2,044 research outputs found
Multiwavelength perspective of AGN evolution
Discovering and studying obscured AGN at z>1-3 is important not only to
complete the AGN census, but also because they can pinpoint galaxies where
nuclear accretion and star-formation are coeval, and mark the onset of AGN
feedback. We present the latest results on the characterization of z=1-3
galaxies selected for their high mid-infrared to optical flux ratio, showing
that they are massive and strongly star-forming galaxies, and that many do host
highly obscured AGN. We present a pilot program to push the search of
moderately obscured AGN up to z=5-6 and discuss the perspectives of this line
of research.Comment: Invited talk at the conference: X-Ray Astronomy 2009, Present Status,
multiwavelength approach and future perspectives, September 2009, Bologn
Warm Absorbers in the ROSAT Spectra of Quasars
We present two ROSAT PSPC observations of the radio-loud, lobe-dominated quasar 3C 351, which shows an 'ionized absorber' in its X-ray spectrum. The factor 1.7 change in flux in the approx. 2 years between the observations allows a test of of models for this ionized absorber. The absorption feature at approx. 0.7 keV (quasar frame) is present in both spectra but with a lower optical depth when the source intensity - and hence the ionizing flux at the absorber - is higher, in accordance with a simple, single-zone, equilibrium photoionization model. Detailed modeling confirms this agreement quantitatively. The maximum response time of 2 years allows us to limit the gas density: n(sub e) greater than 2 x 10(exp 4)cm(exp -3); and the distance of the ionized gas from the central source R less than 19 pc. This produces a strong test for a photoionized absorber in 3C 351: a factor 2 flux change in approx. 1 week in this source must show non-equilibrium effects in the ionized absorber
Are the X-ray Spectra of Flat-spectrum Radio Quasars and BL Lacertae Objects Different?
We study the X-ray spectra of 114 flat-spectrum radio quasars (FSRQ) using
the hardness ratios as given in the WGA catalogue of ROSAT sources. This sample
includes all WGA FSRQ with high-quality data and comprises about 20 per cent of
presently known such objects, which makes this the largest FSRQ sample ever
studied in the X-ray band. We find that FSRQ have a distribution of energy
spectral indices ranging between 0 and 3 with a mean value alpha_x ~ 1. This is
consistent with that of low-energy cutoff BL Lacs (LBL; alpha_x ~ 1.1),
generally found in radio surveys, but significantly different from that of
high-energy cutoff BL Lacs (HBL), normally selected in the X-ray band, which
display steeper X-ray spectra (alpha_x ~ 1.5). The shape of the
optical-to-X-ray continuum is concave (that is alpha_x < alpha_ox) for the
majority of FSRQ, as found for LBL, supporting a dominance of inverse Compton
emission in the X-ray band in most objects. Our results are at odds with
previous studies of the X-ray spectra of FSRQ, which were however plagued by
low spectral resolution and/or small number statistics and selection effects,
and have important implications for the proposed connections between FSRQ and
BL Lacs.Comment: 8 pages, 5 figures, TeX file, uses mn.tex and psfig.tex. To appear in
MNRAS. TeX and postscript files also available at
http://itovf2.roma2.infn.it/padovani/xray_spectra/diff.htm
A High Resolution Intergalactic Explorer for the Soft X-ray/FUV
We present a mission concept for high resolution X-ray spectroscopy with a
resolving power, R~6000, (c.f. R=<1000 for Chandra, XMM-Newton). This
resolution is physics-driven, since it allows the thermal widths of coronal
X-ray lines to be measured, and astrophysics-driven, since 50 km/s resolves
internal galaxy motions, and galaxy motions within larger structures.
