203 research outputs found
Measuring the Redshift Evolution of Clustering: the Hubble Deep Field South
We present an analysis of the evolution of galaxy clustering in the redshift
interval 0<z<4.5 in the HDF-S. The HST optical data are combined with infrared
ISAAC/VLT observations, and photometric redshifts are used for all the galaxies
brighter than I_AB<27.5. The clustering signal is obtained in different
redshift bins using two different approaches: a standard one, which uses the
best redshift estimate of each object, and a second one, which takes into
account the redshift probability function of each object. This second method
makes it possible to improve the information in the redshift intervals where
contamination from objects with insecure redshifts is important. With both
methods, we find that the clustering strength up to z~3.5 in the HDF-S is
consistent with the previous results in the HDF-N. While at redshift lower than
z~1 the HDF galaxy population is un/anti-biased (b<1) with respect to the
underlying dark matter, at high redshift the bias increases up to b~2-3,
depending on the cosmological model. These results support previous claims
that, at high redshift, galaxies are preferentially located in massive haloes,
as predicted by the biased galaxy formation scenario. The impact of cosmic
errors on our analyses has been quantified, showing that errors in the
clustering measurements in the HDF surveys are indeed dominated by shot-noise
in most regimes. Future observations with instruments like the ACS on HST will
improve the S/N by at least a factor of two and more detailed analyses of the
errors will be required. In fact, pure shot-noise will give a smaller
contribution with respect to other sources of errors, such as finite volume
effects or non-Poissonian discreteness effects.Comment: 17 pages Latex, with 12 PostScript figures, Accepted for publication
in MNRA
The dusty environment of Quasars. Far-IR properties of Optical Quasars
We present the ISO far-IR photometry of a complete sub-sample of optically
selected bright quasars belonging to two complete surveys selected through
multicolour (U,B,V,R,I) techniques. The ISOPHOT camera on board of the ISO
Satellite was used to target these quasars at wavelengths of 7.3, 11.5, 60, 100
and 160 micron. Almost two thirds of the objects were detected at least in one
ISOPHOT band. The detection rate is independent of the source redshift, very
likely due to the negative K-correction of the far-IR thermal emission. More
than a half of the optically selected QSOs show significant emission between 4
and 100 micron in the quasar rest-frame. These fluxes have a very likely
thermal origin, although in a few objects an additional contribution from a
non-thermal component is plausible in the long wavelength bands. In a
colour-colour diagram these objects span a wide range of properties from
AGN-dominated to ULIRG-like. The far-IR composite spectrum of the quasar
population presents a broad far-IR bump between 10 and 30 micron and a sharp
drop at wavelengths greater than 100 micron in the quasar restframe. The amount
of energy emitted in the far-IR, is on average a few times larger than that
emitted in the blue and the ratio L(FIR)/L(B) increases with the bolometric
luminosity. Objects with fainter blue magnitudes have larger ratios between the
far-IR (wavelengths > 60 micron) fluxes and the blue band flux, which is
attributed to extinction by dust around the central source. No relation between
the blue absolute magnitude and the dust colour temperature is seen, suggesting
that the dominant source of FIR energy could be linked to a concurrent
starburst rather than to gravitational energy produced by the central engine.Comment: Astronomical Journal, in pres
The Spectral Slope and Escape Fraction of Bright Quasars at : the Contribution to the Cosmic UV Background
We use a sample of 1669 QSOs (, ) from the BOSS survey to
study the intrinsic shape of their continuum and the Lyman continuum photon
escape fraction (f), estimated as the ratio between the observed flux
and the expected intrinsic flux (corrected for the intergalactic medium
absorption) in the wavelength range 865-885 \AA\ rest-frame. Modelling the
intrinsic QSO continuum shape with a power-law,
, we find a median (with a
dispersion of , no dependence on the redshift and a mild intrinsic
luminosity dependence) and a mean f (independent of the QSO
luminosity and/or redshift). The f distribution shows a peak around
zero and a long tail of higher values, with a resulting dispersion of . If
we assume for the QSO continuum a double power-law shape (also compatible with
the data) with a break located at \AA\ and a softening
at wavelengths shorter than , the mean
f rises to . Combining our and f estimates with
the observed evolution of the AGN luminosity function (LF) we compute the AGN
contribution to the UV ionizing background (UVB) as a function of redshift. AGN
brighter than one tenth of the characteristic luminosity of the LF are able to
produce most of it up , if the present sample is representative of
their properties. At higher redshifts a contribution of the galaxy population
is required. Assuming an escape fraction of Lyman continuum photons from
galaxies between and , independent of the galaxy luminosity and/or
redshift, a remarkably good fit to the observational UVB data up to
is obtained. At lower redshift the extrapolation of our empirical estimate
agrees well with recent UVB observations, dispelling the so-called Photon
Underproduction Crisis.Comment: 8 pages, 9 figures, MNRAS accepte
Multicolor observations of the Hubble Deep Field South
We present a deep multicolor (UBVIJsHKs) catalog of galaxies in the HDF-S,
based on observations obtained with the HST WFPC2 in 1998 and VLT-ISAAC in
1999. The photometric procedures were tuned to derive a catalog optimized for
the estimation of photometric redshifts. In particular we adopted a
``conservative'' detection threshold which resulted in a list of 1611 objects.
