in a publication from 3 weeks ago which reported 23 new cluster discoveries made by the SPT. The SPT group decided that it would be a good idea to point our telescope at the locations of these unconfirmed clusters to see what we could find.

We observed a 2 degree-by-2 degree patch of sky around each location for 4 to 10 hours, depending on the patch. In this amount of observation time, we clearly saw each cluster. Sweet! Done with observations. Next we need to figure out exactly where each detector is pointing on the sky. We do this by looking at stars whose location we know very accurately, and adjust our data accordingly. After this we need to make maps from the raw observational data. This involves filtering the data to remove noise from the atmosphere , then dividing the data into pixels on the sky (think of a digital picture - if you zoom in a lot, you start seeing squares of color. Those are individual pixels).

Once we have maps we run a "cluster finding algorithm" to check if there actually is a cluster there. In technical terms, this is done by multiplying the maps with a filter that passes regions with the a range of sizes that we expect to correspond to the size of clusters, and maximizing the signal-to-noise ratio over filter size and location on the map. In the end, this procedure allows us to calculate the signal-to-noise ratio for each cluster.

Finally, we need to calculate the chance that what we see are false detections. In order for these to be false detections, there would have to be a CMB fluctuation that aligned with a decrement from a point source, such as a star. We simulated a sky including the CMB, point sources, and other effects, then looked for fluctuations that were larger than our smallest potential cluster detection. We found that there was one "false detection" in about one trillion degrees. Therefore, we are confident that what we see corresponds to real galaxy clusters.

I have spent the past week taking the observations and working on the analysis with the other post docs here at Pole. I put together a draft of the paper within 1 week of the idea even being floated, and it looks like we will be able to publish this paper on the time scale of a couple of weeks. I'm pretty excited about it, and it has given all of us down here at the Pole plenty to do!