GeminiFocus 2014 Year in Review | Page 6

Figure 1. Left panel: The velocity of galaxies within the 10 GCLASS clusters relative to the velocity dispersion of each cluster versus the position of each galaxy relative to the virial radius. Quiescent galaxies are plotted as red triangles, star-forming galaxies as inverted blue triangles, and post-starburst galaxies as encircled green stars. Strikingly, the post-starbust galaxies form a “ring” structure at high velocities and intermediate radius. Right panel: Galaxies in a simulated set of clusters (black points). Green stars show galaxies that are “quenched” in the simulation on a timescale of 0.1 — 0.5 Gyr after they first cross about half the virial radius. The distribution of the simulated quenched galaxies is statistically consistent with the observed post-starburst population suggesting this is where and when cluster galaxies are first quenched. 4 The GCLASS Survey at z ~ 1 One additional challenge in observing this quenching process in action is that until very recently we’ve only been able to collect data on fairly nearby clusters. Locally, the average star-forming galaxy is only converting a few solar masses of gas into stars per year. That’s a pretty low level of activity. Therefore, the difference between a recently quenched local galaxy and one that has normal starformation activity is fairly marginal. One nice solution would be to look at galaxies at higher redshifts. The average star-forming galaxy at z ~ 1 is 10 times more active than locally, so if the quenching process is violent, it would be much easier to identify one that has recently been stripped of its gas. From 2009  2011 we carried out a large -  spectroscopic survey with the Gemini MultiObject Spectrograph of 10 massive clusters at z  1. This program, the Gemini CLuster ~  Astrophysics Spectroscopic Survey (GCLASS) — one of the largest surveys ever carried out on the Gemini North and South telescopes — required about 220 hours of observations, but has provided spectra for ~  500 cluster galaxies at z  1. GCLASS marked about a ~  10-fold increase in the number of spectra for cluster galaxies compared to any previous cluster survey at this redshift. GeminiFocus GCLASS had numerous scientific goals. One was to measure how the central galaxies in clusters grow over cosmic time (Lidman et al., 2012 and 2013), as well as how the stars are distributed in high-redshift clusters — both on the galaxy scale (van der Burg et al., 2013) and in the global cluster scale (van der Burg et al., 2014). In addition, it also looked critically at the stellar populations of cluster galaxies at z ~ 1 in Muzzin et al. (2012). New Insights More recently we’ve attacked the problem of linking cluster galaxy dynamics to the quenching process. This has led to some interesting new insights about the quenching process (e.g., Noble et al., 2013; Muzzin et al., 2014). One of the most striking revelations from the GCLASS data is that there appears to be a correlation between the stellar populations of the galaxies and their dynamical state within the cluster. The left panel of Figure 1 plots the GCLASS cluster galaxies relative to “typical” velocities of all galaxies (the velocity dispersion) against their distance from the cluster’s center (which has been normalized to the virial radius of each cluster). Galaxies have been color-coded based on their stel- 2014 Year in Review January 2015