247 research outputs found
Multi-Wavelength Properties of Barred Galaxies in the Local Universe. I: Virgo Cluster
We study in detail how the barred galaxy fraction varies as a function of
luminosity, HI gas mass, morphology and color in the Virgo cluster in order to
provide a well defined, statistically robust measurement of the bar fraction in
the local universe spanning a wide range in luminosity (factor of ~100) and HI
gas mass. We combine multiple public data-sets (UKIDSS near-infrared imaging,
ALFALFA HI gas masses, GOLDMine photometry). After excluding highly inclined
systems, we define three samples where galaxies are selected by their B-band
luminosity, H-band luminosity, and HI gas mass. We visually assign bars using
the high resolution H-band imaging from UKIDSS. When all morphologies are
included, the barred fraction is ~17-24% while for morphologically selected
discs, we find that the barred fraction in Virgo is ~29-34%: it does not depend
strongly on how the sample is defined and does not show variations with
luminosity or HI gas mass. The barred fraction depends most strongly on the
morphological composition of the sample: when the disc populations are
separated into lenticulars (S0--S0/a), early-type spirals (Sa--Sb), and
late-type spirals (Sbc--Sm), we find that the early-type spirals have a higher
barred fraction (~45-50%) compared to the lenticulars and late-type spirals
(~22-36%). This difference may be due to the higher baryon fraction of
early-type discs which makes them more susceptible to bar instabilities. We do
not find any evidence of barred galaxies being preferentially blue.Comment: 13 pages, 14 figures. Submitted to Ap
The Importance of AGN in an Assembling Galaxy Cluster
We present results from our multi-wavelength study of SG1120, a super galaxy
group at z=0.37, that will merge to form a galaxy cluster comparable in mass to
Coma. We have spectroscopically confirmed 174 members in the four X-ray
luminous groups that make up SG1120, and these groups have velocity dispersions
of sigma(1D)=303-580 km/s. We find that the supergroup has an excess of 24
micron members relative to CL1358+62, a rich galaxy cluster at z=0.33. SG1120
also has an increasing fraction of 24 micron members with decreasing local
galaxy density, i.e. an infrared-density relation, that is not observed in the
rich cluster. We detect nine of the group galaxies in VLA 1.4 Ghz imaging, and
comparison of the radio to total infrared luminosities indicates that about 30%
of these radio-detected members have AGN. The radio map also reveals that one
of the brightest group galaxies has radio jets. We are currently analysing the
1.4 Ghz observations to determine if AGN can significantly heat the intrahalo
medium and if AGN are related to the excess of 24 micron members.Comment: 4 page proceedings, "The Monster's Fiery Breath: Feedback in
Galaxies, Groups, and Clusters
A VLT/FORS2 Multi-Slit Search for Lyman-alpha Emitting Galaxies at z~6.5
We present results from a deep spectroscopic search in the 9150A atmospheric
window for z~6.5 Lyman-alpha emitting galaxies using the VLT/FORS2. Our
multi-slit+narrow-band filter survey covers a total spatial area of 17.6 sq.
arcmin in four different fields and reaches fluxes down to 5x10^(-18)
erg/s/cm^2 (7 sigma detection). Our detection limit is significantly fainter
than narrow-band searches at this redshift and fainter also than the unlensed
brightness of Hu et al.'s HCM6A at z=6.56, and thus provides better overlap
with surveys at much lower redshifts. Eighty secure emission line galaxies are
detected. However, based on their clear continuum emission shortward of the
line or the presence of multiple lines, none of these can be Ly-alpha emission
at z~6.5. Our null result of finding no z~6.5 Ly-alpha emitters suggests that
the number density of Ly-alpha emitters with L>2x10^(42) erg/s declines by ~2
between z~3 and z~6.5.Comment: accepted by ApJ Letters (originally submitted June 11, 2004
Survival of Massive Star-forming Galaxies in Cluster Cores Drives Gas-Phase Metallicity Gradients : The Effects of Ram Pressure Stripping
Recent observations of galaxies in a cluster at z=0.35 show that their
integrated gas-phase metallicities increase with decreasing cluster-centric
distance. To test if ram pressure stripping (RPS) is the underlying cause, we
use a semi-analytic model to quantify the "observational bias" that RPS
introduces into the aperture-based metallicity measurements. We take integral
field spectroscopy of local galaxies, remove gas from their outer galactic
disks via RPS, and then conduct mock slit observations of cluster galaxies at
z=0.35. Our RPS model predicts a typical cluster-scale metallicity gradient of
-0.03 dex/Mpc. By removing gas from the outer galactic disks, RPS introduces a
mean metallicity enhancement of +0.02 dex at a fixed stellar mass. This gas
removal and subsequent quenching of star formation preferentially removes low
mass cluster galaxies from the observed star-forming population. As only the
more massive star-forming galaxies survive to reach the cluster core, RPS
produces a cluster-scale stellar mass gradient of -0.05 log(M_*/M_sun)/Mpc.
