22 research outputs found
Detection of correlated galaxy ellipticities on CFHT data: first evidence for gravitational lensing by large-scale structures
We report the detection of a significant (5.5 sigma) excess of correlations
between galaxy ellipticities at scales ranging from 0.5 to 3.5 arc-minutes.
This detection of a gravitational lensing signal by large-scale structure was
made using a composite high quality imaging survey of 6300 arcmin^2 obtained at
the Canada France Hawaii Telescope (CFHT) with the UH8K and CFH12K panoramic
CCD cameras. The amplitude of the excess correlation is 2.2\pm 0.2 % at 1
arcmin scale, in agreement with theoretical predictions of the lensing effect
induced by large-scale structure.We provide a quantitative analysis of
systematics which could contribute to the signal and show that the net effect
is small and can be corrected for. We show that the measured ellipticity
correlations behave as expected for a gravitational shear signal. The
relatively small size of our survey precludes tight constraints on cosmological
models. However the data are in favor of cluster normalized cosmological
models, and marginally reject Cold Dark Matter models with (Omega=0.3,
sigma_8<0.6) or (Omega=1, sigma_8=1). The detection of cosmic shear
demonstrates the technical feasibility of using weak lensing surveys to measure
dark matter clustering and the potential for cosmological parameter
measurements, in particular with upcoming wide field CCD cameras.Comment: 19 pages. 19 Figures. Revised version accepted in A&
Star formation and dust obscuration at z~2: galaxies at the dawn of downsizing
We present first results of a study aimed to constrain the star formation
rate and dust content of galaxies at z~2. We use a sample of BzK-selected
star-forming galaxies, drawn from the COSMOS survey, to perform a stacking
analysis of their 1.4 GHz radio continuum as a function of different stellar
population properties, after removing AGN contaminants from the sample. Dust
unbiased star formation rates are derived from radio fluxes assuming the local
radio-IR correlation. The main results of this work are: i) specific star
formation rates are constant over about 1 dex in stellar mass and up to the
highest stellar mass probed; ii) the dust attenuation is a strong function of
galaxy stellar mass with more massive galaxies being more obscured than lower
mass objects; iii) a single value of the UV extinction applied to all galaxies
would lead to grossly underestimate the SFR in massive galaxies; iv) correcting
the observed UV luminosities for dust attenuation based on the Calzetti recipe
provide results in very good agreement with the radio derived ones; v) the mean
specific star formation rate of our sample steadily decreases by a factor of ~4
with decreasing redshift from z=2.3 to 1.4 and a factor of ~40 down the local
Universe.
These empirical SFRs would cause galaxies to dramatically overgrow in mass if
maintained all the way to low redshifts, we suggest that this does not happen
because star formation is progressively quenched, likely starting from the most
massive galaxies.Comment: 5 pages, 5 figures. Accepted for publication in Astrophysical Journal
Letter
The VIMOS VLT Deep Survey: Evolution of the non-linear galaxy bias up to z=1.5
We present the first measurements of the Probability Distribution Function
(PDF) of galaxy fluctuations in the VIMOS-VLT Deep Survey (VVDS) cone, covering
0.4x0.4 deg between 0.4<z<1.5. The second moment of the PDF, i.e. the rms
fluctuations of the galaxy density field, is with good approximation constant
over the full redshift baseline investigated: we find that, in redshift space,
sigma_8 for galaxies brighter than M=-20+5log h has a mean value of 0.94\pm0.07
in the redshift interval 0.7<z<1.5. The third moment, i.e. the skewness,
increases with cosmic time: we find that the probability of having underdense
regions is greater at z~0.7 than it was at z~1.5. By comparing the PDF of
galaxy density contrasts with the theoretically predicted PDF of mass
fluctuations we infer the redshift-, density-, and scale-dependence of the
biasing function b(z, \delta, R) between galaxy and matter overdensities up to
redshift z=1.5. Our results can be summarized as follows: i) the galaxy bias is
an increasing function of redshift: evolution is marginal up to z~0.8 and more
pronounced for z>0.8; ii) the formation of bright galaxies is inhibited below a
characteristic mass-overdensity threshold whose amplitude increases with
redshift and luminosity; iii) the biasing function is non linear in all the
redshift bins investigated with non-linear effects of the order of a few to 10%
on scales >5Mpc.Comment: 30 pages, 17 figs, Accepted by A&
On the cosmic evolution of the scaling relations between black holes and their host galaxies: Broad Line AGN in the zCOSMOS survey
(Abriged) We report on the measurement of the rest frame K-band luminosity
and total stellar mass of the hosts of 89 broad line Active Galactic Nuclei
detected in the zCOSMOS survey in the redshift range 1<z<2.2. The unprecedented
multiwavelength coverage of the survey field allows us to disentangle the
emission of the host galaxy from that of the nuclear black hole in their
Spectral Energy Distributions. We derive an estimate of black hole masses
through the analysis of the broad Mg II emission lines observed in the
medium-resolution spectra taken with VIMOS/VLT as part of the zCOSMOS project.
We found that, as compared to the local value, the average black hole to host
galaxy mass ratio appears to evolve positively with redshift, with a best fit
evolution of the form (1+z)^{0.68 \pm0.12 +0.6 -0.3}, where the large
asymmetric systematic errors stem from the uncertainties in the choice of IMF,
in the calibration of the virial relation used to estimate BH masses and in the
mean QSO SED adopted. A thorough analysis of observational biases induced by
intrinsic scatter in the scaling relations reinforces the conclusion that an
evolution of the MBH-M* relation must ensue for actively growing black holes at
early times: either its overall normalization, or its intrinsic scatter (or
both) appear to increase with redshift. This can be interpreted as signature of
either a more rapid growth of supermassive black holes at high redshift, a
change of structural properties of AGN hosts at earlier times, or a significant
mismatch between the typical growth times of nuclear black holes and host
galaxies.Comment: 47 pages, 8 figures. Accepted for publication in Ap
Detection of correlated galaxy ellipticities from CFHT data: first evidence for gravitational lensing by large-scale structures
We report the detection of a significant (5.5 sigma) excess of correlations between galaxy ellipticities at scales ranging from 0.5 to 3.5 arc-minutes. This detection of a gravitational lensing signal by large-scale structure was made using a composite high quality imaging sun ey of 6300 arcmin(2) obtained at the Canada France Hawaii Telescope (CFHT) with the UH8K and CFH12K panoramic CCD cameras. The amplitude of the excess correlation is 2.2 +/- 0.2% at 1 arcmin scale, in agreement with theoretical predictions of the lensing effect induced by large-scale structure. We provide a quantitative analysis of systematics which could contribute to the signal and show that the net effect is small and can be corrected for. In particular, we show that the spurious excess of correlations caused by the residual of the anisotropic Point Spread Function (PSF) correction is well below the measured signal. We show that the measured ellipticity correlations behave as expected for a gravitational shear signal. The relatively small size of our survey precludes tight constraints on cosmological models. However the data are in favor of cluster normalized cosmological models, and marginally reject Cold Dark Matter models with (Omega = 0.3, sigma(8) < 0.6) or (Omega = 1, sigma(8) = 1). The detection of cosmic sheer demonstrates the technical feasibility of using weak lensing surveys to measure dark matter clustering and the potential for cosmological parameter measurements, in particular with upcoming wide field CCD cameras