394 research outputs found
The Properties of Radio Galaxies and the Effect of Environment in Large Scale Structures at
In this study we investigate 89 radio galaxies that are
spectroscopically-confirmed to be members of five large scale structures in the
redshift range of . Based on a two-stage classification
scheme, the radio galaxies are classified into three sub-classes: active
galactic nucleus (AGN), hybrid, and star-forming galaxy (SFG). We study the
properties of the three radio sub-classes and their global and local
environmental preferences. We find AGN hosts are the most massive population
and exhibit quiescence in their star-formation activity. The SFG population has
a comparable stellar mass to those hosting a radio AGN but are unequivocally
powered by star formation. Hybrids, though selected as an intermediate
population in our classification scheme, were found in almost all analyses to
be a unique type of radio galaxies rather than a mixture of AGN and SFGs. They
are dominated by a high-excitation radio galaxy (HERG) population. We discuss
environmental effects and scenarios for each sub-class. AGN tend to be
preferentially located in locally dense environments and in the cores of
clusters/groups, with these preferences persisting when comparing to galaxies
of similar colour and stellar mass, suggesting that their activity may be
ignited in the cluster/group virialized core regions. Conversely, SFGs exhibit
a strong preference for intermediate-density global environments, suggesting
that dusty starbursting activity in LSSs is largely driven by galaxy-galaxy
interactions and merging.Comment: 28 pages, 10 figures, accepted to MNRA
The CFHTLS Real Time Analysis System "Optically Selected GRB Afterglows"
We describe a wide and deep search for optical GRB afterglows on images taken
with MegaCAM at the Canada France Hawaii Telescope, within the framework of the
CFHT Legacy Survey. This search is performed in near real-time thanks to a Real
Time Analysis System called "Optically Selected GRB Afterglows", which has been
completely realized and installed on a dedicated computer in Hawaii. This
pipeline automatically and quickly analyzes Megacam images and extracts from
them a list of astrometrically and photometrically variable objects which are
displayed on a web page for validation by a member of the collaboration. In
this paper, we comprehensively describe the RTAS process. We also present
statistical results based on nearly one full year of operation, showing the
quality of the images and the performance of the RTAS. Finally, we compare the
efficiency of this study with similar searches, propose an ideal observational
strategy using simulations, and discuss general considerations on the searches
for GRB afterglows independently of the prompt emission. This is the first of a
series of two papers. A second paper will discuss the characterization of
variable objects we have found, as well as the GRB afterglow candidates and
their nature. We will also estimate or at least constrain the collimation
factor of GRB using the totality of the Very Wide Survey observations.Comment: 11 pages, 10 Figures, 5 Table
NoSOCS in SDSS. II. Mass Calibration of Low Redshift Galaxy Clusters with Optical and X-ray Properties
We use SDSS data to investigate the scaling relations of 127 NoSOCS and 56
CIRS galaxy clusters at low redshift (). We show that richness and
both optical and X-ray luminosities are reliable mass proxies. The scatter in
mass at fixed observable is 40%, depending on the aperture, sample and
observable considered. For example, for the massive CIRS systems
= 0.33 0.05 and = 0.48
0.06. For the full sample = 0.43 0.03 and
= 0.56 0.06. We estimate substructure using two and
three dimensional optical data, verifying that substructure has no significant
effect on the cluster scaling relations (intercepts and slopes), independent of
which substructure test we use. For a subset of twenty-one clusters, we
estimate masses from the M-T relation using temperature measures from BAX.
The scaling relations derived from the optical and X-ray masses are indeed very
similar, indicating that our method consistently estimates the cluster mass and
yields equivalent results regardless of the wavelength from which we measure
mass. For massive systems, we represent the mass-richness relation by a
function with the form , with
M being expressed in units of 10 M. Using the virial
mass, for CIRS clusters, we find A = (1.39 0.07) and B = (1.00
0.11). The relations based on the virial mass have a scatter of
= 0.37 0.05, while = 0.77
0.22 for the caustic mass and = 0.34 0.08
for the temperature based mass (abridged).Comment: 27 pages, 22 figures, 12 tables, Accepted to MNRA
Southern Cosmology Survey II: Massive Optically-Selected Clusters from 70 square degrees of the SZE Common Survey Area
We present a catalog of 105 rich and massive (M>3\times10^{14}M_{\sun})
optically-selected clusters of galaxies extracted from 70 square-degrees of
public archival griz imaging from the Blanco 4-m telescope acquired over 45
nights between 2005 and 2007. We use the clusters' optically-derived properties
to estimate photometric redshifts, optical luminosities, richness, and masses.
