400 research outputs found
Obscured GOODS AGN and Their Host Galaxies at z < 1.25: The Slow Black Hole Growth Phase
We compute black hole masses and bolometric luminosities for 87 obscured AGN
in the redshift range 0.25 < z < 1.25, selected from the GOODS deep
multi-wavelength survey fields via their X-ray emission. We fit the optical
images and obtain morphological parameters for the host galaxy, separating the
galaxy from its central point source, thereby obtaining a four-band optical SED
for each active nucleus. We calculate bolometric luminosities for these AGN by
reddening a normalized mean SED of GOODS broad-line AGN to match the observed
central point-source SED of each obscured AGN. This estimate of Lbol has a
smaller spread than simple bolometric corrections to the X-ray luminosity or
direct integration of the observed multi-wavelength SED, suggesting it is a
better measure. We estimate central black hole masses from the bulge
luminosities. The black hole masses span a wide range, 7 x 10^6 M_sun to 6 x
10^9 M_sun; the median black hole mass is 5 x 10^8 M_sun. The majority of these
AGN have L/L_Edd < 0.01, and we detect no significant evolution of the mean
Eddington ratio to z = 1.25. This implies that the bulk of black hole growth in
these obscured AGN must have occurred at z > 1 and that we are observing these
AGN in a slow- or no-growth state.Comment: Accepted to ApJ; 17 pages, 10 figures, 2 tables; Updated version has
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Optical Spectroscopy Of X-Ray Sources In The Extended Chandra Deep Field South
We present the first results of our optical spectroscopy program aimed to provide redshifts and identifications for the X-ray sources in the Extended Chandra Deep Field South. A total of 339 sources were targeted using the IMACS spectrograph at the Magellan telescopes and the VIMOS spectrograph at the VLT. We measured redshifts for 186 X-ray sources, including archival data and a literature search. We find that the active galactic nucleus (AGN) host galaxies have on average redder rest-frame optical colors than nonactive galaxies, and that they live mostly in the "green valley." The dependence of the fraction of AGNs that are obscured on both luminosity and redshift is confirmed at high significance and the observed AGN spatial density is compared with the expectations from existing luminosity functions. These AGNs show a significant difference in the mid-IR to X-ray flux ratio for obscured and unobscured AGNs, which can be explained by the effects of dust self-absorption on the former. This difference is larger for lower luminosity sources, which is consistent with the dust opening angle depending on AGN luminosity.National Aeronautics and Space Administration PF8-90055, NAS8-03060NSF AST0407295Spitzer JPL RSA1288440Natural Science and Engineering Research Council of Canada (NSERC)National Academy of SciencesNASA/INTEGRAL NNG05GM79GAstronom
A massive, distant proto-cluster at z=2.47 caught in a phase of rapid formation?
Numerical simulations of cosmological structure formation show that the
Universe's most massive clusters, and the galaxies living in those clusters,
assemble rapidly at early times (2.5 < z < 4). While more than twenty
proto-clusters have been observed at z > 2 based on associations of 5-40
galaxies around rare sources, the observational evidence for rapid cluster
formation is weak. Here we report observations of an asymmetric, filamentary
structure at z = 2.47 containing seven starbursting, submillimeter-luminous
galaxies and five additional AGN within a comoving volume of 15000 Mpc.
As the expected lifetime of both the luminous AGN and starburst phase of a
galaxy is ~100 Myr, we conclude that these sources were likely triggered in
rapid succession by environmental factors, or, alternatively, the duration of
these cosmologically rare phenomena is much longer than prior direct
measurements suggest. The stellar mass already built up in the structure is
and we estimate that the cluster mass will exceed that
of the Coma supercluster at . The filamentary structure is in line
with hierarchical growth simulations which predict that the peak of cluster
activity occurs rapidly at z > 2.Comment: 7 pages, 3 figures, 2 tables, accepted in ApJL (small revisions from
previous version
INTEGRAL hard X-ray spectra of the cosmic X-ray background and Galactic ridge emission
We derive the spectra of the cosmic X-ray background (CXB) and of the
Galactic ridge X-ray emission (GRXE) in the ~20-200 keV range from the data of
the IBIS instrument aboard the INTEGRAL satellite obtained during the four
dedicated Earth-occultation observations of early 2006. We analyse the
modulation of the IBIS/ISGRI detector counts induced by the passage of the
Earth through the field of view of the instrument. Unlike previous studies, we
do not fix the spectral shape of the various contributions, but model instead
their spatial distribution and derive for each of them the expected modulation
of the detector counts. The spectra of the diffuse emission components are
obtained by fitting the normalizations of the model lightcurves to the observed
modulation in different energy bins. The obtained CXB spectrum is consistent
with the historic HEAO-1 results and falls slightly below the spectrum derived
