364 research outputs found
Deep Extragalactic X-ray Surveys
Deep surveys of the cosmic X-ray background are reviewed in the context of
observational progress enabled by the Chandra X-ray Observatory and the X-ray
Multi-Mirror Mission-Newton. The sources found by deep surveys are described
along with their redshift and luminosity distributions, and the effectiveness
of such surveys at selecting active galactic nuclei (AGN) is assessed. Some key
results from deep surveys are highlighted including (1) measurements of AGN
evolution and the growth of supermassive black holes, (2) constraints on the
demography and physics of high-redshift AGN, (3) the X-ray AGN content of
infrared and submillimeter galaxies, and (4) X-ray emission from distant
starburst and normal galaxies. We also describe some outstanding problems and
future prospects for deep extragalactic X-ray surveys.Comment: 32 pages; Annu. Rev. Astron. Astrophys., Volume 43 (2005); updated to
match accepted versio
A Puzzling X-Ray Source Found in the chandra Deep Field South
In this letter we report the detection of an extremely strong X-ray emission
line in the 940ks chandra ACIS-I spectrum of CXO CDFS J033225.3-274219. The
source was identified as a Type1 AGN at redshift of z = 1.617, with 2.0 -- 10.0
keV rest frame X-ray luminosity of ~ 10^44 ergs s^-1. The emission line was
detected at 6.2^{+0.2}_{-0.1} keV, with an equivalent width (EW) of
4.4^{+3.2}_{-1.4} keV, both quantities referring to the observed frame. In the
rest frame, the line is at 16.2^{+0.4}_{-0.3} keV with an EW of
11.5^{+8.3}_{-3.7} keV. An X-ray emission line at similar energy (~ 17 keV,
rest frame) in QSO PKS 2149-306 was discovered before using ASCA data. We
reject the possibility that the line is due to a statistical or instrumental
artifact. The line is most likely due to blueshifted Fe-K emission from an
relativistic outflow, probably an inner X-ray jet, with velocities of the order
of ~ 0.6-0.7c. Other possible explanations are also discussed
The Chandra-COSMOS survey IV: X-ray spectra of the bright sample
We present the X-ray spectral analysis of the 390 brightest extragalactic
sources in the Chandra-COSMOS catalog, showing at least 70 net counts in the
0.5-7 keV band. This sample has a 100% completeness in optical-IR
identification, with 75% of the sample having a spectroscopic redshift and 25%
a photometric redshift. Our analysis allows us to accurately determine the
intrinsic absorption, the broad band continuum shape ({\Gamma}) and intrinsic
L(2-10) distributions, with an accuracy better than 30% on the spectral
parameters for 95% of the sample. The sample is equally divided in type-1
(49.7%) and type-2 AGN (48.7%) plus few passive galaxies at low z. We found a
significant difference in the distribution of {\Gamma} of type-1 and type-2,
with small intrinsic dispersion, a weak correlation of {\Gamma} with L(2-10)
and a large population (15% of the sample) of high luminosity, highly obscured
(QSO2) sources. The distribution of the X ray/Optical flux ratio (Log(FX /Fi))
for type-1 is narrow (0 < X/O < 1), while type-2 are spread up to X/O = 2. The
X/O correlates well with the amount of X-ray obscuration. Finally, a small
sample of Compton thick candidates and peculiar sources is presented. In the
appendix we discuss the comparison between Chandra and XMM-Newton spectra for
280 sources in common. We found a small systematic difference, with XMM-Newton
spectra that tend to have softer power-laws and lower obscuration.Comment: 20 pages, 16 figures. Accepted for Pubblication in MNRAS, 2013
February
Diffusion in normal and critical transient chaos
In this paper we investigate deterministic diffusion in systems which are
spatially extended in certain directions but are restricted in size and open in
other directions, consequently particles can escape. We introduce besides the
diffusion coefficient D on the chaotic repeller a coefficient which
measures the broadening of the distribution of trajectories during the
transient chaotic motion. Both coefficients are explicitly computed for
one-dimensional models, and they are found to be different in most cases. We
show furthermore that a jump develops in both of the coefficients for most of
the initial distributions when we approach the critical borderline where the
escape rate equals the Liapunov exponent of a periodic orbit.Comment: 4 pages Revtex file in twocolumn format with 2 included postscript
figure
Ionised outflows in z 2.4 quasar host galaxies
AGN-driven outflows are invoked by galaxy evolutionary models to quench star
formation and to explain the origin of the relations observed locally between
super massive black holes and their host galaxies. This work aims to detect the
presence of extended ionised outflows in luminous quasars where we expect the
maximum activity both in star formation and in black hole accretion. Currently,
there are only a few studies based on spatially resolved observations of
outflows at high redshift, . We analyse a sample of six luminous () quasars at , observed in H-band using the
near-IR integral field spectrometer SINFONI at VLT. We perform a kinematic
analysis of the [OIII] emission line at . [OIII] has a
complex gas kinematic, with blue-shifted velocities of a few hundreds of km/s
and line widths up to 1500 km/s. Using the spectroastrometric method we infer
size of the ionised outflows of up to 2 kpc. The properties of the
ionised outflows, mass outflow rate, momentum rate and kinetic power, are
