121 research outputs found
Tests of the Las Campanas Distant Cluster Survey from Confirmation Observations for the ESO Distant Cluster Survey
The ESO Distant Cluster Survey (EDisCS) is a photometric and spectroscopic
study of the galaxy cluster population at two epochs, z~0.5 and z~0.8, drawn
from the Las Campanas Distant Cluster Survey (LCDCS). We report results from
the initial candidate confirmation stage of the program and use these results
to probe the properties of the LCDCS. Of the 30 candidates targeted, we find
statistically significant overdensities of red galaxies near 28. Of the ten
additional candidates serendipitously observed within the fields of the
targeted 30, we detect red galaxy overdensities near six. We test the
robustness of the published LCDCS estimated redshifts to misidentification of
the brighest cluster galaxy (BCG) in the survey data, and measure the spatial
alignment of the published cluster coordinates, the peak red galaxy
overdensity, and the brightest cluster galaxy. We conclude that for LCDCS
clusters out to z~0.8, 1) the LCDCS coordinates agree with the centroid of the
red galaxy overdensity to within 25'' (~150 h^{-1} kpc) for 34 out of 37
candidates with 3\sigma galaxy overdensities, 2) BCGs are typically coincident
with the centroid of the red galaxy population to within a projected separation
of 200 h^{-1} kpc (32 out of 34 confirmed candidates), 3) the red galaxy
population is strongly concentrated, and 4) the misidentification of the BCG in
the LCDCS causes a redshift error >0.1 in 15-20% of the LCDCS candidates. These
findings together help explain the success of the surface brightness
fluctuations detection method.Comment: 10 pages, 9 figures, accepted for publication in the November 10
issue of 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 `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
Identifying strong lenses with unsupervised machine learning using convolutional autoencoder
In this paper, we develop a new unsupervised machine learning technique comprised of a feature extractor, a convolutional autoencoder, and a clustering algorithm consisting of a Bayesian Gaussian mixture model. We apply this technique to visual band space-based simulated imaging data from the Euclid Space Telescope using data from the strong gravitational lenses finding challenge. Our technique promisingly captures a variety of lensing features such as Einstein rings with different radii, distorted arc structures, etc., without using predefined labels. After the clustering process, we obtain several classification clusters separated by different visual features which are seen in the images. Our method successfully picks up 3c63 per cent of lensing images from all lenses in the training set. With the assumed probability proposed in this study, this technique reaches an accuracy of 77.25 \ub1 0.48 per cent in binary classification using the training set. Additionally, our unsupervised clustering process can be used as the preliminary classification for future surveys of lenses to efficiently select targets and to speed up the labelling process. As the starting point of the astronomical application using this technique, we not only explore the application to gravitationally lensed systems, but also discuss the limitations and potential future uses of this technique
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
The evolution of the density of galaxy clusters and groups: denser environments at higher redshifts
We show that, observationally, the projected local density distribution in
high-z clusters is shifted towards higher values compared to clusters at lower
redshift. To search for the origin of this evolution, we analyze a sample of
haloes selected from the Millennium Simulation and populated using
semi-analytic models, investigating the relation between observed projected
density and physical 3D density, using densities computed from the 10 and 3
closest neighbours. Both observationally and in the simulations, we study the
relation between number of cluster members and cluster mass, and number of
members per unit of cluster mass. We find that the observed evolution of
projected densities reflects a shift to higher values of the physical 3D
density distribution. In turn, this must be related with the globally higher
number of galaxies per unit of cluster volume N/V in the past. We show that the
evolution of N/V is due to a combination of two effects: a) distant clusters
were denser in dark matter (DM) simply because the DM density within R_{200}
(~the cluster virial radius) is defined to be a fixed multiple of the critical
density of the Universe, and b) the number of galaxies per unit of cluster DM
mass is remarkably constant both with redshift and cluster mass if counting
galaxies brighter than a passively evolving magnitude limit. Our results
highlight that distant clusters were much denser environments than today's
clusters, both in galaxy number and mass, and that the density conditions felt
by galaxies in virialized systems do not depend on the system mass.Comment: accepted for publication in MNRA
How special are Brightest Group and Cluster Galaxies?
We use the Sloan Digital Sky Survey to construct a sample of 625 brightest
group and cluster galaxies (BCGs) together with control samples of non-BCGs
matched in stellar mass, redshift, and color. We investigate how the systematic
properties of BCGs depend on stellar mass and on their privileged location near
the cluster center. The groups and clusters that we study are drawn from the C4
catalogue of Miller et al. (2005) but we have developed improved algorithms for
identifying the BCG and for measuring the cluster velocity dispersion. Since
the SDSS photometric pipeline tends to underestimate the luminosities of large
galaxies in dense environments, we have developed a correction for this effect
which can be readily applied to the published catalog data. We find that BCGs
are larger and have higher velocity dispersions than non-BCGs of the same
stellar mass, which implies that BCGs contain a larger fraction of dark matter.
