lar populations: quenched galaxies are plotted as red triangles, and star-forming galaxies appear as inverted blue triangles. These
two populations are distinctly different, with
quenched galaxies tending to be found near
the cluster’s center and moving at slower
velocities. Star-forming galaxies are found
throughout the cluster and tend to be moving at higher velocities.
This had been seen before in lower-redshift
clusters (e.g., Biviano et al., 2002; Poggianti
et al., 2004). It can be explained naturally by
the quenched galaxies having been accreted into the cluster earlier (presumably when
they were still star-forming galaxies). Most
likely they experienced dynamical friction
and lost kinetic energy, gradually slipping
into orbits well within the clusters’ gravitational potential wells. Star-forming galaxies
are likely to be more recently accreted and
hence have higher velocities, as they are on
their first passage through the clusters.
Our team also looked at galaxies that
had spectral signatures indicating recent
quenching. These “post-starburst” galaxies
are shown as the encircled green stars in the
left panel of Figure 1. These high-velocity
objects had quite a striking distribution in
this Figure, as they tend to lie roughly in a
“ring” structure at intermediate radius. In
particular, they very clearly avoid both the
cluster core region, which is dominated by
quenched galaxies, and the large radius region dominated by star-forming galaxies.
This signature has never been seen before,
so modeling it seemed an obvious way to try
to understand more about the quenching
process. We were most interested in getting
some quantitative measurements of both
how long it takes for galaxies to quench
(once the process begins) and where in the
cluster it starts to happen. In particular, the
latter measurement is unprecedented. We
employed some high-resolution dark-mat-
January 2015
ter-only simulations of clusters to see if we
could input these numbers and reproduce
the observed distribution of the post-starbursts in the right panel of Figure 1.
In the simulations, we followed galaxies as
they first fell into the cluster. We then simulated quenching at different locations and
with different timescales. These models easily ruled out many locations and specific
timescales; the details of this can be found
in Muzzin et al. (2014). What worked best
in reproducing the distribution of the poststarbursts was if we quenched galaxies after
they first passed roughly half the virial radius (R ~ 0.5R200) on a timescale of ~ 0.1 - 0.5
Gyr. The resulting distribution of post-starburst galaxies is shown as the right panel of
Figure 1, and is statistically a good match to
the observation.
This was the first time that simultaneous constraints had been set on both the duration
of the quenching process and where it happens. We were concerned that our findings
depended heavily on a simulation, so we also
used the stellar populations of the galaxies to
test the model. Figure 2 shows the average
spectra of both the typical star-forming galaxy in the cluster (top blue) and the average
spectra of the post-starburst galaxies (bottom green). These spectra were fit to a range
of models, with the best-fit star-formation
history shown in the bottom left panel.
Basically, this test showed that star-forming
galaxies continue forming stars for several
Gyr within the cluster before they quench
— and that process needs to be rapid in order to create the stellar populations seen in
the post-starburst galaxies. Not only is this
consistent with what we d W&