673 research outputs found
Is there any evidence that ionised outflows quench star formation in type 1 quasars at z<1?
The aim of this paper is to test the basic model of negative AGN feedback.
According to this model, once the central black hole accretes at the Eddington
limit and reaches a certain critical mass, AGN driven outflows blow out gas,
suppressing star formation in the host galaxy and self-regulating black hole
growth. We consider a sample of 224 quasars selected from the SDSS at z<1
observed in the infrared band by Herschel. We evaluate the star formation rate
in relation to several outflow signatures traced by the [OIII]4959,5007 and
[OII]3726,3729 emission lines in about half of the sample with high quality
spectra. Most of the quasars show asymmetric and broad wings in [OIII], which
we interpret as outflow signatures. We separate the quasars in two groups,
``weakly'' and ``strongly'' outflowing, using three different criteria. When we
compare the mean star formation rate in five redshift bins in the two groups,
we find that the SFRs are comparable or slightly larger in the strongly
outflowing quasars. We estimate the stellar mass from SED fitting and the
quasars are distributed along the star formation main sequence, although with a
large scatter. The scatter from this relation is uncorrelated with respect to
the kinematic properties of the outflow. Moreover, for quasars dominated in the
infrared by starburst or by AGN emission, we do not find any correlation
between the star formation rate and the velocity of the outflow, a trend
previously reported in the literature for pure starburst galaxies. We conclude
that the basic AGN negative feedback scenario seems not to agree with our
results. Although we use a large sample of quasars, we did not find any
evidence that the star formation rate is suppressed in the presence of AGN
driven outflows on large scale. A possibility is that feedback is effective
over much longer timescales than those of single episodes of quasar activity.Comment: 18 pages, new version that implements the suggestions of the referee
and matches the AA published versio
A Sensitive Quantification of HHV-6B by Real-time PCR
Human herpesvirus (HHV)-6B is a pathogen causing latent infection in virtually all humans. Nevertheless, the interaction of HHV-6B with its host cells is poorly understood. Although HHV-6B is approximately 90% homologous to HHV-6A, it expresses certain B-specific genes. In order to quantify the amount of expressed viral mRNA we have developed a method using real-time PCR on a LightCycler instrument. Here we describe an assay for the detection of the HHV-6B B6 mRNA, but our approach can easily be extended to involve other mRNAs. This method is useful during the study of HHV-6B biology and offers reliable and reproducible, quantitative detection of viral mRNA below the attomol range
AGN X-ray variability in the XMM-COSMOS survey
We took advantage of the observations carried out by XMM in the COSMOS field
during 3.5 years, to study the long term variability of a large sample of AGN
(638 sources), in a wide range of redshift (0.1<z<3.5) and X-ray luminosity
(L(2-10)). Both a simple statistical method to asses the
significance of variability, and the Normalized Excess Variance
() parameter, where used to obtain a quantitative measurement
of the variability. Variability is found to be prevalent in most AGN, whenever
we have good statistic to measure it, and no significant differences between
type-1 and type-2 AGN were found. A flat (slope -0.23+/-0.03) anti-correlation
between and X-ray luminosity is found, when significantly
variable sources are considered all together. When divided in three redshift
bins, the anti-correlation becomes stronger and evolving with z, with higher
redshift AGN being more variable. We prove however that this effect is due to
the pre-selection of variable sources: considering all the sources with
available measurement, the evolution in redshift disappears.
For the first time we were also able to study the long term X-ray variability
as a function of and Eddington ratio, for a large sample of AGN
spanning a wide range of redshift. An anti-correlation between
and is found, with the same slope of the
anti-correlation between and X-ray luminosity, suggesting
that the latter can be a byproduct of the former one. No clear correlation is
found between and the Eddington ratio in our sample.
