A Spectroscopically Confirmed Excess of
Dusty Star Formation in a Super Galaxy Group at z~0.37
[1]
Kim-Vy Tran (Texas A&M, U. of Zürich), Amélie Saintonge (MPE),
John Moustakas (UC San Diego), Lei Bei (OCIW), Anthony Gonzalez (U. Florida),
B. Holden (UC Santa Cruz), D. Zaritsky (U. Arizona), & S. Kautsch (U. Florida)
Introduction:
To trace how dust-obscured star formation varies with
environment, we compare the fraction of Spitzer/MIPS 24
micron sources in a Super Galaxy Group to the field and a
rich galaxy cluster at z~0.35. We draw on multi-wavelength
observations that include extensive optical spectroscopy
(>1800 redshifts) to isolate galaxies in each environment and
thus ensure a uniform analysis. We focus on the four galaxy
groups in Super Group 1120-12 that will merge to form a
galaxy cluster comparable in mass to Coma [2].
Fig. 1: HST/ACS F814 images (15’‘x15’’) of the Super Group
galaxies with infrared (IR) star formation rates (SFR) > 3
M/yr. Each image lists the absolute Vvega magnitude,
estimated log(M), Hubble Type, and 24 micron SFR.
[-23.07, 11.68]
[-5, 4.35]
[-22.83, 11.49]
[5.2, 151.4]
[-22.44, 11.04]
[-0.5, 59.97]
[-21.9, 10.72]
[-21.88, 10.84]
[-21.84, 10.65]
[2, 14.43]
[2, 13.77]
[2, 11.74]
[-21.23, 10.46]
[-21.22, 10.49]
[-21.17, 10.59]
[2, 4.86]
[-20.87, 10.06]
[2, 3.99]
[2, 4.02]
[-20.86, 10.28]
[5.2, 16.96]
[5.2, 25.67]
[-20.79, 10.35]
[2, 5.91]
[-22.18, 11.02]
[-22.06, 11.04]
[-22.05, 10.98]
[2, 15.87]
[2, 11.95]
[2, 30.9]
[-21.38, 10.56]
[-21.36, 10.47]
[-21.3, 10.54]
[2, 9.61]
[-21.13, 10.89]
[2, 9.31]
[-20.77, 10]
[10.5, 4.79]
[7.8, 41.71]
[-21.09, 10.18]
[4.5, 5.01]
[-20.77, 10.67]
[1, 4.42]
[2, 7.06]
[-20.99, 10.49]
[2, 5.42]
[-20.72, 10.12]
[2, 8.48]
[-21.96, 11.22]
[-5, 5.5]
[-21.26, 10.28]
[2, 7.14]
[-20.96, 10.51]
[6.5, 5.2]
Fig. 2: Rest-frame color-magnitude (CM) diagram of
galaxies in the FIELD (top), SUPER GROUP (middle), and
CLUSTER (bottom). The galaxies with IR SFR > 3 M/yr are
shown as filled circles; the corresponding fractions are
listed. The CM relation (solid line[3]) is normalized to the
red sequence in CL1358, and the same CM relation is shown
in each panel; the dotted
line is the Butcher-Oemler
color cut for blue galaxies.
At z~0.35, the fraction
of IR-bright galaxies in
the groups (z~0.37) is
nearly as high as in the
field and is four times
higher than in the cluster
(32% vs. 7%).
[-21.94, 10.86]
[2, 7.72]
[-21.23, 10.6]
[2, 13.88]
[-20.96, 10.74]
[2, 3.74]
[-20.54, 10.43]
[-5, 4.89]
Fig. 3: Relative fraction of absorption-line (circles),
emission-line (triangles), and IR-bright (squares) members
as a function of local galaxy surface density in the SUPER
GROUP (left panels) and CLUSTER (right panels). We show
luminosity-selected (V<-20.5; top panels) and mass-selected
10
(M>4x10 M; bottom) members. The long-dashed, dotted,
and short-dashed lines respectively show the absorptionline, emission-line, and IR-bright fractions in the FIELD.
The Super Groupʼs IR
luminosity function
confirms that it has a
higher density of IR
members compared to the
cluster and includes bright
IR sources not found in
clusters at z<0.35.
The Super Group 1120 shows a strong trend of
decreasing IR fraction with increasing galaxy density
(IR-density relation) not observed in the cluster. These
dramatic differences are surprising because the early-type
fraction in the supergroup is already as high as in clusters
[4]
, i.e. the timescales for morphological transformation
cannot be strongly coupled to when the star formation is
completely quenched.
Low-mass IR-bright members: The supergroup has a
significant fraction (~17%) of luminous, low-mass, IR-bright
members that are outside the group cores; once their star
formation is quenched, most will evolve into faint red
galaxies. Our analysis indicates that the supergroupʼs 24
micron population also differs from that in the field.
References:
[1]
Tran et al. 2009, ApJ, 705, 809
[2]
Gonzalez et al. 2005, ApJL, 624, 73
[3]
van Dokkum et al. 1998, ApJ, 500, 714
[4]
Kautsch et al. 2008, ApJL, 688, 5
Contact: kimvy.tran@gmail.com & www.kimvytran.org