Environment and Galaxy Mass
Quench Star Formation at z ~1
environments and galaxy masses, so either
the mass or the environment can be fixed to
isolate the effects of the other. The team obtained the new observations with the Gemini
Multi-Object Spectrograph instruments on
Gemini North and Gemini South. Complete results appear in A. Muzzin et al. (The Astrophysical Journal, 746: 188, 2012).
Exploring the Early Stages of
Massive Galaxy Formation in
the Local Universe
Figure 5.
Left panel: The
fraction of starforming galaxies as
a function of galaxy
stellar mass for
galaxies in different
environments. Right
panel: The fraction
of star forminggalaxies as a function
of environment for
galaxies with different
stellar masses.
Both stellar mass
and environment
determine the
likelihood of finding
star-forming galaxies.
What determines the history of star formation
in galaxies? Locally, both intrinsic characteristics of galaxies, namely mass and the environment, are correlated with properties such
as star formation rate and galaxy color. Adam
Muzzin (Yale University and Leiden Observatory, The Netherlands) and collaborators pursued this question in the earlier universe with
observations of 10 massive clusters at z ~1, as
part of the Gemini Cluster Astrophysics Spectroscopic Survey (GCLASS). They conclude
that in the early universe, both mass and environment play roles to determine a galaxy’s
star formation, but the effects are separable.
According to Muzzin, “While both stellar mass
and environment determine whether a galaxy
is ‘on’ (i.e., forming stars), once it is ‘on,’ the rate
at which it consumes gas and forms stars is
completely self-regulated by its mass.”
Environment still has a strong effect on
quenching star formation, suppressing it in
the denser regions at the cluster centers. The
team argues that this environmental effect occurs rapidly, so no signature exists to link environment to certain properties such as specific
star formation rate (Figure 5).
The GCLASS sample is selected based on observations at 3.6 microns, which corresponds
to the H-band in the rest frame of these galaxies and is thus a good indicator of stellar mass.
The clusters offer the advantage of a range of
24
GeminiFocus
Among the most important discoveries in
extragalactic astronomy during the past few
years is that massive, spheroidal-like galaxies
(M* > 1011 MSun) at redshift z > 1 are significantly more compact (by a factor of 4) than their
local equivalent counterparts.
Recent theoretical models show that some of
these massive, high-redshift compact galaxies could have possibly survived untouched
since their formation epoch at z > 3. If these
models are correct, the existence of a population of nearby, old, compact massive galaxies
presents the possibility of detailed studies of
galaxy formation mechanisms in the early universe using a sample of local galaxies.
Ignacio Trujillo (Instituto de Astrofísica de Canarias, Spain) and collaborators performed
the first systematic search of these galaxies
using the New York University Value-Added
Galaxy Catalog from Sloan Digital Sky Survey
Data Release 6 (The Astrophysical Journal, 692:
118, 2009). They find that the fraction of local
massive compact galaxies, similar to those
found at high redshift (i.e. with a re < 1.5 kiloparsec (5,000 light-years) and M* > 1011 MSun),
represent only 0.03 percent of the massive
galaxies in the nearby universe (z < 0.2). Moreover, these galaxies are relatively young and
metal-rich.
Detailed analysis of the morphological properties of these nearby massive compact gal-
June2012