Finally, no correlation is found between the X-ray and the
optical variability.Comment: 14 Pages, 13 figures. Accepted to the Astrophysical Journal on
December 6, 201
The Mean Star-Forming Properties of QSO Host Galaxies
Quasi-stellar objects (QSOs) occur in galaxies in which supermassive black
holes (SMBHs) are growing substantially through rapid accretion of gas. Many
popular models of the co-evolutionary growth of galaxies and SMBHs predict that
QSOs are also sites of substantial recent star formation, mediated by important
processes, such as major mergers, which rapidly transform the nature of
galaxies. A detailed study of the star-forming properties of QSOs is a critical
test of such models. We present a far-infrared Herschel/PACS study of the mean
star formation rate (SFR) of a sample of spectroscopically observed QSOs to z~2
from the COSMOS extragalactic survey. This is the largest sample to date of
moderately luminous AGNs studied using uniform, deep far-infrared photometry.
We study trends of the mean SFR with redshift, black hole mass, nuclear
bolometric luminosity and specific accretion rate (Eddington ratio). To
minimize systematics, we have undertaken a uniform determination of SMBH
properties, as well as an analysis of important selection effects within
spectroscopic QSO samples that influence the interpretation of SFR trends. We
find that the mean SFRs of these QSOs are consistent with those of normal
massive star-forming galaxies with a fixed scaling between SMBH and galaxy mass
at all redshifts. No strong enhancement in SFR is found even among the most
rapidly accreting systems, at odds with several co-evolutionary models.
Finally, we consider the qualitative effects on mean SFR trends from different
assumptions about the star-forming properties of QSO hosts and redshift
evolution of the SMBH-galaxy relationship. While limited currently by
uncertainties, valuable constraints on AGN-galaxy co-evolution can emerge from
our approach.Comment: 10 figures, 1 table; accepted for publication in Astronomy &
Astrophysic
The incidence of obscuration in active galactic nuclei
We study the incidence of nuclear obscuration on a complete sample of 1310
AGN selected on the basis of their rest-frame 2-10 keV X-ray flux from the
XMM-COSMOS survey, in the redshift range 0.3<z<3.5. We classify the AGN as
obscured or un-obscured on the basis of either the optical spectral properties
and the overall SED or the shape of the X-ray spectrum. The two classifications
agree in about 70% of the objects, and the remaining 30% can be further
subdivided into two distinct classes: at low luminosities X-ray un-obscured AGN
do not always show signs of broad lines or blue/UV continuum emission in their
optical spectra, most likely due to galaxy dilution effects; at high
luminosities broad line AGN may have absorbed X-ray spectra, which hints at an
increased incidence of small-scale (sub-parsec) dust-free obscuration. We
confirm that the fraction of obscured AGN is a decreasing function of the
intrinsic X-ray luminosity, while the incidence of absorption shows significant
evolution only for the most luminous AGN, which appear to be more commonly
obscured at higher redshift. We find no significant difference between the mean
stellar masses and star formation rates of obscured and un-obscured AGN hosts.
We conclude that the physical state of the medium responsible for obscuration
in AGN is complex, and mainly determined by the radiation environment (nuclear
luminosity) in a small region enclosed within the gravitational sphere of
influence of the central black hole, but is largely insensitive to the wider
scale galactic conditions.Comment: 18 pages, 17 figures, 2 tables. Accepted for publication by MNRA
Compton Thick AGN in the XMM-COSMOS survey
Heavily obscured, Compton Thick (CT, NH>10^24 cm^-2) AGN may represent an
important phase in AGN/galaxy co-evolution and are expected to provide a
significant contribution to the cosmic X-ray background (CXB). Through direct
X-ray spectra analysis, we selected 39 heavily obscured AGN (NH>3x10^23 cm^-2)
in the 2 deg^2 XMM-COSMOS survey. After selecting CT AGN based on the fit of a
simple absorbed two power law model to the XMM data, the presence of CT AGN was
confirmed in 80% of the sources using deeper Chandra data and more complex
models. The final sample of CT AGN comprises 10 sources spanning a large range
of redshift and luminosity. We collected the multi-wavelength information