Such a mission could be small and have a rapid response allowing us to `X-ray
the Universe' using the afterglows of Gamma-ray Bursts (GRBs) as strong
background sources of X-rays, and so illuminate the `Cosmic Web'. The Cosmic
Web is predicted to contain most of the normal matter (baryons) in the nearby
Universe.Comment: Paper presented at the August 2002 SPIE meeting "Astronomical
Telescopes and Instrumentation", Kona, Hawai
Narrow Line Seyfert 1s in the IBISCO sample
We present the broad-band soft and hard X-ray spectral analysis of 8 Narrow
Line Seyfert 1 galaxies extracted from the IBISCO Sample. The study also
focuses on the properties of the NLS1 in our sample in relation to those of the
IBISCO parent Seyfert population. The IBISCO sample comprises 57 AGN selected
from the INTEGRAL IBIS AGN catalogue (in the 20-100 keV band), with z0.05
and covering a wide range of luminosities, BH masses and absorption. All AGN
have also measurements of the molecular gas (H) content of their host
galaxies, through the detection of CO emission lines. The main goals of this
analysis are to accurately determine the X-ray continuum emission, investigate
the presence of absorption features around 7 keV (indicative of the presence of
outflows) and measure the bolometric luminosity in order to study the accretion
parameters of the eight IBISCO NLS1, and study the accretion mechanisms and
investigate the feeding and feedback cycle in these peculiar AGN. Preliminary
results show that NLS1 tend to have higher Eddington ratios and larger
molecular gas fractions than their parent Seyfert population in the IBISCO
sample. Nuclear (AGN) vs. host galaxy properties scaling relations of NLS1 in
relation to the parent Seyfert population are also explored.Comment: 4 pages, 2 figures, 1 table. Proceedings of the conference
"Revisiting narrow-line Seyfert 1 galaxies and their place in the Universe ",
Padova 9-13 April 201
The interaction-driven starburst contribution to the cosmic star formation rate density
An increasing amount of observational evidence supports the notion that there
are two modes of star formation: a quiescent mode in disk-like galaxies, and a
starburst mode, which is generally interpreted as driven by merging. Using a
semi-analytic model of galaxy formation, we derive the relative contribution to
the cosmic star formation rate density of quiescently starforming and starburst
galaxies, predicted under the assumption that starburst events are triggered by
galaxy encounters (merging and fly-by kind) during their merging histories. We
show that, within this framework, quiescently starforming galaxies dominate the
cosmic star formation rate density at all redshifts. The contribution of the
burst-dominated starforming galaxies increases with redshift, rising from <5%
at low redshift (z5. We estimated that the fraction of the
final (z=0) galaxy stellar mass which is formed through the burst component of
star formation is ~10% for 10^10 M_\odot<M_*<10^11.5 M_\odot. Starburst
galaxies, selected according to their distance from the galaxy main sequence,
account for ~10% of the star formation rate density in the redshift interval
1.5<z<2.5, i.e. at the cosmic peak of the star formation activity.Comment: 11 pages, 8 figures. Accepted for publication in A&
The Building Up of the Black Hole Mass - Stellar Mass Relation
We derive the growth of SMBHs relative to the stellar content of their host
galaxy predicted under the assumption of BH accretion triggered by galaxy
encounters occurring during their merging histories. We show that, within this
framework, the ratio Gamma=(M_BH/M_*)(z)/(M_BH/M_*)(z=0) between the Black Hole
mass and the galactic stellar mass (normalized to the local value) depends on
both BH mass and redshift. While the average value and the spread of Gamma(z)
increase with z, such an effect is larger for massive BHs, reaching values
Gamma=5 for massive Black Holes (M>10^9 M_{\odot}) at z>4, in agreement with
recent observations of high-redshift QSOs; this is due to the effectiveness of
interactions in triggering BH accretion in high-density environments at high
redshifts. To test such a model against observations, we worked out specific
predictions for sub-samples of the simulated galaxies corresponding to the
different observational samples for which measurements of Gamma have been
obtained. We found that for Broad Line AGNs at 1<z<2 values of Gamma=2 are
expected, with a mild trend toward larger value for increasing BH mass.
Instead, when we select from our Monte Carlo simulations only extremely gas
rich, rapidly star forming galaxies at 2<z<3, we find low values 0.3<Gamma<1.5,
consistent with recent observational findings on samples of sub-mm galaxies; in
the framework of our model, these objects end up at z=0 in low-to-intermediate
mass BHs (M<10^9 M_{\odot}), and they do not represent typical paths leading to
local massive galaxies. The latter have formed preferentially through paths
passing above the local M_*-M_BH relation. We discuss how the global picture
emerging from the model is consistent with a downsizing scenario, where massive
BHs accrete a larger fraction of their final mass at high redshifts z>4.Comment: 13 pages, 9 figures. Accepted to MNRA
Feasibility Study on a Decentralised System Architecture for Animal Transport Tracing Systems (DEAR-TRACE)
The objective of the study was to demonstrate whether the data recorded by the tracing systems of long journey animal transport vehicles for the official controls under Regulation 1/2005 and managed by different private service providers for trucks fitted with their systems could be made remotely available to competent authorities in this decentralised system architecture.
The concept was tested for around 2 months with 2 tracing systems from different service providers, with volunteering transport companies using these systems and with the participation of competent authorities in 3 Member States with a substantial share in the international animal transport.
Overall the experiment proved that with limited software development resources the integration of commercial tracing systems in animal transports would be feasible and offer several advantages to the competent authorities both in respect of animal welfare and traceability of animal movement.JRC.DG.G.4-Maritime affair
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