61 research outputs found
Abell 2384: the galaxy population of a cluster post-merger
We combine multi-object spectroscopy from the 2dF and EFOSC2 spectrographs
with optical imaging of the inner 30'x30' of A2384 taken with the ESO Wide
Field Imager. We carry out a kinematical analysis using the EMMIX algorithm and
biweight statistics. We address the possible presence of cluster substructures
with the Dressler-Shectman test. Cluster galaxies are investigated with respect
to [OII] and H{\alpha} equivalent width. Galaxies covered by our optical
imaging observations are additionally analysed in terms of colour, star
formation rate and morphological descriptors such as Gini coefficient and M20
index. We study cluster galaxy properties as a function of clustercentric
distance and investigate the distribution of various galaxy types in
colour-magnitude and physical space. The Dressler-Shectman test reveals a
substructure in the east of the 2dF field-of-view. We determine the mass ratio
between the northern and southern subcluster to be 1.6:1. In accordance with
other cluster studies, we find that a large fraction of the disk galaxies close
to the cluster core show no detectable star formation. Probably these are
systems which are quenched due to ram-pressure stripping. The sample of
quenched disks populates the transition area between the blue cloud and the red
sequence in colour-magnitude space. We also find a population of
morphologically distorted galaxies in the central cluster region. The
substructure in the east of A2384 might be a group of galaxies falling onto the
main cluster. We speculate that our sample of quenched spirals represents an
intermediate phase in the ram-pressure driven transformation of infalling field
spirals into cluster S0s. This is motivated by their position in
colour-magnitude space. The occurrence of morphologically distorted galaxies in
the cluster core complies with the hypothesis of A2384 representing a post
merger system.Comment: 14 pages, 18 figures, A&A accepte
The star formation history of intermediate‐redshift late‐type galaxies
We combine the latest observations of disc galaxy photometry and rotation curves at moderate redshift from the FORS Deep Field (FDF) with simple models of chemical enrichment. Our method describes the build‐up of the stellar component through infall of gas and allows for gas and metal outflows. In this framework, we keep a minimum number of constraints and we search a large volume of parameter space, looking for the models that best reproduce the photometric observations in the observed redshift range (0.5 < z < 1). We find that star formation efficiency correlates well with vmax, so that massive discs are more efficient in the formation of stars and have a smaller spread in stellar ages. This trend presents a break at around vmax∼ 140 km s−1. Galaxies on either side of this threshold have significantly different age distributions. This break has been already suggested by several authors in connection with the contribution from either gravitational instabilities or supernova‐driven turbulence to star formation. The gas infall time‐scale and gas outflows also present a correlation with galaxy mass, so that massive discs have shorter infall time‐scales and smaller outflow fractions. The model presented in this paper suggests that massive discs have formation histories resembling those of early‐type galaxies, with highly efficient and short‐lived bursts, in contrast with low‐mass discs, which have a more extended star formation history. The ages correlate well with galaxy mass or luminosity, and the predicted gas‐phase metallicities are consistent with the observations of local and moderate‐redshift galaxies. One option to explain the observed shallow slope of the Tully-Fisher relation at intermediate redshift could be small episodes of star formation in low‐mass disc
Ram pressure and dusty red galaxies - key factors in the evolution of the multiple cluster system Abell 901/902
We present spectroscopic observations of 182 disk galaxies (96 in the cluster
and 86 in the field environment) in the region of the Abell 901/902 multiple
cluster system, which is located at a redshift of . The presence
of substructures and non-Gaussian redshift distributions indicate that the
cluster system is dynamically young and not in a virialized state. We find
evidence for two important galaxy populations. \textit{Morphologically
distorted galaxies} are probably subject to increased tidal interactions. They
show pronounced rotation curve asymmetries at intermediate cluster-centric
radii and low rest-frame peculiar velocities. \textit{Morphologically
undistorted galaxies} show the strongest rotation curve asymmetries at high
rest-frame velocities and low cluster-centric radii. Supposedly, this group is
strongly affected by ram-pressure stripping due to interaction with the