We complement the optical measurements with archival XMM-Newton and ROSAT X-ray
data which provide additional luminosity and mass constraints on a modest
fraction of the cluster sample. Two of our clusters show clear evidence for
central lensing arcs; one of these has a spectacular large-diameter,
nearly-complete Einstein Ring surrounding the brightest cluster galaxy. A
strong motivation for this study is to identify the massive clusters that are
expected to display prominent signals from the Sunyaev-Zeldovich Effect (SZE)
and therefore be detected in the wide-area mm-band surveys being conducted by
both the Atacama Cosmology Telescope and the South Pole Telescope. The optical
sample presented here will be useful for verifying new SZE cluster candidates
from these surveys, for testing the cluster selection function, and for
stacking analyzes of the SZE data.Comment: 13 pages, 7 Figures. Accepted for publication to ApJSS. Full
resolution plots and additional material available at
http://peumo.rutgers.edu/~felipe/e-prints
"Orphan" afterglows in the Universal Structured Jet Model for gamma-ray bursts
The paucity of reliable achromatic breaks in Gamma-Ray Burst afterglow light
curves motivates independent measurements of the jet aperture. Serendipitous
searches of afterglows, especially at radio wavelengths, have long been the
classic alternative. These survey data have been interpreted assuming a
uniformly emitting jet with sharp edges (``top-hat'' jet), in which case the
ratio of weakly relativistically beamed afterglows to GRBs scales with the jet
solid angle. In this paper, we consider, instead, a very wide outflow with a
luminosity that decreases across the emitting surface. In particular, we adopt
the universal structured jet (USJ) model, that is an alternative to the top-hat
model for the structure of the jet. However, the interpretation of the survey
data is very different: in the USJ model we only observe the emission within
the jet aperture and the observed ratio of prompt emission rate to afterglow
rate should solely depend on selection effects. We compute the number and rate
of afterglows expected in all-sky snapshot observations as a function of the
survey sensitivity. We find that the current (negative) results for OA searches
are in agreement with our expectations. In radio and X-ray bands this was
mainly due to the low sensitivity of the surveys, while in the optical band the
sky-coverage was not sufficient. In general we find that X-ray surveys are poor
tools for OA searches, if the jet is structured. On the other hand, the FIRST
radio survey and future instruments like the Allen Telescope Array (in the
radio band) and especially GAIA, Pan-Starrs and LSST (in the optical band) will
have chances to detect afterglows.Comment: 10 pages, 8 figures. MNRAS accepted. Moderate revision
A GMBCG Galaxy Cluster Catalog of 55,424 Rich Clusters from SDSS DR7
We present a large catalog of optically selected galaxy clusters from the
application of a new Gaussian Mixture Brightest Cluster Galaxy (GMBCG)
algorithm to SDSS Data Release 7 data. The algorithm detects clusters by
identifying the red sequence plus Brightest Cluster Galaxy (BCG) feature, which
is unique for galaxy clusters and does not exist among field galaxies. Red
sequence clustering in color space is detected using an Error Corrected
Gaussian Mixture Model. We run GMBCG on 8240 square degrees of photometric data
from SDSS DR7 to assemble the largest ever optical galaxy cluster catalog,
consisting of over 55,000 rich clusters across the redshift range from 0.1 < z
< 0.55. We present Monte Carlo tests of completeness and purity and perform
cross-matching with X-ray clusters and with the maxBCG sample at low redshift.
