599 research outputs found
A possible bias on the estimate of Lbol/Ledd in AGN as a function of luminosity and redshift
The BH mass (and the related Eddington ratio) in broad line AGN is usually
evaluated by combining estimates (often indirect) of the BLR radius and of the
FWHM of the broad lines, under the assumption that the BLR clouds are in
Keplerian motion around the BH. Such an evaluation depends on the geometry of
the BLR. There are two major options for the BLR configuration: spherically
symmetric or ``flattened''. In the latter case the inclination to the line of
sight becomes a relevant parameter. This paper is devoted to evaluate the bias
on the estimate of the Eddington ratio when a spherical geometry is assumed
(more generally when inclination effects are ignored), while the actual
configuration is ``flattened'', as some evidence suggests. This is done as a
function of luminosity and redshift, on the basis of recent results which show
the existence of a correlation between the fraction of obscured AGN and these
two parameters up to at least z=2.5. The assumed BLR velocity field is akin to
the ``generalized thick disk'' proposed by Collin et al. (2006). Assuming an
isotropic orientation in the sky, the mean value of the bias is calculated as a
function of luminosity and redshift. It is demonstrated that, on average, the
Eddington ratio obtained assuming a spherical geometry is underestimated for
high luminosities, and overestimated for low luminosities. This bias converges
for all luminosities at z about 2.7, while nothing can be said on this bias at
larger redshifts due to the lack of data. The effects of the bias, averaged
over the luminosity function of broad line AGN, have been calculated. The
results imply that the bias associated with the a-sphericity of the BLR make
even worse the discrepancy between the observations and the predictions of
evolutionary models.Comment: 6 pages, 3 figures, accepted for publication in A&
The environments of z~1 Active Galactic Nuclei at 3.6um
We present an analysis of a large sample of AGN environments at z~1 using
stacked Spitzer data at 3.6um. The sample contains type-1 and type-2 AGN in the
form of quasars and radio galaxies, and spans a large range in both optical and
radio luminosity. We find, on average, that 2 to 3 massive galaxies containing
a substantial evolved stellar population lie within a 200-300 kpc radius of the
AGN, constituting a >8-sigma excess relative to the field. Secondly, we find
evidence for the environmental source density to increase with the radio
luminosity of AGN, but not with black-hole mass. This is shown first by
dividing the AGN into their classical AGN types, where we see more significant
over-densities in the fields of the radio-loud AGN. If instead we dispense with
the classical AGN definitions, we find that the source over-density as a
function of radio luminosity for all our AGN exhibits a positive correlation.
One interpretation of this result is that the Mpc-scale environment is in some
way influencing the radio emission that we observe from AGN. This could be
explained by the confinement of radio jets in dense environments leading to
enhanced radio emission or, alternatively, may be linked to more rapid
black-hole spin brought on by galaxy mergers.Comment: 13 pages, 12 figures, accepted by MNRA
Linking stellar mass and star formation in Spitzer/MIPS 24 micron galaxies
We present deep Ks<21.5 (Vega) identifications, redshifts and stellar masses
for most of the sources composing the bulk of the 24 micron background in the
GOODS/CDFS. Our identified sample consists of 747 Spitzer/MIPS 24 micron
objects, and includes ~94% of all the 24 micron sources in the GOODS-South
field which have fluxes Snu(24)>83 microJy (the 80% completeness limit of the
Spitzer/GTO 24 micron catalog). 36% of our galaxies have spectroscopic
redshifts (mostly at z<1.5) and the remaining ones have photometric redshifts
of very good quality, with a median of |dz|=|zspec-zphot|/(1+zspec)=0.02. We
find that MIPS 24 micron galaxies span the redshift range z~0-4, and that a
substantial fraction (28%) lie at high redshifts z>1.5. We determine the
existence of a bump in the redshift distribution at z~1.9, indicating the
presence of a significant population of galaxies with PAH emission at these
redshifts. Massive (M>10^11 Msun) star-forming galaxies at redshifts 2<z<3 are
