212 research outputs found
Halpha-Derived Star-Formation Rates For Three z ~ 0.75 EDisCS Galaxy Clusters
We present Halpha-derived star-formation rates (SFRs) for three z ~ 0.75
galaxy clusters. Our 1 sigma flux limit corresponds to a star-formation rate of
0.10-0.24 solar mass per year, and our minimum reliable Halpha + [N II]
rest-frame equivalent width is 10\AA. We show that Halpha narrowband imaging is
an efficient method for measuring star formation in distant clusters. In two
out of three clusters, we find that the fraction of star-forming galaxies
increases with projected distance from the cluster center. We also find that
the fraction of star-forming galaxies decreases with increasing local galaxy
surface density in the same two clusters. We compare the median rate of star
formation among star-forming cluster galaxies to a small sample of star-forming
field galaxies from the literature and find that the median cluster SFRs are
\~50% less than the median field SFR. We characterize cluster evolution in
terms of the mass-normalized integrated cluster SFR and find that the z ~ 0.75
clusters have more SFR per cluster mass on average than the z <= 0.4 clusters
from the literature. The interpretation of this result is complicated by the
dependence of the mass-normalized SFR on cluster mass and the lack of
sufficient overlap in the mass ranges covered by the low and high redshift
samples. We find that the fraction and luminosities of the brightest starburst
galaxies at z ~ 0.75 are consistent with their being progenitors of the
post-starburst galaxies at z ~ 0.45 if the post-starburst phase lasts several
(~5) times longer than the starburst phase.Comment: Accepted for publication in ApJ, 20 pages, 24 figure
The build-up of the colour-magnitude relation in galaxy clusters since z~0.8
Using galaxy clusters from the ESO Distant Cluster Survey, we study how the
distribution of galaxies along the colour-magnitude relation has evolved since
z~0.8. While red-sequence galaxies in all these clusters are well described by
an old, passively evolving population, we confirm our previous finding of a
significant evolution in their luminosity distribution as a function of
redshift. When compared to galaxy clusters in the local Universe, the high
redshift EDisCS clusters exhibit a significant "deficit" of faint red galaxies.
Combining clusters in three different redshift bins, and defining as `faint'
all galaxies in the range 0.4 > L/L* > 0.1, we find a clear decrease in the
luminous-to-faint ratio of red galaxies from z~0.8 to z~0.4. The amount of such
a decrease appears to be in qualitative agreement with predictions of a model
where the blue bright galaxies that populate the colour-magnitude diagram of
high redshift clusters, have their star formation suppressed by the hostile
cluster environment. Although model results need to be interpreted with
caution, our findings clearly indicate that the red-sequence population of
high-redshift clusters does not contain all progenitors of nearby red-sequence
cluster galaxies. A significant fraction of these must have moved onto the
red-sequence below z~0.8.Comment: 15 pages, 10 figures, accepted for publication in MNRA
The relation between star formation, morphology and local density in high redshift clusters and groups
We investigate how the [OII] properties and the morphologies of galaxies in
clusters and groups at z=0.4-0.8 depend on projected local galaxy density, and
compare with the field at similar redshifts and clusters at low-z. In both
nearby and distant clusters, higher-density regions contain proportionally
fewer star-forming galaxies, and the average [OII] equivalent width of
star-forming galaxies is independent of local density. However, in distant
clusters the average current star formation rate (SFR) in star-forming galaxies
seems to peak at densities ~15-40 galaxies Mpc^{-2}. At odds with low-z
results, at high-z the relation between star-forming fraction and local density
varies from high- to low-mass clusters. Overall, our results suggest that at
high-z the current star formation (SF) activity in star-forming galaxies does
not depend strongly on global or local environment, though the possible SFR
peak seems at odds with this conclusion. We find that the cluster SFR
normalized by cluster mass anticorrelates with mass and correlates with the
star-forming fraction. These trends can be understood given a) that the average
star-forming galaxy forms about 1 Msun/yr in all clusters; b) that the total
number of galaxies scales with cluster mass and c) the dependence of
star-forming fraction on cluster mass. We present the morphology-density (MD)