correlated with the AGN luminosity. The increase in outflow rate with
increasing AGN luminosity is consistent with the idea that a luminous AGN
pushes away the surrounding gas through fast outflows driven by radiation
pressure, which depends on the emitted luminosity. We derive mass outflow rates
of about 6-700 M/yr for our sample, which are lower than those
observed in molecular outflows. Indeed physical properties of ionised outflows
show dependences on AGN luminosity which are similar to those of molecular
outflows but indicating that the mass of ionised gas is smaller than that of
the molecular one. Alternatively, this discrepancy between ionised and
molecular outflows could be explained with different acceleration mechanisms.Comment: 13 pages, 11 figures; accepted for publication in A&
Luminosity-dependent evolution of soft X-ray selected AGN: New Chandra and XMM-Newton surveys
We present new results on the cosmological evolution of unabsorbed (type-1)
active galactic nuclei (AGN) selected in the soft (0.5-2 keV) X-ray band. From
a variety of ROSAT, XMM-Newton and Chandra surveys we selected a total of ~1000
AGN with an unprecedented spectroscopic and photometric optical/NIR
identification completeness. For the first time we are able to derive reliable
space densities for low-luminosity (Seyfert-type) X-ray sources at cosmological
redshifts. The evolutionary behaviour of AGN shows a strong dependence on X-ray
luminosity: while the space density of high-luminosity AGN reaches a peak
around z~2, similar to that of optically selected QSO, the space density of
low-luminosity AGNs peaks at redshifts below z=1. This confirms previous ROSAT
findings of a luminosity-dependent density evolution. Using a rigorous
treatment of the optical identification completeness we are able to show that
the space density of AGN with X-ray luminosities L_x < 10^45 erg s^-1 declines
significantly towards high redshifts.Comment: 21 pages, 13 figures, A&A (in press
Evidence for feedback in action from the molecular gas content in the z~1.6 outflowing QSO XID2028
Gas outflows are believed to play a pivotal role in shaping galaxies, as they regulate both star formation and black hole growth. Despite their ubiquitous presence, the origin and the acceleration mechanism of such powerful and extended winds is not yet understood. Direct observations of the cold gas component in objects with detected outflows at other wavelengths are needed to assess the impact of the outflow on the host galaxy interstellar medium (ISM). We observed with the Plateau de Bure Interferometer an obscured quasar at z~1.5, XID2028, for which the presence of an ionised outflow has been unambiguously signalled by NIR spectroscopy. The detection of CO(3-2) emission in this source allows us to infer the molecular gas content and compare it to the ISM mass derived from the dust emission. We then analyze the results in the context of recent insights on scaling relations, which describe the gas content of the overall population of star-forming galaxies at a similar redshifts. The Star formation efficiency (~100) and gas mass (M_gas=2.1-9.5x10^{10} M_sun) inferred from the CO(3-2) line depend on the underlying assumptions on the excitation of the transition and the CO-to-H2 conversion factor. However, the combination of this information and the ISM mass estimated from the dust mass suggests that the ISM/gas content of XID2028 is significantly lower than expected for its observed M⋆, sSFR and redshift, based on the most up-to-date calibrations (with gas fraction <20% and depletion time scale <340 Myr). Overall, the constraints we obtain from the far infrared and millimeter data suggest that we are observing QSO feedback able to remove the gas from the host
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
The Chandra survey of the COSMOS field II: source detection and photometry
The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8 Ms, Chandra program,
that covers the central contiguous ~0.92 deg^2 of the COSMOS field. C-COSMOS is
the result of a complex tiling, with every position being observed in up to six
overlapping pointings (four overlapping pointings in most of the central ~0.45
deg^2 area with the best exposure, and two overlapping pointings in most of the
surrounding area, covering an additional ~0.47 deg^2). Therefore, the full
exploitation of the C-COSMOS data requires a dedicated and accurate analysis
focused on three main issues: 1) maximizing the sensitivity when the PSF
changes strongly among different observations of the same source (from ~1
arcsec up to ~10 arcsec half power radius); 2) resolving close pairs; and 3)
obtaining the best source localization and count rate. We present here our
treatment of four key analysis items: source detection, localization,
photometry, and survey sensitivity. Our final procedure consists of a two step
procedure: (1) a wavelet detection algorithm, to find source candidates, (2) a
maximum likelihood Point Spread Function fitting algorithm to evaluate the
source count rates and the probability that each source candidate is a
fluctuation of the background. We discuss the main characteristics of this
procedure, that was the result of detailed comparisons between different
detection algorithms and photometry tools, calibrated with extensive and
dedicated simulations.Comment: Accepted for publication in The Astrophysical Journal Supplement
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