Friday, October 26, 2012

Studying Galaxy Evolution with CANDELS Observations of High Redshift Clusters

Galaxy clusters and superclusters represent the largest bound structures in the universe. Because of this they are key probes of the large scale physics driving the expansion of the universe and the coalescence of matter which ultimately spawns the galaxies we observe. Studies of galaxy clusters also played an important role in the discovery of dark matter, the bullet cluster ultimately being a smoking gun. The topic of this blog post, however, is using galaxy clusters as laboratories for studying galaxy evolution.

By the early 1980's astronomers were finding strong evidence that the morphology of galaxies is correlated with their environment (the density of galaxies in the vicinity). Galaxies observed in high density environments are far more likely to be massive, elliptical, and passively evolving (little or no ongoing star formation), but the physical origins of this relationship are still not entirely known. It is likely related to interactions between the galaxies and the cluster and/or interactions between the cluster galaxies themselves. Galaxy-galaxy interactions include galaxy mergers which can significantly disrupt and randomize the orbits of their stars, growing and transforming the galaxy in the process.

Top: false colour image of the cluster combining
images from Subaru and the Spitzer Space telescope.
Bottom: Detection image for the cluster showing
the overdensity of galaxies at the redshift of the cluster.
Recently a technique has been developed for discovering galaxy clusters at high redshift which exploits the fact that as you observe galaxies at increasing distances certain spectral features redshift into redder photometric bands. Using data from large scale, deep photometric surveys it is possible to measure the density of galaxies which have similar colors measured in various bands. However, because these discoveries are based on photometry, the certainty you have that clusters you find are not just chance alignments of unassociated galaxies is relatively low (more on this unfortunate fact can be found in this previous post). Nevertheless, this technique was used by CANDELS team member Casey Papovich and collaborators to discover a galaxy cluster at a redshift of 1.62, meaning light we detect from these objects was emitted when the universe was less than a third of its current age!

Since its discovery, this cluster has been extensively studied. Spectroscopic observations were undertaken in order to confirm the narrow range in redshifts for potential cluster members. Despite difficulties in observing strong spectral features at this redshift (due to overlap with strong night sky emission), redshifts for 11 galaxies were obtained and found to lie close to the expected value for this cluster. X-ray observations were also taken using NASA's Chandra X-ray Observatory finding that the x-ray emission from this cluster is dominated by a single point source with weak extended emission. This, along with the spatial distribution of cluster members, supports a hypothesis in which this is a young cluster still in the process of forming (it has also been dubbed a "proto-cluster").

In 2011, I joined the team studying this cluster led by Casey Papovich, just as the first Hubble imaging from the CANDELS project was being released. The depth and resolution of these images finally made it possible to study the structures and morphologies of these galaxies. Because the large field of view of these images allows cluster and field populations to be examined in a single uniform data set, CANDELS imaging is ideal for studies of environmental trends. Using a code called GALFIT, I was able to measure sizes of these galaxies, as well as a parameter which measures how compact these galaxies are: the Sersic index. A general rule of thumb is disky galaxies have Sersic indices around 1 while spheroidal galaxies have Sersic indices between 2.5 and 4.

Hubble Space Telescope coverage from CANDELS. The cluster sits at the edge of a much larger field allowing us to construct a large comparison sample of non-cluster galaxies from a single uniform dataset.

In the first paper published using the results of these fits, we looked primarily at the sizes of passive galaxies. We first found that, at this redshift, in both the cluster and the field, passive galaxies have smaller sizes than similar galaxies in the local universe. This means that these galaxies must evolve in size over the subsequent ~10 billion years. Second, in the cluster there is a dearth of compact passive galaxies, possibly indicating that the galaxies within the cluster are experiencing a accelerated evolution compared with those in the field. This accelerated evolution could be related to an enhanced rate of mergers in the higher density environments. Evidence supporting this scenario was found in a study by Jennifer Lotz who shows that members of this cluster are far more likely to have very close neighboring galaxies than members of the field population (this study will be the topic of an upcoming blog post).

We extended the analysis of our first paper by folding in the structure of these galaxies using the Sersic index. If this cluster were to follow a trend similar to the morphology density relation we see locally, one would expect passive cluster galaxies to be more compact (and therefor have larger Sersic indices). When we looked at this, however, we found just the opposite! The passive galaxies in the cluster were found to have more disk like structures and larger sizes (when compared with other passive galaxies in our sample), attributes which are more characteristic of galaxies which are actively forming stars. The spatial distribution of these galaxies was also found to be very interesting: the structural differences we observe appear to be driven by passive galaxies in the outskirts of the cluster. Passive galaxies in the cluster core and the field appear to make up a more uniform sample of compact spheroids.

So what's going on here? One possibility is that these galaxies were actively star forming, disk galaxies which were recently accreted into the cluster environment. During the process of accretion, interactions with the cluster itself stopped their star formation. This could be caused by gas (which fuels star formation) being stripped from the galaxy or being heated and thus preventing it from collapsing into stars. This heating could possibly be caused by intergalactic shock waves that are propagated as a cluster is in the process of collapsing. These shock waves are formed when the galaxies being rapidly pulled inwards meet the ambient medium surrounding the cluster. While these shocks are largely theoretical at this point, there has been recent evidence in their favour in the nearby Coma cluster.

One caveat to this study is that there is a small number of cluster galaxies which are bright enough for us to observe and many of our distance measures have a large uncertainty. Because of this, our results are at the edge of statistical significance. However, similar results were found by Ruth Grützbauch and collaborators for a more highly evolved cluster at a slightly lower redshift. Their cluster, XMMU J2235.3-2557 at a redshift of 1.39, is found to have what they describe as a "quenching radius". Any galaxy within this distance from the cluster is extremely passive, but as you move further out the star formation rate increases. A picture is emerging in which galaxy clusters influence the star formation of galaxies at extremely large distances from the cluster core, much further out than previous estimates. To confirm this result will require more similar studies of high redshift clusters, so stay tuned!

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