600 research outputs found
The X-ray emission of z>2.5 active galactic nuclei can be obscured by their host galaxies
We present a multi-wavelength study of seven AGN at spectroscopic redshift
>2.5 in the 7 Ms Chandra Deep Field South, selected to have good FIR/sub-mm
detections. Our aim is to investigate the possibility that the obscuration
observed in the X-rays can be produced by the interstellar medium (ISM) of the
host galaxy. Based on the 7 Ms Chandra spectra, we measured obscuring column
densities N in excess of 7x10 cm and intrinsic X-ray
luminosities L>10 erg s for our targets, as well as
equivalent widths for the Fe K emission line EW>0.5-1 keV. We built the
UV-to-FIR spectral energy distributions by using broad-band photometry from
CANDELS and Herschel catalogs. By means of an SED decomposition technique, we
derived stellar masses (M~10 Msun), IR luminosities
(L>10 Lsun), star formation rates (SFR~190-1680 Msun yr)
and AGN bolometric luminosities (L~10 erg s) for our
sample. We used an empirically-calibrated relation between gas masses and
FIR/sub-mm luminosities and derived M~0.8-5.4x10 Msun.
High-resolution (0.3-0.7'') ALMA data (when available, CANDELS data otherwise)
were used to estimate the galaxy size and hence the volume enclosing most of
the ISM under simple geometrical assumptions. These measurements were then
combined to derive the column density associated with the ISM of the host, on
the order of N~10 cm. The comparison between the
ISM column densities and those measured from the X-ray spectral analysis shows
that they are similar. This suggests that, at least at high redshift,
significant absorption on kpc scales by the dense ISM in the host likely adds
to or substitutes that produced by circumnuclear gas on pc scales (i.e., the
torus of unified models). The lack of unobscured AGN among our ISM-rich targets
supports this scenario.Comment: 15 pages, 3 figures. Accepted for publication in A&
Evolution in the iron abundance of the ICM
We present a Chandra analysis of the X-ray spectra of 56 clusters of galaxies
at , which cover a temperature range of keV. Our analysis
is aimed at measuring the iron abundance in the ICM out to the highest redshift
probed to date. We find that the emission-weighted iron abundance measured
within in clusters below 5 keV is, on average, a factor of
higher than in hotter clusters, following , which confirms the trend seen in local samples. We made use of
combined spectral analysis performed over five redshift bins at
to estimate the average emission weighted iron abundance. We find a constant
average iron abundance as a function of redshift,
but only for clusters at . The emission-weighted iron abundance is
significantly higher () in the redshift range
, approaching the value measured locally in the inner radii for a mix of cool-core and non cool-core clusters in the
redshift range . The decrease in with can be
parametrized by a power law of the form . The observed
evolution implies that the average iron content of the ICM at the present epoch
is a factor of larger than at . We confirm that the ICM is
already significantly enriched () at a look-back time
of 9 Gyr. Our data provide significant constraints on the time scales and
physical processes that drive the chemical enrichment of the ICM.Comment: 4 pages, 4 figures, to appear in the Proceedings of "The Extreme
Universe in the Suzaku Era", Dicember 2006, Kyoto (Japan
Tracing the evolution in the iron content of the ICM
We present a Chandra analysis of the X-ray spectra of 56 clusters of galaxies
at z>0.3, which cover a temperature range of 3>kT>15 keV. Our analysis is aimed
at measuring the iron abundance in the ICM out to the highest redshift probed
to date. We find that the emission-weighted iron abundance measured within
(0.15-0.3)R_vir in clusters below 5 keV is, on average, a factor of ~2 higher
than in hotter clusters, following Z(T)~0.88T^-(0.47)Z_o, which confirms the
trend seen in local samples. We made use of combined spectral analysis
performed over five redshift bins at 0.3>z>1.3 to estimate the average emission
weighted iron abundance. We find a constant average iron abundance Z_Fe~0.25Z_o
as a function of redshift, but only for clusters at z>0.5. The
emission-weighted iron abundance is significantly higher (Z_Fe~0.4Z_o) in the
redshift range z~0.3-0.5, approaching the value measured locally in the inner
0.15R_vir radii for a mix of cool-core and non cool-core clusters in the
redshift range 0.1<z<0.3. The decrease in Z_Fe with redshift can be
parametrized by a power law of the form ~(1+z)^(-1.25). The observed evolution
implies that the average iron content of the ICM at the present epoch is a
factor of ~2 larger than at z=1.2. We confirm that the ICM is already
significantly enriched (Z_Fe~0.25Z_o) at a look-back time of 9 Gyr. Our data
provide significant constraints on the time scales and physical processes that
drive the chemical enrichment of the ICM.Comment: 6 pages, 6 figures, to appear in the Proceedings of "Heating vs.
