7 research outputs found
LoCuSS: Exploring the selection of faint blue background galaxies for cluster weak-lensing
Cosmological constraints from galaxy clusters rely on accurate measurements
of the mass and internal structure of clusters. An important source of
systematic uncertainty in cluster mass and structure measurements is the secure
selection of background galaxies that are gravitationally lensed by clusters.
This issue has been shown to be particular severe for faint blue galaxies. We
therefore explore the selection of faint blue background galaxies, by reference
to photometric redshift catalogs derived from the COSMOS survey and our own
observations of massive galaxy clusters at z~0.2. We show that methods relying
on photometric redshifts of galaxies in/behind clusters based on observations
through five filters, and on deep 30-band COSMOS photometric redshifts are both
inadequate to identify safely faint blue background galaxies. This is due to
the small number of filters used by the former, and absence of massive galaxy
clusters at redshifts of interest in the latter. We therefore develop a
pragmatic method to combine both sets of photometric redshifts to select a
population of blue galaxies based purely on photometric analysis. This sample
yields stacked weak-lensing results consistent with our previously published
results based on red galaxies. We also show that the stacked clustercentric
number density profile of these faint blue galaxies is consistent with
expectations from consideration of the lens magnification signal of the
clusters. Indeed, the observed number density of blue background galaxies
changes by ~10-30 per cent across the radial range over which other surveys
assume it to be flat.Comment: submitted to MNRA
The AGN Population in X-ray Selected Galaxy Groups at
We use Chandra data to study the incidence and properties of Active Galactic
Nuclei (AGN) in 16 intermediate redshift () X-ray-selected
galaxy groups in the Chandra Deep Field-South. We measure an AGN fraction of
at ,
approximately a factor of two higher than the AGN fraction found for rich
clusters at comparable redshift. This extends the trend found at low redshift
for groups to have higher AGN fractions than clusters. Our estimate of the AGN
fraction is also more than a factor of 3 higher than that of low redshift
X-ray-selected groups. Using optical spectra from various surveys, we also
constrain the properties of emission-line selected AGN in these groups.
Contrary to the large population of X-ray AGN ( erg/s) =
25), we find only 4 emission-line AGN, 3 of which are also X-ray bright.
Furthermore, most of the X-ray AGN in our groups are optically-dull (i.e. lack
strong emission-lines) similar to those found in low redshift X-ray groups and
clusters of galaxies. This contrasts with the AGN population found in low
redshift optically-selected groups which are dominated by emission-line AGN.
The differences between the optically and X-ray-selected AGN populations in
groups are consistent with a scenario where most AGN in the densest
environments are currently in a low accretion state.Comment: 8 pages, 4 figures, accepted for publication in Ap
The lack of star formation gradients in galaxy groups up to z~1.6
In the local Universe, galaxy properties show a strong dependence on
environment. In cluster cores, early type galaxies dominate, whereas
star-forming galaxies are more and more common in the outskirts. At higher
redshifts and in somewhat less dense environments (e.g. galaxy groups), the
situation is less clear. One open issue is that of whether and how the star
formation rate (SFR) of galaxies in groups depends on the distance from the
centre of mass. To shed light on this topic, we have built a sample of X-ray
selected galaxy groups at 0<z<1.6 in various blank fields (ECDFS, COSMOS,
GOODS). We use a sample of spectroscopically confirmed group members with
stellar mass M >10^10.3 M_sun in order to have a high spectroscopic
completeness. As we use only spectroscopic redshifts, our results are not
affected by uncertainties due to projection effects. We use several SFR
indicators to link the star formation (SF) activity to the galaxy environment.
Taking advantage of the extremely deep mid-infrared Spitzer MIPS and
far-infrared Herschel PACS observations, we have an accurate, broad-band
measure of the SFR for the bulk of the star-forming galaxies. We use
multi-wavelength SED fitting techniques to estimate the stellar masses of all
objects and the SFR of the MIPS and PACS undetected galaxies. We analyse the
dependence of the SF activity, stellar mass and specific SFR on the
group-centric distance, up to z~1.6, for the first time. We do not find any
correlation between the mean SFR and group-centric distance at any redshift. We
do not observe any strong mass segregation either, in agreement with
predictions from simulations. Our results suggest that either groups have a
much smaller spread in accretion times with respect to the clusters and that
the relaxation time is longer than the group crossing time.Comment: Accepted for publication in MNRA
Gemini Observations of Galaxies in Rich Early Environments (GOGREEN) I : survey description
We describe a new Large Program in progress on the Gemini North and South telescopes: Gemini Observations of Galaxies in Rich Early Environments (GOGREEN). This is an imaging and deep spectroscopic survey of 21 galaxy systems at 1 10 in halo mass. The scientific objectives include measuring the role of environment in the evolution of low-mass galaxies, and measuring the dynamics and stellar contents of their host haloes. The targets are selected from the SpARCS, SPT, COSMOS, and SXDS surveys, to be the evolutionary counterparts of today's clusters and groups. The new red-sensitive Hamamatsu detectors on GMOS, coupled with the nod-and-shuffle sky subtraction, allow simultaneous wavelength coverage over lambda similar to 0.6-1.05 mu m, and this enables a homogeneous and statistically complete redshift survey of galaxies of all types. The spectroscopic sample targets galaxies with AB magnitudes z' <24.25 and [3.6] mu m <22.5, and is therefore statistically complete for stellar masses M* greater than or similar to 10(10.3) M-circle dot, for all galaxy types and over the entire redshift range. Deep, multiwavelength imaging has been acquired over larger fields for most systems, spanning u through K, in addition to deep IRAC imaging at 3.6 mu m. The spectroscopy is similar to 50 per cent complete as of semester 17A, and we anticipate a final sample of similar to 500 new cluster members. Combined with existing spectroscopy on the brighter galaxies from GCLASS, SPT, and other sources, GOGREEN will be a large legacy cluster and field galaxy sample at this redshift that spectroscopically covers a wide range in stellar mass, halo mass, and clustercentric radius.Peer reviewe