These tests indicate high completeness and purity across the full redshift
range for clusters with 15 or more members.Comment: Updated to match the published version. The catalog can be accessed
from: http://home.fnal.gov/~jghao/gmbcg_sdss_catalog.htm
An automated archival VLA transients survey
In this paper we present the results of a survey for radio transients using
data obtained from the Very Large Array archive. We have reduced, using a
pipeline procedure, 5037 observations of the most common pointings - i.e. the
calibrator fields. These fields typically contain a relatively bright point
source and are used to calibrate `target' observations: they are therefore
rarely imaged themselves. The observations used span a time range ~ 1984 - 2008
and consist of eight different pointings, three different frequencies (8.4, 4.8
and 1.4 GHz) and have a total observing time of 435 hours. We have searched for
transient and variable radio sources within these observations using components
from the prototype LOFAR transient detection system. In this paper we present
the methodology for reducing large volumes of Very Large Array data; and we
also present a brief overview of the prototype LOFAR transient detection
algorithms. No radio transients were detected in this survey, therefore we
place an upper limit on the snapshot rate of GHz frequency transients > 8.0 mJy
to rho less than or equal to 0.032 deg^-2 that have typical timescales 4.3 to
45.3 days. We compare and contrast our upper limit with the snapshot rates -
derived from either detections or non-detections of transient and variable
radio sources - reported in the literature. When compared with the current Log
N - Log S distribution formed from previous surveys, we show that our upper
limit is consistent with the observed population. Current and future radio
transient surveys will hopefully further constrain these statistics, and
potentially discover dominant transient source populations. In this paper we
also briefly explore the current transient commissioning observations with
LOFAR, and the impact they will make on the field.Comment: Accepted for publication in MNRA
Possible evidence of the radio AGN quenching of neighbouring galaxies at z ⌠1
Using 57 radio active galactic nuclei (RAGNs) at 0.55 †z †1.3 drawn from five fields of the Observations of Redshift Evolution in Large-Scale Environments (ORELSE) survey, we study the effect of injection of energy from outbursts of RAGN on their spectroscopically confirmed neighbouring galaxies (SNGs). We observe an elevated fraction of quenched neighbours (f_q) within 500 kpc projected radius of RAGN in the most dense local environments compared to those of non-RAGN control samples matched to the RAGN population in colour, stellar mass, and local environment at 2Ï significance. Further analyses show that there are offsets at similar significance between f_qs of RAGN-SNGs and the appropriate control samples for galaxies specifically in cluster environments and those hosted by most massive cluster galaxies, which tentatively suggests that some negative feedback from the RAGN is occurring in these dense environments. In addition, we find that the median radio power of RAGN increases with increasing local overdensity, an effect which may lend itself to the quenching of neighbouring galaxies. Furthermore, we find that, in the highest local overdensities, the f_q of the sub-sample of lower stellar mass RAGN-SNGs is larger than that of the higher stellar mass RAGN-SNGs sub-sample, which indicates a more pronounced effect from RAGN on lower stellar mass galaxies. We propose a scenario in which RAGN residing within clusters might heat the intracluster medium (ICM) affecting both in situ star formation and any inflowing gas that remains in their neighbouring galaxies
The structure function of variable 1.4 GHz radio sources based on NVSS and FIRST observations
We augment the two widest/deepest 1.4 GHz radio surveys: the NRAO VLA Sky
Survey (NVSS) and the Faint Images of the Radio Sky at Twenty-Centimeters
(FIRST), with the mean epoch in which each source was observed. We use these
catalogs to search for unresolved sources which vary between the FIRST and NVSS
epochs. We find 43 variable sources (0.1% of the sources) which vary by more
than 4 sigma, and we construct the mean structure function of these objects.
This enables us to explore radio variability on time scales between several
months and about five years. We find that on these time scales, the mean
structure function of the variable sources is consistent with a flat structure
function. A plausible explanation to these observations is that a large
fraction of the variability at 1.4 GHz is induced by scintillations in the
interstellar medium, rather than by intrinsic variability. Finally, for a sub
sample of the variables for which the redshift is available, we do not find
strong evidence for a correlation between the variability amplitude and the
source redshift.Comment: 7 pages, 7 figures, ApJ in pres
- âŠ