intra-cluster medium. Among the morphologically undistorted galaxies, dusty red
galaxies have particularly strong rotation curve asymmetries, suggesting ram
pressure is an important factor in these galaxies. Furthermore, dusty red
galaxies on average have a bulge-to-total ratio higher by a factor of two than
cluster blue cloud and field galaxies. The fraction of kinematically distorted
galaxies is 75% higher in the cluster than in the field environment. This
difference mainly stems from morphological undistorted galaxies, indicating a
cluster-specific interaction process that only affects the gas kinematics but
not the stellar morphology. Also the ratio between gas and stellar scale length
is reduced for cluster galaxies compared to the field sample. Both findings
could be best explained by ram-pressure effects.Comment: Electronic version published in Astronomy and Astrophysics Volume
549, Page 0; 19 pages, 21 figure
Tully-Fisher analysis of the multiple cluster system Abell 901/902
We derive rotation curves from optical emission lines of 182 disk galaxies
(96 in the cluster and 86 in the field) in the region of Abell 901/902 located
at . We focus on the analysis of B-band and stellar-mass
Tully-Fisher relations. We examine possible environmental dependencies and
differences between normal spirals and "dusty red" galaxies, i.e. disk galaxies
that have red colors due to relatively low star formation rates. We find no
significant differences between the best-fit TF slope of cluster and field
galaxies. At fixed slope, the field population with high-quality rotation
curves (57 objects) is brighter by \Delta M_{B}=-0\fm42\pm0\fm15 than the
cluster population (55 objects). We show that this slight difference is at
least in part an environmental effect. The scatter of the cluster TFR increases
for galaxies closer to the core region, also indicating an environmental
effect. Interestingly, dusty red galaxies become fainter towards the core at
given rotation velocity (i.e. total mass). This indicates that the star
formation in these galaxies is in the process of being quenched. The
luminosities of normal spiral galaxies are slightly higher at fixed rotation
velocity for smaller cluster-centric radii. Probably these galaxies are
gas-rich (compared to the dusty red population) and the onset of ram-pressure
stripping increases their star-formation rates. The results from the TF
analysis are consistent with and complement our previous findings. Dusty red
galaxies might be an intermediate stage in the transformation of infalling
field spiral galaxies into cluster S0s, and this might explain the well-known
increase of the S0 fraction in galaxy clusters with cosmic time.Comment: Accepted for publication in Astronomy and Astrophysics; 16 pages, 14
figure
Kinematic and Structural Evolution of Field and Cluster Spiral Galaxies
To understand the processes that build up galaxies we investigate the stellar
structure and gas kinematics of spiral and irregular galaxies out to redshift
1. We target 92 galaxies in four cluster (z = 0.3 & 0.5) fields to study the
environmental influence. Their stellar masses derived from multiband VLT/FORS
photometry are distributed around but mostly below the characteristic
Schechter-fit mass. From HST/ACS images we determine morphologies and
structural parameters like disk length, position angle and ellipticity.
Combining the spectra of three slit positions per galaxy using the MXU mode of
VLT/FORS2 we construct the two-dimensional velocity field from gas emission
lines for 16 cluster members and 33 field galaxies. The kinematic position
angle and flatness are derived by a Fourier expansion of elliptical velocity
profiles. To trace possible interaction processes, we define three irregularity
indicators based on an identical analysis of local galaxies from the SINGS
project. Our distant sample displays a higher fraction of disturbed velocity
fields with varying percentages (10%, 30% and 70%) because they trace different
features. While we find far fewer candidates for major mergers than the SINS
sample at z~2, our data are sensitive enough to trace less violent processes.
Most irregular signatures are related to star formation events and less massive
disks are affected more than Milky-Way type objects. We detect similarly high
fractions of irregular objects both for the distant field and cluster galaxies
with similar distributions. We conclude that we may witness the building-up of
disk galaxies still at redshifts z~0.5 via minor mergers and gas accretion,
while some cluster members may additionally experience stripping, evaporation
or harassment interactions.Comment: 4 pages, 2 colour figures, to appear in the ASP Conference Series
Proceedings of "Galaxies in Isolation: Exloring Nature vs. Nurture", Granada,
200
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