Cooling in Galaxies and Clusters of Galaxies", August 2006, Garching
(Germany
The space density of Compton-thick AGN at z~0.8 in the zCOSMOS-Bright Survey
The obscured accretion phase in BH growth is a key ingredient in many models
linking the AGN activity with the evolution of their host galaxy. At present, a
complete census of obscured AGN is still missing. The purpose of this work is
to assess the reliability of the [NeV] emission line at 3426 A to pick up
obscured AGN up to z~1 by assuming that [NeV] is a reliable proxy of the
intrinsic AGN luminosity and using moderately deep X-ray data to characterize
the amount of obscuration. A sample of 69 narrow-line (Type 2) AGN at
z=0.65-1.20 were selected from the 20k-zCOSMOS Bright galaxy sample on the
basis of the presence of the [NeV] emission. The X-ray properties of these
galaxies were then derived using the Chandra-COSMOS coverage of the field; the
X-ray-to-[NeV] flux ratio, coupled with X-ray spectral and stacking analyses,
was then used to infer whether Compton-thin or Compton-thick absorption were
present in these sources. Then the [NeV] luminosity function was computed to
estimate the space density of Compton-thick (CT) AGN at z~0.8. Twenty-three
sources were detected by Chandra, and their properties are consistent with
moderate obscuration (on average, ~a few 10^{22} cm^-2). The X-ray properties
of the remaining 46 X-ray undetected Type 2 AGN were derived using X-ray
stacking analysis. Current data indicate that a fraction as high as ~40% of the
present sample is likely to be CT. The space density of CT AGN with
logL_2-10keV>43.5 at z=0.83 is (9.1+/-2.1) 10^{-6} Mpc^{-3}, in good agreement
with both XRB model expectations and the previously measured space density for
objects in a similar redshift and luminosity range. We regard our selection
technique for CT AGN as clean but not complete, since even a mild extinction in
the NLR can suppress [NeV] emission. Therefore, our estimate of their space
density should be considered as a lower limit.Comment: 10 pages, 7 figures, 2 tables, A&A, in pres
Photometric Redshift of X-Ray Sources in the Chandra Deep Field South
Based on the photometry of 10 near-UV, optical, and near-infrared bands of
the Chandra Deep Field South, we estimate the photometric redshifts for 342
X-ray sources, which constitute ~99% of all the detected X-ray sources in the
field. The models of spectral energy distribution are based on galaxies and a
combination of power-law continuum and emission lines. Color information is
useful for source classifications: Type-I AGN show non-thermal spectral
features that are distinctive from galaxies and Type-II AGN. The hardness ratio
in X-ray and the X-ray-to-optical flux ratio are also useful discriminators.
Using rudimentary color separation techniques, we are able to further refine
our photometric redshift estimations. Among these sources, 137 have reliable
spectroscopic redshifts, which we use to verify the accuracy of photometric
redshifts and to modify the model inputs. The average relative dispersion in
redshift distribution is ~8%, among the most accurate for photometric surveys.
The high reliability of our results is attributable to the high quality and
broad coverage of data as well as the applications of several independent
methods and a careful evaluation of every source. We apply our redshift
estimations to study the effect of redshift on broadband colors and to study
the redshift distribution of AGN. Our results show that both the hardness ratio
and U-K color decline with redshift, which may be the result of a K-correction.
The number of Type-II AGN declines significantly at z>2 and that of galaxies
declines at z>1. However, the distribution of Type-I AGN exhibits less redshift
dependence. As well, we observe a significant peak in the redshift distribution
at z=0.6. We demonstrate that our photometric redshift estimation produces a
reliable database for the study of X-ray luminosity of galaxies and AGN.Comment: 40 pages, 11 figures. Accepted for publication in the Astrophysical
Journa
Chandra and XMM-Newton Observations of RDCS1252.9-2927, A Massive Cluster at z=1.24
We present deep Chandra and XMM obervations of the galaxy cluster
RDCS1252.9-2927, which was selected from the ROSAT Deep Cluster Survey (RDCS)
and confirmed by extensive spectroscopy with the VLT at redshift z=1.237. With
the Chandra data, the X-ray emission from the intra-cluster medium is well
resolved and traced out to 500 kpc, thus allowing a measurement of the physical
properties of the gas with unprecedented accuracy at this redshift. We detect a
clear 6.7 keV Iron K line in the Chandra spectrum providing a redshift within
1% of the spectroscopic one. By augmenting our spectroscopic analysis with the
XMM data (MOS detectors only), we significantly narrow down the 1 sigma error
bar to 10% for the temperature and 30% for the metallicity, with best fit
values kT = 6.0(+0.7,-0.5) keV, Z = 0.36(+0.12,-0.10) Z_sun. In the likely
hypothesis of hydrostatic equilibrium, we measure a total mass of M_{500} =
(1.9+-0.3)10^14 h_70^{-1}M_sun within R_{Delta=500} = 536 kpc. Overall, these
observations imply that RDCS1252.9-2927 is the most X-ray luminous and likely
the most massive bona-fide cluster discovered to date at z>1. When combined
with current samples of distant clusters, these data lend further support to a
mild evolution of the cluster scaling relations, as well the metallicity of the
intra-cluster gas. Inspection of the cluster mass function in the current
cosmological concordance model (h,Omega_m,Omega_Lambda)=(0.7,0.3,0.7) and
sigma_8=0.7-0.8 shows that RDCS1252.9-2927 is an M* cluster at z=1.24, in
keeping with number density expectations in the RDCS survey volume.Comment: 9 pages, 1 color figure (fig6.jpg). The Astronomical Journal in press
(Jan 2004). Full resolution preprint available at
http://www.eso.org/~prosati/RDCS1252
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
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
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