This mass segregation drives the predicted cluster-scale metallicity gradient
of -0.03 dex/Mpc. However, the effects of RPS alone can not explain the higher
metallicities measured in cluster galaxies at z=0.35. We hypothesize that
additional mechanisms including steep internal metallicity gradients and
self-enrichment due to gas strangulation are needed to reproduce our
observations at z=0.35.Comment: 17 pages, 21 figures, accepted for publication Ap
Forming Early-type Galaxies in Groups Prior to Cluster Assembly
We study a unique proto-cluster of galaxies, the supergroup SG1120-1202. We
quantify the degree to which morphological transformation of cluster galaxies
occurs prior to cluster assembly in order to explain the observed early-type
fractions in galaxy clusters at z=0. SG1120-1202 at z~0.37 is comprised of four
gravitationally bound groups that are expected to coalesce into a single
cluster by z=0. Using HST ACS observations, we compare the morphological
fractions of the supergroup galaxies to those found in a range of environments.
We find that the morphological fractions of early-type galaxies (~60 %) and the
ratio of S0 to elliptical galaxies (0.5) in SG1120-1202 are very similar to
clusters at comparable redshift, consistent with pre-processing in the group
environment playing the dominant role in establishing the observed early-type
fraction in galaxy clusters.Comment: 5 pages, 2 figures, 2 tables. Accepted for publication in ApJ Letter
The mass distribution in an assembling super galaxy group at
We present a weak gravitational lensing analysis of supergroup SG11201202,
consisting of four distinct X-ray-luminous groups, that will merge to form a
cluster comparable in mass to Coma at . These groups lie within a
projected separation of 1 to 4 Mpc and within km s and
form a unique protocluster to study the matter distribution in a coalescing
system.
Using high-resolution {\em HST}/ACS imaging, combined with an extensive
spectroscopic and imaging data set, we study the weak gravitational distortion
of background galaxy images by the matter distribution in the supergroup. We
compare the reconstructed projected density field with the distribution of
galaxies and hot X-ray emitting gas in the system and derive halo parameters
for the individual density peaks.
We show that the projected mass distribution closely follows the locations of
the X-ray peaks and associated brightest group galaxies. One of the groups that
lies at slightly lower redshift () than the other three groups
() is X-ray luminous, but is barely detected in the
gravitational lensing signal. The other three groups show a significant
detection (up to in mass), with velocity dispersions between
and km s and masses between
and , consistent with independent measurements. These groups are
associated with peaks in the galaxy and gas density in a relatively
straightforward manner. Since the groups show no visible signs of interaction,
this supports the picture that we are catching the groups before they merge
into a cluster.Comment: 10 pages, 10 figures, accepted for publication by Astronomy &
Astrophysic
Spectroscopic Confirmation of Multiple Red Galaxy-Galaxy Mergers in MS1054-03 (z=0.83)
We present follow-up spectroscopy of the galaxy cluster MS1054-03 (z=0.83)
confirming that at least six of the nine merging galaxy pairs identified by van
Dokkum et al. (1999) are indeed bound systems: they have projected separations
of R_s<10 kpc and relative line-of sight velocities of dv<165 km/s. For the
remaining three pairs, we were unable to obtain redshifts of both constituent
galaxies. To identify a more objective sample of merging systems, we select
bound red galaxy pairs (R_s<=30 kpc, dv<=300 km/s) from our sample of 121
confirmed cluster members: galaxies in bound red pairs make up 15.7+/-3.6% of
the cluster population. The (B-K_s) color-magnitude diagram shows that the pair
galaxies are as red as the E/S0 members and have a homogeneous stellar
population. The red pair galaxies span a large range in luminosity and internal
velocity dispersion to include some of the brightest, most massive members
(L>L*, sigma>200 km/s); these bound galaxy pairs must evolve into E/S0 members
by z~0.7. These results combined with MS1054's high merger fraction and
reservoir of likely future mergers indicates that most, if not all, of its
early-type members evolved from (passive) galaxy-galaxy mergers at z<~1.Comment: accepted by ApJ Letters; high resolution version of Fig. 2 available
at http://www.exp-astro.phys.ethz.ch/tran/outgoing/ms1054mgrs.ps.g
First Measurement of a Rapid Increase in the AGN Fraction in High-Redshift Clusters of Galaxies
We present the first measurement of the AGN fraction in high-redshift
clusters of galaxies (z~0.6) with spectroscopy of one cluster and archival data
for three additional clusters. We identify 8 AGN in all four of these clusters
from the Chandra data, which are sensitive to AGN with hard X-ray (2-10keV)
luminosity L_{X,H} > 10^43 erg/s in host galaxies more luminous than a rest
frame M_R < -20 mag. This stands in sharp contrast to the one AGN with L_{X,H}
> 10^43 erg/s we discovered in our earlier study of eight low-redshift clusters
with z=0.06-0.31 (average z~0.2). Three of the four high-redshift cluster
datasets are sensitive to nearly L_{X,H} > 10^42 erg/s and we identify seven
AGN above this luminosity limit, compared to two in eight, low-redshift
clusters. Based on membership estimates for each cluster, we determine that the
AGN fraction at z~0.6 is f_A(L_X>10^42;M_R<-20) = 0.028 (+0.019/-0.012) and
f_A(L_X>10^43;M_R<-20) = 0.020 (+0.012/-0.008). These values are approximately
a factor of 20 greater than the AGN fractions in lower-redshift (average z~0.2)
clusters of galaxies and represent a substantial increase over the factors of
1.5 and 3.3 increase, respectively, in the measured space density evolution of
the hard X-ray luminosity function over this redshift range. Potential
systematic errors would only increase the significance of our result. The
cluster AGN fraction increases more rapidly with redshift than the field and
the increase in cluster AGN indicates the presence of an AGN Butcher-Oemler
Effect.Comment: ApJL Accepted, 5 pages, 2 figure
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