In contrast to non-BCGs, the dynamical mass-to-light ratio of BCGs does not
vary as a function of galaxy luminosity. Hence BCGs lie on a different
fundamental plane than ordinary elliptical galaxies. BCGs also follow a steeper
Faber-Jackson relation than non-BCGs, as suggested by models in which BCGs
assemble via dissipationless mergers along preferentially radial orbits. We
find tentative evidence that this steepening is stronger in more massive
clusters. BCGs have similar mean stellar ages and metallicities to non-BCGs of
the same mass, but they have somewhat higher alpha/Fe ratios, indicating that
star formation may have occurred over a shorter timescale in the BCGs. Finally,
we find that BCGs are more likely to host radio-loud active galactic nuclei
than other galaxies of the same mass, but are less likely to host an optical
AGN. The differences we find are more pronounced for the less massive BCGs.Comment: Replaced with slightly modified version accepted by MNRAS. 28 pages,
25 figures. Version with full resolution figures available at
http://www.mpa-garching.mpg.de/~anja/bcgs_avdl.pd
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.
Optical and near-infrared observations of the GRB020405 afterglow
(Abridged) We report on observations of the optical and NIR afterglow of
GRB020405. Ground-based optical observations started about 1 day after the GRB
and spanned a period of ~10 days; archival HST data extended the coverage up to
70 days after the GRB. We report the first detection of the afterglow in NIR
bands. The detection of emission lines in the optical spectrum indicates that
the GRB is located at z = 0.691. Absorptions are also detected at z = 0.691 and
at z = 0.472. The latter system is likely caused by clouds in a galaxy located
2 arcsec southwest of the GRB host. Hence, for the first time, the galaxy
responsible for an intervening absorption system in the spectrum of a GRB
afterglow is identified. Optical and NIR photometry indicates that the decay in
all bands follows a single power law of index alpha = 1.54. The late-epoch VLT
and HST points lie above the extrapolation of this power law, so that a plateau
is apparent in the VRIJ light curves at 10-20 days after the GRB. The light
curves at epochs later than day ~20 after the GRB are consistent with a
power-law decay with index alphaprime = 1.85. We suggest that this deviation
can be modeled with a SN having the same temporal profile as SN2002ap, but 1.3
mag brighter at peak, and located at the GRB redshift. Alternatively, a shock
re-energization may be responsible for the rebrightening. A polarimetric R-band
measurement shows that the afterglow is polarized, with P = 1.5 % and theta =
172 degrees. Optical-NIR spectral flux distributions show a change of slope
across the J band which we interpret as due to the presence of nu_c. The
analysis of the multiwavelength spectrum within the fireball model suggests
that a population of relativistic electrons produces the optical-NIR emission
via synchrotron in an adiabatically expanding blastwave, and the X-rays via IC.Comment: 17 pages, 10 figures, 4 tables, accepted for publication on A&A, main
journa
Global Law as Intercontextuality and as Interlegality
Since the 1990s the effects of globalization on law and legal developments has been a central topic of scholarly debate. To date, the debate is however marked by three substantial deficiencies which this chapter seeks to remedy through a reconceptualization of global law as a law of inter-contextuality expressed through inter-legality and materialized through a particular body of legal norms which can be characterized as connectivity norms.
The first deficiency is a historical and empirical one. Both critics as well as advocates of ânon-state lawâ share the assumption that âlaw beyond the stateâ and related legal norms have gained in centrality when compared with previous historical times. While global law, including both public and private global governance law as well as regional occurrences such as EU law, has undergone profound transformations since the structural transformations which followed the de-colonialization processes of the mid-twentieth century, we do not have more global law relatively to other types of law today than in previous historical times.
The second deficiency is a methodological one. The vast majority of scholarship on global law is either of an analytical nature, drawing on insights from philosophy, or empirically observing the existence of global law and the degree of compliance with global legal norms at a given moment in time. While both approaches bring something to the table they remain static approaches incapable of explaining and evaluating the transformation of global law over time.
The third deficiency is a conceptual-theoretical one. In most instances, global law is understood as a unitary law producing singular legal norms with a planetary reach, or, alternatively, a radical pluralist perspective is adopted dismissing the existence of singular global norms. Both of these approaches however misapprehend the structural characteristics, function and societal effects of global law. Instead a third positon between unitary and radical pluralist perspectives can be adopted through an understanding of global law and its related legal norms as a de-centred kind of inter-contextual law characterised by inter-legality
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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