with Swift/BAT. A 10% higher normalization of the CXB cannot be completely
excluded, but it would imply an unrealistically high albedo of the Earth. The
derived spectrum of the GRXE confirms the presence of a minimum around 80 keV
with improved statistics and yields an estimate of ~0.6 M_Sun for the average
mass of white dwarfs in the Galaxy. The analysis also provides updated
normalizations for the spectra of the Earth's albedo and the cosmic-ray induced
atmospheric emission.Comment: 13 pages, 13 figures, minor changes to text, A&A in pres
The Multiwavelength Survey by Yale-Chile (MUSYC): Deep Medium-Band optical imaging and high quality 32-band photometric redshifts in the ECDF-S
We present deep optical 18-medium-band photometry from the Subaru telescope
over the ~30' x 30' Extended Chandra Deep Field-South (ECDF-S), as part of the
Multiwavelength Survey by Yale-Chile (MUSYC). This field has a wealth of
ground- and space-based ancillary data, and contains the GOODS-South field and
the Hubble Ultra Deep Field. We combine the Subaru imaging with existing
UBVRIzJHK and Spitzer IRAC images to create a uniform catalog. Detecting
sources in the MUSYC BVR image we find ~40,000 galaxies with R_AB<25.3, the
median 5 sigma limit of the 18 medium bands. Photometric redshifts are
determined using the EAZY code and compared to ~2000 spectroscopic redshifts in
this field. The medium band filters provide very accurate redshifts for the
(bright) subset of galaxies with spectroscopic redshifts, particularly at 0.1 <
z 3.5. For 0.1 < z < 1.2, we find a 1 sigma scatter in \Delta
z/(1+z) of 0.007, similar to results obtained with a similar filter set in the
COSMOS field. As a demonstration of the data quality, we show that the red
sequence and blue cloud can be cleanly identified in rest-frame color-magnitude
diagrams at 0.1 < z < 1.2. We find that ~20% of the red-sequence-galaxies show
evidence of dust-emission at longer rest-frame wavelengths. The reduced images,
photometric catalog, and photometric redshifts are provided through the public
MUSYC website.Comment: 19 pages, 14 image
Chandra Observations of Galaxy Zoo Mergers: Frequency of Binary Active Nuclei in Massive Mergers
We present the results from a Chandra pilot study of 12 massive galaxy
mergers selected from Galaxy Zoo. The sample includes major mergers down to a
host galaxy mass of 10 that already have optical AGN
signatures in at least one of the progenitors. We find that the coincidences of
optically selected active nuclei with mildly obscured ( cm) X-ray nuclei are relatively common (8/12), but the
detections are too faint ( counts per nucleus; erg s cm) to reliably separate starburst and
nuclear activity as the origin of the X-ray emission. Only one merger is found
to have confirmed binary X-ray nuclei, though the X-ray emission from its
southern nucleus could be due solely to star formation. Thus, the occurrences
of binary AGN in these mergers are rare (0-8%), unless most merger-induced
active nuclei are very heavily obscured or Compton thick.Comment: 8 pages, including 5 figures and 1 table. Accepted by Ap
The XMM deep survey in the CDF-S II. a 9-20 keV selection of heavily obscured active galaxies at z>1.7
We present results on a search of heavily obscured active galaxies z>1.7
using the rest-frame 9-20 keV excess for X-ray sources detected in the deep
XMM-CDFS survey. Out of 176 sources selected with the conservative detection
criteria (>8 sigma) in the first source catalogue of Ranalli et al., 46 objects
lie in the redshift range of interest with the median redshift z~2.5. Their
typical rest-frame 10-20 keV luminosity is 1e+44 erg/s, as observed. Among
optically faint objects that lack spectroscopic redshift, four were found to be
strongly absorbed X-ray sources, and the enhanced Fe K emission or absorption
features in their X-ray spectra were used to obtain X-ray spectroscopic
redshifts. Using the X-ray colour-colour diagram based on the rest-frame 3-5
keV, 5-9 keV, and 9-20 keV bands, seven objects were selected for their 9-20
keV excess and were found to be strongly absorbed X-ray sources with column
density of nH > 0.6e+24 cm-2, including two possible Compton thick sources.
While they are emitting at quasar luminosity, ~3/4 of the sample objects are
found to be absorbed by nH > 1e+22 cm-2. A comparison with local AGN at the
matched luminosity suggests an increasing trend of the absorbed source fraction
for high-luminosity AGN towards high redshifts.Comment: 9 pages, 7 figures. Accepted for publication in A&
Black hole growth and host galaxy morphology
We use data from large surveys of the local Universe (SDSS+Galaxy Zoo) to
show that the galaxy-black hole connection is linked to host morphology at a
fundamental level. The fraction of early-type galaxies with actively growing
black holes, and therefore the AGN duty cycle, declines significantly with
increasing black hole mass. Late-type galaxies exhibit the opposite trend: the
fraction of actively growing black holes increases with black hole mass.Comment: 4 pages, 2 figures. Proceedings of the IAU Symposium no. 267,
"Co-Evolution of Central Black Holes and Galaxies: Feeding and Feedback",
eds. B.M. Peterson, R.S. Somerville and T. Storchi-Bergman
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