Liza Sazonova
In the local Universe, dense clusters of galaxies have a striking over-abundance of red-and-dead spheroidal galaxies compared to star-forming disks. This is widely known as the the morphology-density relationship. However, the physical mechanisms that transform disk galaxies in clusters and quench their star formation are still disputed. The time period when this happens is also unknown. To answer these questions, star formation and morphology must be studied in tandem in more distant, early Universe clusters, where these environments just began to assemble.
We present a study of the morphology of galaxies in four distant (1 < z < 2) clusters with Hubble Space Telescope imaging. We find that two out of four clusters (z = 1.19 and z = 1.75) have already built up a population of compact spheroidal galaxies, and so have an established morphology-density relationship. We find that specifically the low mass (log M < 10.5) galaxies are more compact in clusters. We also find an intriguing system of two clusters at a remarkably small 3D separation of 73 Mpc that show no morphology-density relationship but whose galaxies have stronger merger signatures.
We conclude that there is a mechanism responsible for establishing the morphology-density relationship that is active in some clusters as early as z=1.75, and that this mechanism forms compact, spheroidal galaxies unlike gas removal processes that are prevalent in the local Universe. However, we find a significant intracluster variance that may depend on the larger cosmological environment in which the cluster is embedded.
An important factor affecting this evolution is the cosmic environment of a galaxy: whether it resides in a dense region (cluster of galaxies) or a less dense one (“field”). This is known as the morphology-density relationship: all nearby galaxy clusters almost entirely host quiescent elliptical galaxies. Therefore, nearby clusters must have some mechanism that quenches star formation in their galaxies and transforms their structure (morphology). We call this environmental quenching. The primary environmental quenching mechanism in local clusters is “ram pressure stripping”, where the star forming gas of a cluster galaxy is stripped away by the hot gas in the cluster.
However, galaxy clusters also exist in a more distant, and hence younger, Universe. Many have been identified at redshifts z > 1 (more than 8 Gyr ago), but it is unclear if these dense environments played a similarly important role in galaxy evolution then. To test this, we studied 4 distant galaxy clusters (1 < z < 2 ) and compared the morphology of cluster galaxies to those in the less dense field.
We obtained long exposure rest-frame optical imaging of cluster galaxies with the Hubble Space Telescope (HST), and used existing HST observations of field galaxies from the CANDELS survey. To ensure the comparison is robust, we constructed control samples of field galaxies that matched redshift and mass distribution for each cluster.
Optical imaging traces the stars in a galaxy, so we used these data to determine the stellar structure of cluster and field galaxies. We used an open-source code STATMORPH and Principal Component Analysis to parameterize the structure of a galaxy: its bulge strength (how spheroidal it is), compactness, asymmetry, and disturbance (deviation from a typical galaxy shape). We then compared field and cluster populations using Monte Carlo analysis. The two goals of our study were to determine if cluster galaxies are already more spheroidal than field ones at 1 < z < 2, and whether disturbance and compactness of cluster galaxies can give insight into the dominant environmental quenching mechanisms.
We found that galaxies in two out of four clusters are significantly more bulge-dominated and compact than field galaxies. This shows that the morphology-density relationship is already established in some clusters as early as 10 Gyr ago. However, the cluster cannot transform infalling galaxies into compact spheroids by gas removal alone. Therefore ram pressure stripping, dominant in the local Universe, cannot be the only important process in these distant clusters. Instead, more violent processes are required, for example, a merger of galaxies or tidal interactions that perturb the infalling galaxies. We further investigated the dependence of the morphology-density relationship on the galaxy mass, structural type, and distance from the cluster center. This gave us deeper constraints on the possible environmental quenching mechanisms, described in detail in our paper.
On the other hand, we found no evidence for the morphology-density relationship in the remaining two clusters, showing large variability between different clusters in the distant Universe. However, these two clusters are located remarkably close to each other, which complicated our analysis and added uncertainty, so the apparent lack of morphology-density relationship there is less conclusive. The details of the cluster-cluster variation in the morphology-density relationship at high redshift is a promising area of research to explore with the unprecedented capabilities of the upcoming James Webb Space Telescope.