characterized by very high star-formation rates (SFR>500 Msun/yr), and some of
them are able to construct a mass of 10^10-10^11 Msun in a single burst
lifetime (~0.01-0.1 Gyr). At lower redshifts z<2, massive star-forming galaxies
are also present, but appear to be building their stars on long timescales,
either quiescently or in multiple modest burst-like episodes. At redshifts
z~1-2, the ability of the burst-like mode to produce entire galaxies in a
single event is limited to some lower (M<7x10^10 Msun) mass systems, and it is
basically negligible at z<1. Our results support a scenario where
star-formation activity is differential with assembled stellar mass and
redshift, and where the relative importance of the burst-like mode proceeds in
a down-sizing way from high to low redshifts. (abridged)Comment: Accepted for publication in the ApJ. 19 pages, 10 figures. Uses
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Regulation of p53 Translation and Induction after DNA Damage by Ribosomal Protein L26 and Nucleolin
SummaryIncreases in p53 protein levels after DNA damage have largely been attributed to an increase in the half-life of p53 protein. Here we demonstrate that increased translation of p53 mRNA is also a critical step in the induction of p53 protein in irradiated cells. Ribosomal protein L26 (RPL26) and nucleolin were found to bind to the 5âČ untranslated region (UTR) of p53 mRNA and to control p53 translation and induction after DNA damage. RPL26 preferentially binds to the 5âČUTR after DNA damage, and its overexpression enhances association of p53 mRNA with heavier polysomes, increases the rate of p53 translation, induces G1 cell-cycle arrest, and augments irradiation-induced apoptosis. Opposite effects were seen when RPL26 expression was inhibited. In contrast, nucleolin overexpression suppresses p53 translation and induction after DNA damage, whereas nucleolin downregulation promotes p53 expression. These findings demonstrate the importance of increased translation of p53 in DNA-damage responses and suggest critical roles for RPL26 and nucleolin in affecting p53 induction
The environment of active objects in the nearby universe
We study the galaxy environment of active galaxies, radio-loud and
radio-quiet quasars in the redshift range . We use APM
galaxies in order to explore the local galaxy overdensity and the
colour distribution of neighbouring galaxies of these target samples. For
comparison, we perform similar analysis on samples of Abell clusters with X-ray
emission, and samples of Abell clusters with richness R=1 and R=0. The
projected cross-correlations show that the samples of quasars and active
galaxies reside in regions of galaxy density enhancements lower than those
typical of R=0 clusters. We also find that in the nearby universe the local
galaxy overdensity of radio-loud and radio-quiet quasars are comparable. The
analysis of the distribution of galaxy colour indexes suggests that the
environment of quasars is not strongly dominated by a population of red
galaxies, characteristic of rich Abell cluster, an effect that is more clearly
appreciated for our sample of radio-loud quasars.Comment: 5 pages, 3 figures, to appear in MNRA
Number counts and clustering properties of bright Distant Red Galaxies in the UKIDSS Ultra Deep Survey Early Data Release
We describe the number counts and spatial distribution of 239 Distant Red
Galaxies (DRGs), selected from the Early Data Release of the UKIDSS Ultra Deep
Survey. The DRGs are identified by their very red infrared colours with
(J-K)AB>1.3, selected over 0.62 sq degree to a 90% completeness limit of
KAB~20.7. This is the first time a large sample of bright DRGs has been studied
within a contiguous area, and we provide the first measurements of their number
counts and clustering. The population shows strong angular clustering,
intermediate between those of K-selected field galaxies and
optical/infrared-selected Extremely Red Galaxies. Adopting the redshift
distributions determined from other recent studies, we infer a high correlation
length of r0~11 h-1 Mpc. Such strong clustering could imply that our galaxies
are hosted by very massive dark matter halos, consistent with the progenitors
of present-day L>L* elliptical galaxies.Comment: 5 pages, 4 figures, revised version accepted to MNRAS.
Higher-resolution figures available from the authors on reques
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