32 research outputs found
The Chandra Deep Field South: the 1 Million Second
We present the main results from our 940 ksec observation of the Chandra Deep
Field South (CDFS), using the source catalog described in an accompanying paper
(Giacconi et al. 2001). We extend the measurement of source number counts to
5.5e-17 erg/cm^2/s in the soft 0.5-2 keV band and 4.5e-16 erg/cm^2/s in the
hard 2-10 keV band. The hard band LogN-LogS shows a significant flattening
(slope~=0.6) below ~1e-14 erg/cm^2/s, leaving at most 10-15% of the X-ray
background (XRB) to be resolved, the main uncertainty lying in the measurement
of the total flux of the XRB. On the other hand, the analysis in the very hard
5-10 keV band reveals a relatively steep LogN-LogS (slope ~=1.3) down to 1e-15
erg/cm^2/s. Together with the evidence of a progressive flattening of the
average X-ray spectrum near the flux limit, this indicates that there is still
a non negligible population of faint hard sources to be discovered at energies
not well probed by Chandra, which possibly contribute to the 30 keV bump in the
spectrum of the XRB. We use optical redshifts and identifications, obtained
with the VLT, for one quarter of the sample to characterize the combined
optical and X-ray properties of the CDFS sample. Different source types are
well separated in a parameter space which includes X-ray luminosity, hardness
ratio and R-K color. Type II objects, while redder on average than the field
population, have colors which are consistent with being hosted by a range of
galaxy types. Type II AGN are mostly found at z<~1, in contrast with
predictions based on AGN population synthesis models, thus suggesting a
revision of their evolutionary parameters.Comment: Accepted by The Astrophysical Journal, 24 pages, 8 figures, 1 color
jpg plate (fig.1
Ensemble Learning Independent Component Analysis of Normal Galaxy Spectra
In this paper, we employe a new statistical analysis technique, Ensemble
Learning for Independent Component Analysis (EL-ICA), on the synthetic galaxy
spectra from a newly released high resolution evolutionary model by Bruzual &
Charlot. We find that EL-ICA can sufficiently compress the synthetic galaxy
spectral library to 6 non-negative Independent Components (ICs), which are good
templates to model huge amount of normal galaxy spectra, such as the galaxy
spectra in the Sloan Digital Sky Survey (SDSS). Important spectral parameters,
such as starlight reddening, stellar velocity dispersion, stellar mass and star
formation histories, can be given simultaneously by the fit. Extensive tests
show that the fit and the derived parameters are reliable for galaxy spectra
with the typical quality of the SDSS.Comment: 41 pages, 23 figures, to be published in A
A Classic Type 2 QSO
In the Chandra Deep Field South 1Msec exposure we have found, at redshift
3.700 +- 0.005, the most distant Type 2 AGN ever detected. It is the source
with the hardest X-ray spectrum with redshift z>3. The optical spectrum has no
detected continuum emission to a 3sigma detection limit of ~3 10^{-19}
ergs/s/cm^2/AA and shows narrow lines of Ly_alpha, CIV, NV, HeII, OVI, [OIII],
and CIII]. Their FWHM line widths have a range of ~700-2300 km/s with an
average of approximately ~1500 km/s. The emitting gas is metal rich (Z ~2.5-3
Z_solar). In the X-ray spectrum of 130 counts in the 0.5-7 keV band there is
evidence for intrinsic absorption with N_H > 10^{24} cm^{-2}. An iron K_alpha
line with rest frame energy and equivalent width of ~6.4 keV and ~1 keV,
respectively, in agreement with the obscuration scenario, is detected at a
2sigma level. If confirmed by our forthcoming XMM observations this would be
the highest redshift detection of FeK_alpha. Depending on the assumed cosmology
and the X-ray transfer model, the 2-10 keV rest frame luminosity corrected for
absorption is ~10^{45 +- 0.5} ergs/s, which makes our source a classic example
of the long sought Type 2 QSOs. From standard population synthesis models,
these sources are expected to account for a relevant fraction of the
black-hole-powered QSO distribution at high redshift.Comment: 24 LaTeX pages including 6 postscript figures. Revised version,
accepted by Ap
Regular and Irregular States in Generic Systems
In this work we present the results of a numerical and semiclassical analysis
of high lying states in a Hamiltonian system, whose classical mechanics is of a
generic, mixed type, where the energy surface is split into regions of regular
and chaotic motion. As predicted by the principle of uniform semiclassical
condensation (PUSC), when the effective tends to 0, each state can be
classified as regular or irregular. We were able to semiclassically reproduce
individual regular states by the EBK torus quantization, for which we devise a
new approach, while for the irregular ones we found the semiclassical
prediction of their autocorrelation function, in a good agreement with
numerics. We also looked at the low lying states to better understand the onset
of semiclassical behaviour.Comment: 25 pages, 14 figures (as .GIF files), high quality figures available
upon reques
The X-Ray Derived Cosmological Star Formation History and the Galaxy X-Ray Luminosity Functions in the Chandra Deep Fields North and South
The cosmological star formation rate in the combined Chandra Deep Fields
North and South is derived from our X-Ray Luminosity Function for Galaxies in
these Deep Fields. Mild evolution is seen up to redshift order unity with SFR ~
(1 + z)^{2.7}. This is the first directly observed normal star-forming galaxy
X-ray luminosity function (XLF) at cosmologically interesting redshifts (z>0).
This provides the most direct measure yet of the X-ray derived cosmic
star-formation history of the Universe. We make use of Bayesian statistical
methods to classify the galaxies and the two types of AGN, finding the most
useful discriminators to be the X-ray luminosity, X-ray hardness ratio, and
X-ray to optical flux ratio. There is some residual AGN contamination in the
sample at the bright end of the luminosity function. Incompleteness slightly
flattens the XLF at the faint end of the luminosity function. The XLF has a
lognormal distribution and agrees well with the radio and infrared luminosity
functions. However, the XLF does not agree with the Schechter luminosity
function for the H-alpha LF indicating that additional and different physical
processes may be involved in the establishment of the lognormal form of the
XLF. The agreement of our star formation history points with the other star
formation determinations in different wavebands (IR, Radio, H-alpha) gives an
interesting constraint on the IMF, and X-rays may be measuring directly the
binary star formation history of the Universe. X-ray studies will continue to
be useful for probing the star formation history of the universe by avoiding
problems of obscuration. Star formation may therefore be measured in more
detail by deep surveys with future x-ray missions.Comment: Accepted for publication in ApJ. 19 pages with 10 figures formatted
with emulateapj. Version with B/W only figures available at
http://www.pha.jhu.edu/~ptak/paper
Structural, Electronic, and Magnetic Properties of Bimetallic Ni m Nb n (m + n ≤ 8) Clusters: First Principle Study
Structural, electronic, and magnetic properties of bimetallic NimNbn(m + n≤8) clusters have been investigated using the particle swarm optimization coupled with density functional theory. Ninety-seven stable structures of Ni mNb n clusters were found. Among these Ni mNb n clusters, most of the ground state of bimetallic clusters prefers compact structures when m + n>3. The HOMO–LUMO gaps of these bimetallic Ni mNb n clusters were found in the range of 0.1–0.5 eV. The exchange splitting exists in most of these binary clusters and results in non-zero magnetization in these clusters. Most of the clusters show their magnetic moment strongly depending on the size, the symmetry, the configuration, and the composition. An interesting finding is that the magnetic moment per atom in NiNb and Ni2Nb clusters shows a larger value (i.e., 1.5 and 1.0 μB) than that of the corresponding size of pure Ni clusters.</p