Understanding the behaviors of galaxies in clusters is a large and complex problem that has not daunted Adam Muzzin of the Leiden Observatory at Leiden University in The Netherlands. Muzzin led an international team using data from the Gemini Cluster Astrophysics Spectroscopic Survey (GCLASS) in order to explore galaxies that have recently stopped (quenched) the formation of stars.
Left panel: the velocity vs. clustercentric radius phase space of galaxies in the nine GCLASS clusters. The velocities are in units relative to the individual cluster velocity dispersions and the radii are relative to the position of the brightest cluster galaxy scaled by the R200 of the cluster. The shaded regions are arbitrarily defined but are indicative of increasing time since infall (see text). Quiescent galaxies (red triangles), star forming galaxies (blue triangles), and poststarburst galaxies (green stars) all occupy distinct locations in phase space. Right panels: the ratio of quiescent and poststarburst galaxies compared to star-forming galaxies separated into the three radial bins marked by the dotted lines (top panel), and the three phase space bins marked by the shaded regions (bottom panel). The error bars are 1σ Poisson errors. Poststarburst galaxies are distributed fairly uniformly in the cluster by radius (top panel), with a peak in the middle bin; however, in phase space they are most prevalent in the middle bin and completely absent in the inner bin (bottom panel).
Their findings reveal that these galaxies are very different from other cluster galaxies, and for the first time show that these quenched galaxies tend to be closer to the cluster’s center and moving especially fast. As a critical part of their results, the team established unprecedented constraints on how long this quenching takes, and where it happens. It’s quick, by astrophysical timescales – between 100-500 million years, and happens roughly halfway out from the center of the cluster.