relation for our z=0.4-0.8 clusters, and uncover that the decline of the spiral
fraction with density is entirely driven by galaxies of types Sc or later. For
galaxies of a given Hubble type, we see no evidence that SF properties depend
on local environment. In contrast with recent findings at low-z, in our distant
clusters the SF-density relation and the MD-relation are equivalent, suggesting
that neither of the two is more fundamental than the other.(abr.)Comment: 21 pages, 14 figures, accepted for publication in Ap
Building Communities in Tense Times: Fostering Connectedness Between Cultures and Generations through Community Arts
The worldwide upsurge in social polarizations generates intercommunity tensions that challenge the social fabric of urban neighborhoods and undermine the relationships between their members. Because community arts can foster the creation of connections between people that would not have been in contact otherwise, they are often perceived as being powerful tools to foster community resilience. Through a multiple case study approach, this article describes how three community arts projects, carried out in two socioeconomically deprived neighborhoods of Montreal (Canada), influenced the social relationships between participants from diverse ethnocultural backgrounds and generations. Using participant observation and arts-based data collection methods (photography, video, and arts productions), the authors examine how the three projects illustrate (a) the interactive processes at play, (b) the transmission and hybridization of stories and images of adversity and resiliency, and (c) the access to a collective voice
The SPLASH Survey: Kinematics of Andromeda's Inner Spheroid
The combination of large size, high stellar density, high metallicity, and
Sersic surface brightness profile of the spheroidal component of the Andromeda
galaxy (M31) within R_proj ~ 20 kpc suggest that it is unlike any subcomponent
of the Milky Way. In this work we capitalize on our proximity to and external
view of M31 to probe the kinematical properties of this "inner spheroid." We
employ a Markov chain Monte Carlo (MCMC) analysis of resolved stellar
kinematics from Keck/DEIMOS spectra of 5651 red giant branch stars to
disentangle M31's inner spheroid from its stellar disk. We measure the mean
velocity and dispersion of the spheroid in each of five spatial bins after
accounting for a locally cold stellar disk as well as the Giant Southern Stream
and associated tidal debris. For the first time, we detect significant spheroid
rotation (v_rot ~ 50 km/s) beyond R_proj ~ 5 kpc. The velocity dispersion
decreases from about 140 km/s at R_proj = 7 kpc to 120 km/s at R_proj = 14 kpc,
consistent to 2 sigma with existing measurements and models. We calculate the
probability that a given star is a member of the spheroid and find that the
spheroid has a significant presence throughout the spatial extent of our
sample. Lastly, we show that the flattening of the spheroid is due to velocity
anisotropy in addition to rotation. Though this suggests that the inner
spheroid of M31 more closely resembles an elliptical galaxy than a typical
spiral galaxy bulge, it should be cautioned that our measurements are much
farther out (2 - 14 r_eff) than for the comparison samples.Comment: Accepted for publication in Ap
The build-up of the colour-magnitude relation in galaxy clusters since z⌠0.8
Using galaxy clusters from the ESO Distant Cluster Survey, we study how the distribution of galaxies along the colour-magnitude relation has evolved since z⌠0.8. While red-sequence galaxies in all these clusters are well described by an old, passively evolving population, we confirm our previous finding of a significant evolution in their luminosity distribution as a function of redshift. When compared to galaxy clusters in the local Universe, the high-redshift EDisCS clusters exhibit a significant deficit of faint red galaxies. Combining clusters in three different redshift bins, and defining as âfaint' all galaxies in the range 0.4 âłL/L*âł 0.1, we find a clear decrease in the luminous-to-faint ratio of red galaxies from z⌠0.8 to âŒ0.4. The amount of such a decrease appears to be in qualitative agreement with predictions of a model where the blue bright galaxies that populate the colour-magnitude diagram of high-redshift clusters, have their star formation suppressed by the hostile cluster environment. Although model results need to be interpreted with caution, our findings clearly indicate that the red-sequence population of high-redshift clusters does not contain all progenitors of nearby red-sequence cluster galaxies. A significant fraction of these must have moved on to the red sequence below z⌠0.
