9 research outputs found
Optical dropout galaxies lensed by the cluster A2667
We investigate the nature and the physical properties of z, Y and J-dropout
galaxies selected behind the lensing cluster A2667. This field is part of our
project aimed at identifying z~7-10 candidates accessible to spectroscopic
studies, based on deep photometry with ESO/VLT HAWK-I and FORS2 (zYJH and
Ks-band images, AB(3 sigma)~26-27) on a sample of lensing clusters extracted
from our multi-wavelength combined surveys with SPITZER, HST, and Herschel. In
this paper we focus on the complete Y and J-dropout sample, as well as the
bright z-dropouts fulfilling the selection criteria by Capak et al. (2011). 10
candidates are selected within the common field of ~33 arcmin2 (effective area
once corrected for contamination and lensing dilution). All of them are
detected in H and Ks bands in addition to J and/or IRAC 3.6/4.5, with
H(AB)~23.4 to 25.2, and have modest magnification factors. Although best-fit
photometric redshifts place all these candidates at high-z, the contamination
by low-z interlopers is estimated at 50-75% level based on previous studies,
and the comparison with the blank-field WIRCAM Ultra-Deep Survey (WUDS). The
same result is obtained when photometric redshifts include a luminosity prior,
allowing us to remove half of the original sample as likely z~1.7-3 interlopers
with young stellar pulations and strong extinction. Two additional sources
among the remaining sample could be identified at low-z based on a detection at
24 microns and on the HST z_850 band. These low-z interlopers are not well
described by current templates given the large break, and cannot be easily
identified based solely on optical and near-IR photometry. Given the estimated
dust extinction and high SFRs, some of them could be also detected in the IR or
sub-mm bands. After correction for likely contaminants, the observed counts at
z>7.5 seem to be in agreement with an evolving LF. (abridged)Comment: 18 pages, 11 figures. Accepted for publication in A&
LoCuSS: First Results from Strong-lensing Analysis of 20 Massive Galaxy Clusters at z~0.2
We present a statistical analysis of a sample of 20 strong lensing clusters
drawn from the Local Cluster Substructure Survey (LoCuSS), based on high
resolution Hubble Space Telescope imaging of the cluster cores and follow-up
spectroscopic observations using the Keck-I telescope. We use detailed
parameterized models of the mass distribution in the cluster cores, to measure
the total cluster mass and fraction of that mass associated with substructures
within R<250kpc.These measurements are compared with the distribution of
baryons in the cores, as traced by the old stellar populations and the X-ray
emitting intracluster medium. Our main results include: (i) the distribution of
Einstein radii is log-normal, with a peak and 1sigma width of
=1.16+/-0.28; (ii) we detect an X-ray/lensing mass discrepancy of
=1.3 at 3 sigma significance -- clusters with larger substructure
fractions displaying greater mass discrepancies, and thus greater departures
from hydrostatic equilibrium; (iii) cluster substructure fraction is also
correlated with the slope of the gas density profile on small scales, implying
a connection between cluster-cluster mergers and gas cooling. Overall our
results are consistent with the view that cluster-cluster mergers play a
prominent role in shaping the properties of cluster cores, in particular
causing departures from hydrostatic equilibrium, and possibly disturbing cool
cores. Our results do not support recent claims that large Einstein radius
clusters present a challenge to the CDM paradigm.Comment: 28 pages, 14 figures, accepted for publication in MNRAS, replaced
with accepted versio
The bright-end of the luminosity function at z~9
We report new constraints on the galaxy luminosity function at z~9 based on
observations carried out with ESO/VLT FORS2, HAWK-I and X-Shooter around the
lensing cluster A2667, as part of our project aimed at selecting z~7-10
candidates accessible to spectroscopy. Only one J-dropout source was selected
in this field fulfilling the color and magnitude criteria. This source was
recently confirmed as a mid-z interloper based on X-Shooter spectroscopy. The
depth and the area covered by our survey are well suited to set strong
constraints on the bright-end of the galaxy luminosity function and hence on
the star formation history at very high redshift. The non-detection of reliable
J-dropout sources over the ~36arcmin2 field of view towards A2667 was used to
carefully determine the lens-corrected effective volume and the corresponding
upper-limit on the density of sources. The strongest limit is obtained for
Phi(M_{1500}=-21.4+/-0.50)<6.70x10^{-6}Mpc^{-3}mag^{-1} at z~9. A
maximum-likelihood fit of the luminosity function using all available data
points including the present new result yields M*>-19.7 with fixed alpha=-1.74
and Phi*=1.10x10^{-3}Mpc^{-3}. The corresponding star formation rate density
should be rho_{SFR}<5.97x10^{-3}M_{solar}/yr/Mpc^{3} at z~9. These results are
in good agreement with the most recent estimates already published in this
range of redshift and for this luminosity domain. This new result confirms the
decrease in the density of luminous galaxies at very high-redshift, hence
providing strong constraints for the design of future surveys aiming to explore
the very high-redshift Universe.Comment: 4 pages, 2 figures, accepted for publication in A&A Letter
Evolution of the observed Ly-alpha luminosity function from z = 6.5 to z = 7.7: evidence for the epoch of reionization ?
