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