The Rest-Frame Optical Luminosity Function of Cluster Galaxies at z<0.8 and the Assembly of the Cluster Red Sequence
We present the rest-frame optical luminosity function (LF) of red sequence
galaxies in 16 clusters at 0.4<z<0.8 drawn from the ESO Distant Cluster Survey
(EDisCS). We compare our clusters to an analogous sample from the Sloan Digital
Sky Survey (SDSS) and match the EDisCS clusters to their most likely
descendants. We measure all LFs down to M M* + (2.5 - 3.5). At z<0.8, the
bright end of the LF is consistent with passive evolution but there is a
significant build-up of the faint end of the red sequence towards lower
redshift. There is a weak dependence of the LF on cluster velocity dispersion
for EDisCS but no such dependence for the SDSS clusters. We find tentative
evidence that red sequence galaxies brighter than a threshold magnitude are
already in place, and that this threshold evolves to fainter magnitudes toward
lower redshifts. We compare the EDisCS LFs with the LF of co-eval red sequence
galaxies in the field and find that the bright end of the LFs agree. However,
relative to the number of bright red galaxies, the field has more faint red
galaxies than clusters at 0.6<z<0.8 but fewer at 0.4<z<0.6, implying
differential evolution. We compare the total light in the EDisCS cluster red
sequences to the total red sequence light in our SDSS cluster sample. Clusters
at 0.4<z<0.8 must increase their luminosity on the red sequence (and therefore
stellar mass in red galaxies) by a factor of 1-3 by z=0. The necessary
processes that add mass to the red sequence in clusters predict local clusters
that are over-luminous as compared to those observed in the SDSS. The predicted
cluster luminosities can be reconciled with observed local cluster luminosities
by combining multiple previously known effects.Comment: Accepted for publication in the Astrophysical Journal. 36 pages, 16
figures, 10 table
Ionized gas discs in elliptical and S0 galaxies at z < 1
We analyse the extended, ionized-gas emission of 24 early-type galaxies (ETGs) at 0 < z < 1 from the ESO Distant Cluster Survey (EDisCS). We discuss different possible sources of ionization and favour star formation as the main cause of the observed emission. 10 galaxies have disturbed gas kinematics, while 14 have rotating gas discs. In addition, 15 galaxies are in the field, while 9 are in the infall regions of clusters. This implies that, if the gas has an internal origin, this is likely stripped as the galaxies get closer to the cluster centre. If the gas instead comes from an external source, then our results suggest that this is more likely acquired outside the cluster environment, where galaxyâgalaxy interactions more commonly take place. We analyse the TullyâFisher relation of the ETGs with gas discs, and compare them to EDisCS spirals. Taking a matched range of redshifts, MB < â20, and excluding galaxies with large velocity uncertainties, we find that, at fixed rotational velocity, ETGs are 1.7 mag fainter in MB than spirals. At fixed stellar mass, we also find that ETGs have systematically lower specific star formation rates than spirals. This study constitutes an important step forward towards the understanding of the evolution of the complex ISM in ETGs by significantly extending the look-back-time baseline explored so far
The ESO Distant Cluster Sample: galaxy evolution and environment out to z=1
The ESO Distant Cluster Survey (EDisCS, P.I. Simon D.M. White, LP 166.A-0162)
is an ESO large programme aimed at studying clusters and cluster galaxies at
z=0.4-1. How different is the evolution of the star formation activity in
clusters, in groups and in the field? Does it depend on cluster mass and/or the
local galaxy density? How relevant are starburst and post-starburst galaxies in
the different environments? Is there an evolution in the galaxies' structures,
and if so, is this related to the changes in their star formation activity?
These are some of the main questions that have been investigated using the
EDisCS dataset.Comment: to appear in The Messenger, issue June 200
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