Aims. Ly-alpha emitters (LAEs) can be detected out to very high redshifts
during the epoch of reionization. The evolution of the LAE luminosity function
with redshift is a direct probe of the Ly-alpha transmission of the
intergalactic medium (IGM), and therefore of the IGM neutral-hydrogen fraction.
Measuring the Ly-alpha luminosity function (LF) of LAEs at redshift z = 7.7
therefore allows us to constrain the ionizing state of the Universe at this
redshift.
Methods. We observed three 7.5'x7.5' fields with the HAWK-I instrument at the
VLT with a narrow band filter centred at 1.06 m and targeting LAEs at
redshift z ~ 7.7. The fields were chosen for the availability of
multiwavelength data. One field is a galaxy cluster, the Bullet Cluster, which
allowed us to use gravitational amplification to probe luminosities that are
fainter than in the field. The two other fields are subareas of the GOODS
Chandra Deep Field South and CFHTLS-D4 deep field. We selected z=7.7 LAE
candidates from a variety of colour criteria, in particular from the absence of
detection in the optical bands.
Results. We do not find any LAE candidates at z = 7.7 in ~2.4 x 10^4 Mpc^3
down to a narrow band AB magnitude of ~ 26, which allows us to infer robust
constraints on the Ly-alpha LAE luminosity function at this redshift.
Conclusions. The predicted mean number of objects at z = 6.5, derived from
somewhat different LFs of Hu et al. (2010), Ouchi et al. (2010), and Kashikawa
et al. (2011) are 2.5, 13.7, and 11.6, respectively. Depending on which of
these LFs we refer to, we exclude a scenario with no evolution from z = 6.5 to
z = 7.7 at 85% confidence without requiring a strong change in the IGM Ly-alpha
transmission, or at 99% confidence with a significant quenching of the IGM
Ly-alpha transmission, possibly from a strong increase in the high
neutral-hydrogen fraction between these two redshifts.Comment: 14 pages, 6 figures, accepted for publication in A&A. Added
references, minor changes applied in text and figures after the first referee
repor
Characterization of the Molecular Determinants of Primary HIV-1 Vpr Proteins: Impact of the Q65R and R77Q Substitutions on Vpr Functions
Although HIV-1 Vpr displays several functions in vitro, limited information exists concerning their relevance during infection. Here, we characterized Vpr variants isolated from a rapid and a long-term non-progressor (LTNP). Interestingly, vpr alleles isolated from longitudinal samples of the LTNP revealed a dominant sequence that subsequently led to diversity similar to that observed in the progressor patient. Most of primary Vpr proteins accumulated at the nuclear envelope and interacted with host-cell partners of Vpr. They displayed cytostatic and proapoptotic activities, although a LTNP allele, harboring the Q65R substitution, failed to bind the DCAF1 subunit of the Cul4a/DDB1 E3 ligase and was inactive. This Q65R substitution correlated with impairment of Vpr docking at the nuclear envelope, raising the possibility of a functional link between this property and the Vpr cytostatic activity. In contradiction with published results, the R77Q substitution, found in LTNP alleles, did not influence Vpr proapoptotic activity
Looking for the first galaxies: lensing or blank fields?
Context. The identification and study of the first galaxies remains one of the most
exciting topics in observational cosmology. The determination of the best
possible observing strategies is a very important choice in order to
build up a representative sample of spectroscopically confirmed sources
at high-z (z\ga 7), beyond the limits of present-day
observations.
Aims. This paper is intended to precisely adress the relative efficiency of
lensing and blank fields in the identification and study of galaxies at
6 \laz\la 12.
Methods. The detection efficiency and field-to-field variance are estimated from
direct simulations of both blank and lensing fields observations. Present
known luminosity functions in the UV are used to determine the expected
distribution and properties of distant samples at z\ga 6 for a
variety of survey configurations. Different models for well known lensing
clusters are used to simulate in details the magnification and dilution
effects on the backgound distant population of galaxies.
Results. The presence of a strong-lensing cluster along the line of sight has a
dramatic effect on the number of observed sources, with a positive
magnification bias in typical ground-based “shallow” surveys
(AB \la 25.5). The positive magnification bias increases with the
redshift of sources and decreases with both depth of the survey and the
size of the surveyed area. The maximum efficiency is reached for lensing
clusters at z ~ 0.1-0.3. Observing blank fields in shallow surveys is
particularly inefficient as compared to lensing fields if the UV LF for
LBGs is strongly evolving at z\ga 7. Also in this case, the number
of z≥ 8 sources expected at the typical depth of JWST (AB ~ 28-29)
is much higher in lensing than in blank fields (e.g. a factor of ~10
for AB \la 28).
All these results have been obtained assuming that number counts derived
in clusters are not dominated by sources below the limiting surface brightness
of observations, which in turn depends on the reliability of the usual
scalings applied to the size of high-z sources.
Conclusions. Blank field surveys with a large field of view are needed to prove the
bright end of the LF at z\ga 6-7, whereas lensing clusters are
particularly useful for exploring the mid to faint end of the LF