The behavior of the observed source counts is in general agreement with the
result of Casertano et al. (2000) in the HDF-S and Williams et al. (1996) in
the HDF-N, while the corresponding counts in the HDF-N provided by
Fernandez-Soto et al. (1999) are systematically lower by a factor 1.5 beyond
I_AB=26. After correcting for the incompleteness of the source counts, the
object surface density at I_AB<27.5 is estimated to be 220 per square arcmin,
providing an estimate of the Extragalactic Background Light in the I band
consistent with the work of Madau & Pozzetti(2000). The comparison between the
median V-I color in the HDF-North and South shows a significant difference
around I_AB~26, possibly due to the presence of large scale structure at z~1 in
the HDF-N. High-z galaxy candidates (90 U dropout and 17 B dropout) were
selected by means of color diagrams, down to a magnitude I_AB=27, with a
surface density of (21+-1) and (3.9+-0.9) per square arcmin, respectively. 11
EROs (with (I-K)_AB>2.7) were selected down to K_AB=24, plus 3 objects whose
upper limit to the Ks flux is still compatible with the selection criterion.
The corresponding surface density of EROs is (2.5+-0.8) per sq.arcmin
((3.2+-0.9) per sq.arcmin if we include the three Ks upper limits). They show a
remarkably non-uniform spatial distribution and are classified with roughly
equal fractions in the categories of elliptical and starburst galaxies.Comment: 36 pages Latex, with 12 PostScript figures. Accepted for publication
in Astronomical Journa
The Optical Identification of a Primeval Galaxy at z >~ 4.4
We have obtained with the SUSI CCD camera on the ESO 3.5m NTT deep images in
the BVRI bands of the field centered on the QSO BRI 1202-0725 (). In the final combined frames the stellar images have FWHM of
1,1,0.6 and 0.65 arcsec respectively. The R and I images show clearly a galaxy
from the QSO, corresponding to kpc at .
Possible identification with the metal absorption systems seen in the line of
sight to the QSO, including the highest redshift damped system known to date at
, are discussed. We conclude that its colours can be reconciled only
with the spectrum of a primeval galaxy at z >~ 4.4, making it the most distant
galaxy detected so far. From its magnitudes and models of young galaxy
evolution we deduce that it is forming stars at a rate
yr and has an estimated age of the order of yr or less, implying
that the bulk of the stellar population formed at .Comment: 5 pages, 1 b/w + 1 color Postcript figure (6.5 Mb after
decompression). Uses mn.sty (included). To appear in MNRAS Letter
Field tests for the ESPRESSO data analysis software
The data analysis software (DAS) for VLT ESPRESSO is aimed to set a new
benchmark in the treatment of spectroscopic data towards the
extremely-large-telescope era, providing carefully designed, fully interactive
recipes to take care of complex analysis operations (e.g. radial velocity
estimation in stellar spectra, interpretation of the absorption features in
quasar spectra). A few months away from the instrument's first light, the DAS
is now mature for science validation, with most algorithms already implemented
and operational. In this paper, I will showcase the DAS features which are
currently employed on high-resolution HARPS and UVES spectra to assess the
scientific reliability of the recipes and their range of application. I will
give a glimpse on the science that will be possible when ESPRESSO data become
available, with a particular focus on the novel approach that has been adopted
to simultaneously fit the emission continuum and the absorption lines in the
Lyman-alpha forest of quasar spectra.Comment: 4 pages, 1 figure; proceedings of ADASS XXVI, accepted by ASP
Conference Serie
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