THE NATURE OF STAR FORMATION AT 24 μm IN THE GROUP ENVIRONMENT AT 0.3 ≲z≲ 0.55
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abstract
Galaxy star formation rates (SFRs) are sensitive to the local environment;
for example, the high-density regions at the cores of dense clusters are known
to suppress star formation. It has been suggested that galaxy transformation
occurs largely in groups, which are the intermediate step in density between
field and cluster environments. In this paper, we use deep MIPS 24 micron
observations of intermediate-redshift (0.3 < z < 0.55) group and field galaxies
from the Group Environment and Evolution Collaboration (GEEC) subset of the
Second Canadian Network for Observational Cosmology (CNOC2) survey to probe the
moderate-density environment of groups, wherein the majority of galaxies are
found. The completeness limit of our study is log(L_TIR (L_sun)) > 10.5,
corresponding to SFR > 2.7 M_sun/yr. We find that the group and field galaxies
have different distributions of morphologies and mass. However, individual
group galaxies have star-forming properties comparable to those of field
galaxies of similar mass and morphology; that is, the group environment does
not appear to modify the properties of these galaxies directly. There is a
relatively large number of massive early-type group spirals, along with E/S0
galaxies, that are forming stars above our detection limit. These galaxies
account for the nearly comparable level of star-forming activity in groups as
compared with the field, despite the differences in mass and morphology
distributions between the two environments. The distribution of specific SFRs
(SFR/M_*) is shifted to lower values in the groups, reflecting the fact that
groups contain a higher proportion of massive and less active galaxies.
Considering the distributions of morphology, mass, and SFR, the group members
appear to lie between field and cluster galaxies in overall properties.
