549 research outputs found
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
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&
On Properties of Boundaries and Electron Conductivity in Mesoscopic Polycrystalline Silicon Films for Memory Devices
We present the results of molecular dynamics modeling on the structural
properties of grain boundaries (GB) in thin polycrystalline films. The
transition from crystalline boundaries with low mismatch angle to amorphous
boundaries is investigated. It is shown that the structures of the GBs satisfy
a thermodynamical criterion. The potential energy of silicon atoms is closely
related with a geometrical quantity -- tetragonality of their coordination with
their nearest neighbors. A crossover of the length of localization is observed.
To analyze the crossover of the length of localization of the single-electron
states and properties of conductance of the thin polycrystalline film at low
temperature, we use a two-dimensional Anderson localization model, with the
random one-site electron charging energy for a single grain (dot), random
non-diagonal matrix elements, and random number of connections between the
neighboring grains. The results on the crossover behavior of localization
length of the single-electron states and characteristic properties of
conductance are presented in the region of parameters where the transition from
an insulator to a conductor regimes takes place.Comment: 8 pages, 3 figure
The VLA Survey of the Chandra Deep Field South: I. Overview of the Radio Data
We report 20 and 6 cm VLA deep observations of the CDF-S including the
Extended CDF-S. We discuss the radio properties of 266 cataloged radio sources,
of which 198 are above a 20 cm completeness level reaching down to 43 microJy
at the center of the field. Survey observations made at 6 cm over a more
limited region covers the original CDF-S to a comparable level of sensitivity
as the 20 cm observations.
Of 266 cataloged radio sources, 52 have X-ray counterparts in the CDF-S and a
further 37 in the E-CDF-S area not covered by the 1 Megasecond exposure. Using
a wide range of material, we have found optical or infrared counterparts for
254 radio sources, of which 186 have either spectroscopic or photometric
redshifts (Paper II). Three radio sources have no apparent counterpart at any
other wavelength. Measurements of the 20 cm radio flux density at the position
of each CDF-S X-ray source detected a further 30 radio sources above a
conservative 3-sigma detection limit.
X-ray and sub-mm observations have been traditionally used as a measure of
AGN and star formation activity, respectively. These new observations probe the
faint end of both the star formation and radio galaxy/AGN population, as well
as the connection between the formation and evolution of stars and SMBHs. Both
of the corresponding gravitational and nuclear fusion driven energy sources can
lead to radio synchrotron emission. AGN and radio galaxies dominate at high
flux densities. Although emission from star formation becomes more prominent at
the microjansky levels reached by deep radio surveys, even for the weakest
sources, we still find an apparent significant contribution from low luminosity
AGN as well as from star formation.Comment: Accpted for publication in the Astrophysical Journal supplements with
3 tables and 18 figure
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
Recommended from our members
Prediction and characterization of complex systems
Complex systems are difficult to characterize and to simulate. By considering a series of explicit systems, through experiments and analysis, this project has shown that dynamical systems can be used to model complex systems. A complex dynamical system requires an exponential amount of computer work to simulate accurately. Direct methods are not practical and it is only by an hierarchical approach that one can gain control over the exponential behavior. This allows the development of efficient methods to study fluid flow and to simulate biological systems. There are two steps in the hierarchical approach. First, one must characterize the complex system as a collection of large domains or objects that have their own forms of interactions. This is done by considering coherent structures, such as solitons, spirals, and propagating fronts and determining their interactions. Second, one must be able to predict the properties of the resulting low-dimensional dynamical system.This is accomplished by an understanding of the topology of the orbits of the dynamical system. The coherent structure description was carried out in fluid and reaction diffusion systems. It was shown that very simple models from statistical mechanics could characterize a rotating Rayleigh-Benard system and that patters in reaction-diffusion systems are well described by soliton-like solutions. The studies of dynamical systems showed that simple characterizations of the phase space can be used to determine long time bounds. Also, that periodic orbit theory can be used to demonstrate that Monte Carlo simulations will converge to incorrect results
Constraining the thermal history of the Warm-Hot Intergalactic Medium
We have identified a large-scale structure traced by galaxies at z=0.8,
within the Lockman Hole, by means of multi-object spectroscopic observations.
By using deep XMM images we have investigated the soft X-ray emission from the
Warm-Hot Intergalactic Medium (WHIM) expected to be associated with this
large-scale structure and we set a tight upper limit to its flux in the very
soft 0.2-0.4 keV band. The non-detection requires the WHIM at these redshifts
to be cooler than 0.1 keV. Combined with the WHIM emission detections at lower
redshift, our result indicates that the WHIM temperature is rapidly decreasing
with redshift, as expected in popular cosmological models.Comment: 10 pages, 5 figures, 1 appendix. A&A accepte
- âŠ