available for all these sources, in order to study the distribution of SMBH and
host properties, such as BH mass (M_BH), Eddington ratio (\lambda_Edd), stellar
mass (M*), specific star formation rate (sSFR) in comparison with a sample of
unobscured AGN. We find that highly obscured sources tend to have significantly
smaller M_BH and higher \lambda_edd with respect to unobscured ones, while a
weaker evolution in M* is observed. The sSFR of highly obscured sources is
consistent with the one observed in the main sequence of star forming galaxies,
at all redshift. We also present optical spectra, spectral energy distribution
(SED) and morphology for the sample of 10 CT AGN: all the available optical
spectra are dominated by the stellar component of the host galaxy, and a highly
obscured torus component is needed in the SED of the CT sources. Exploiting the
high resolution Hubble-ACS images available, we conclude that these highly
obscured sources have a significantly larger merger fraction with respect to
other X-ray selected samples of AGN. Finally we discuss implications in the
context of AGN/galaxy co-evolutionary models, and compare our results with the
predictions of CXB synthesis models.Comment: Revised version after referee comments. Accepted for publication in
Astronomy & Astrophysics on 25 November 2014. 23 pages, 2 tables, 16 figure
The most luminous blue quasars at 3.0 < z < 3.3. III. LBT spectra and accretion parameters
We present the analysis of the rest frame ultraviolet and optical spectra of 30 bright blue quasars at z ⌠3, selected to examine the suitability of active galactic nuclei as cosmological probes. In our previous works, based on pointed XMM-Newton observations, we found an unexpectedly high fraction (â25%) of X-ray weak quasars in the sample. The latter sources also display a flatter UV continuum and a broader and fainter C IV profile in the archival UV data with respect to their X-ray normal counterparts. Here we present new observations with the Large Binocular Telescope in both the zJ (covering the rest frame â2300-3100 AÌ) and the KS (â4750-5350 AÌ) bands. We estimated black hole masses (MBH) and Eddington ratios (λEdd) from the available rest frame optical and UV emission lines (HÎČ, Mg II), finding that our zâŒ3 quasars are on average highly accreting (λEddâ 1.2 and MBHâ 109.7 Mâ), with no difference in λEdd or MBH between X-ray weak and X-ray normal quasars. From the zJ spectra, we derived the properties (e.g. flux, equivalent width) of the main emission lines (Mg II, Fe II), finding that X-ray weak quasars display higher Fe II/Mg II ratios with respect to typical quasars. Fe II/Mg II ratios of X-ray normal quasars are instead consistent with other estimates up to z â 6.5, corroborating the idea of already chemically mature broad line regions at early cosmic time. From the KS spectra, we find that all the X-ray weak quasars present generally weaker [O III] emission (EW < 10 AÌ) than the normal ones. The sample as a whole, however, abides by the known X-ray-[O III] luminosity correlation, hence the different [O III] properties are likely due to an intrinsically weaker [O III] emission in X-ray weak objects, associated to the shape of the spectral energy distribution. We interpret these results in the framework of accretion-disc winds
Spectral Energy Distributions of Type 1 AGN in the COSMOS Survey I - The XMM-COSMOS Sample
The "Cosmic Evolution Survey" (COSMOS) enables the study of the Spectral
Energy Distributions (SEDs) of Active Galactic Nuclei (AGN) because of the deep
coverage and rich sampling of frequencies from X-ray to radio. Here we present
a SED catalog of 413 X-ray (\xmm) selected type 1 (emission line FWHM km
s) AGN with Magellan, SDSS or VLT spectrum. The SEDs are corrected for
the Galactic extinction, for broad emission line contributions, constrained
variability, and for host galaxy contribution. We present the mean SED and the
dispersion SEDs after the above corrections in the rest frame 1.4 GHz to 40
keV, and show examples of the variety of SEDs encountered. In the near-infrared
to optical (rest frame -- 4000\AA), the photometry is complete for
the whole sample and the mean SED is derived from detections only. Reddening
and host galaxy contamination could account for a large fraction of the
observed SED variety. The SEDs are all available on-line.Comment: 22 pages, 22 figures, ApJ accepted, scheduled to be published October
20th, 2012, v75
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