Figure 2.
Top Panel: The average
spectra of star-forming
(blue) and poststarburst galaxies
(green) in the GCLASS
clusters. The black lines
are the best-fit spectra
for both types based
on the star-formation
history in the lowerleft panel. Lower-left
panel: The best-fit
star-formation history
of the post-starburst
galaxies under the
assumption that the
star-forming galaxies
are their progenitors.
This suggests that
the average poststarburst spends ~
2.3 Gyr forming stars
before it is quenched
on a timescale of ~
0.1 ± 0.4 Gyr. This
is fully consistent
with the quenching
model derived from
dynamics of the galaxy
populations in Figure 1.
Towards a Physical Model of Satellite Quenching
The overall picture that emerges from this
analysis is that when star-forming galaxies
fall into clusters they continue forming stars
for 0.5 - 1.5 Gyr — about the time it takes to
fall far enough into the cluster to cross about
half of the virial radius. Once this happens,
the quenching process begins, and it is quite
rapid, perhaps even violent. Despite having concrete numbers to work with, there
are still challenges in understanding exactly
what is happening.
For instance, because the quenching process
appears to be so rapid, it’s tempting to believe
that all gas — both hot and cold — is being
removed from the galaxy. But that may not
6
GeminiFocus
be the full story. Galaxies at z ~ 1 are forming
stars at a much higher rate than those at z ~
0. If only the hot gas was removed, the higher-redshift galaxies would consume most of
their cold gas quite quickly, on a timescale of
~ 0.5 Gyr, which is the upper limit of what the
models permit.
So, there is clearly still work to do. Since
we know that the cold gas consumption
time-scales move to longer values at lower
redshift, measuring any evolution in the
quenching timescale with redshift is crucial
for breaking this degeneracy. Some studies
have been done at lower-redshift, and they
tend to indicate slightly longer timescales
than we measured at z ~ 1 (e.g., Wetzel et al.,
2013; Haines et al., 2013). This would implicate hot gas stripping, but the analysis was
2014 Year in Review
January 2015