160 research outputs found
Dissecting Photometric Redshift for Active Galactic Nucleus Using XMM- and Chandra-COSMOS Samples
In this paper, we release accurate photometric redshifts for 1692 counterparts to Chandra sources in the central square degree of the Cosmic Evolution Survey (COSMOS) field. The availability of a large training set of spectroscopic redshifts that extends to faint magnitudes enabled photometric redshifts comparable to the highest quality results presently available for normal galaxies. We demonstrate that morphologically extended, faint X-ray sources without optical variability are more accurately described by a library of normal galaxies (corrected for emission lines) than by active galactic nucleus (AGN) dominated templates, even if these sources have AGN-like X-ray luminosities. Preselecting the library on the bases of the source properties allowed us to reach an accuracy Ï_(Îz/(1+z(spec))~0.015 with a fraction of outliers of 5.8% for the entire Chandra-COSMOS sample. In addition, we release revised photometric redshifts for the 1735 optical counterparts of the XMM-detected sources over the entire 2 deg^2 of COSMOS. For 248 sources, our updated photometric redshift differs from the previous release by Îz > 0.2. These changes are predominantly due to the inclusion of newly available deep H-band photometry (H_(AB) = 24 mag). We illustrate once again the importance of a spectroscopic training sample and how an assumption about the nature of a source together, with the number and the depth of the available bands, influences the accuracy of the photometric redshifts determined for AGN. These considerations should be kept in mind when defining the observational strategies of upcoming large surveys targeting AGNs, such as eROSITA at X-ray energies and the Australian Square Kilometre Array Pathfinder Evolutionary Map of the Universe in the radio band
Early evolution of CV reduced-type parent body.
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The Stellar Population of Lyman-alpha Emitting Galaxies at z ~ 5.7
We present a study of three Lyman-alpha emitting galaxies (LAEs), selected
via a narrow-band survey in the GOODS northern field, and spectroscopically
confirmed to have redshifts of z ~ 5.65. Using HST ACS and Spitzer IRAC data,
we constrain the rest-frame UV-to-optical spectral energy distributions (SEDs)
of the galaxies. Fitting stellar population synthesis models to the observed
SEDs, we find best-fit stellar populations with masses between ~ 10^9 - 10^10
M_sun and ages between ~ 5 - 100 Myr, assuming a simple starburst star
formation history. However, stellar populations as old as 700 Myr are
admissible if a constant star formation rate model is considered. Very deep
near-IR observations may help to narrow the range of allowed models by
providing extra constraints on the rest-frame UV spectral slope. Our
narrow-band selected objects and other IRAC-detected z ~ 6 i'-dropout galaxies
have similar 3.6 um magnitudes and z' - [3.6] colors, suggesting that they
posses stellar populations of similar masses and ages. This similarity may be
the result of a selection bias, since the IRAC-detected LAEs and i'-dropouts
probably only sample the bright end of the luminosity function. On the other
hand, our LAEs have blue i' - z' colors compared to the i'-dropouts, and would
have been missed by the i'-dropout selection criterion. A better understanding
of the overlap between the LAE and the i'-dropout populations is necessary in
order to constrain the properties of the overall high-redshift galaxy
population, such as the total stellar mass density at z ~ 6.Comment: 10 pages, 8 figures. Accepted for publication in Ap
The stellar content of the COSMOS field as derived from morphological and SED based gtar/galaxy Separation
We report on the stellar content of the COSMOS two degree field, as derived
from a rigorous star/galaxy separation approach developed for using stellar
sources to define the point spread function variation map used in a study of
weak galaxy lensing. The catalog obtained in one filter from the ACS (Advanced
Camera for Survey on the Hubble Space Telescope) is cross-identified with
ground based multi-wavelength catalogs. The classification is reliable to
magnitude and the sample is complete even fainter. We construct a
color-magnitude diagram and color histograms and compare them with predictions
of a standard model of population synthesis. We find features corresponding to
the halo subdwarf main sequence turnoff, the thick disk, and the thin disk.
This data set provides constraints on the thick disk and spheroid density laws
and on the IMF at low mass. We find no evidence of a sharp spheroid edge out to
this distance. We identify a blue population of white dwarfs with counts that
agree with model predictions. We find a hint for a possible slight stellar
overdensity at about 22-34 kpc but the data are not strong enough at present to
claim detection of a stream feature in the halo (abridged).Comment: 32 pages, 13 figures, accepted in APJ Suppl COSMOS special issue,
replaced by larger figures. A full resolution figure preprint can be found at
ftp://ftp.obs-besancon.fr/pub/outgoing/annie/star-cosmos.pd
Rest-UV Absorption Lines as Metallicity Estimator: the Metal Content of Star-Forming Galaxies at z~5
We measure a relation between the depth of four prominent rest-UV absorption
complexes and metallicity for local galaxies and verify it up to z~3. We then
apply this relation to a sample of 224 galaxies at 3.5 = 4.8) in
COSMOS, for which unique UV spectra from DEIMOS and accurate stellar masses
from SPLASH are available. The average galaxy population at z~5 and log(M/Msun)
> 9 is characterized by 0.3-0.4 dex (in units of 12+log(O/H)) lower
metallicities than at z~2, but comparable to z~3.5. We find galaxies with
weak/no Ly-alpha emission to have metallicities comparable to z~2 galaxies and
therefore may represent an evolved sub-population of z~5 galaxies. We find a
correlation between metallicity and dust in good agreement with local galaxies
and an inverse trend between metallicity and star-formation rate (SFR)
consistent with observations at z~2. The relation between stellar mass and
metallicity (MZ relation) is similar to z~3.5, however, there are indications
of it being slightly shallower, in particular for the young, Ly-alpha emitting
galaxies. We show that, within a "bathtub" approach, a shallower MZ relation is
expected in the case of a fast (exponential) build-up of stellar mass with an
e-folding time of 100-200 Myr. Due to this fast evolution, the process of dust
production and metal enrichment as a function of mass could be more stochastic
in the first billion years of galaxy formation compared to later times.Comment: 20 pages, 13 figures, 4 tables; Submitted to Ap
The DEIMOS 10k spectroscopic survey catalog of the COSMOS field
We present a catalog of 10718 objects in the COSMOS field observed through
multi-slit spectroscopy with the Deep Imaging Multi-Object Spectrograph
(DEIMOS) on the Keck II telescope in the wavelength range ~5500-9800A. The
catalog contains 6617 objects with high-quality spectra (two or more spectral
features), and 1798 objects with a single spectroscopic feature confirmed by
the photometric redshift. For 2024 typically faint objects we could not obtain
reliable redshifts. The objects have been selected from a variety of input
catalogs based on multi-wavelength observations in the field, and thus have a
diverse selection function, which enables the study of the diversity in the
galaxy population. The magnitude distribution of our objects is peaked at
I_AB~23 and K_AB~21, with a secondary peak at K_AB~24. We sample a broad
redshift distribution in the range 0<z<6, with one peak at z~1, and another one
around z~4. We have identified 13 redshift spikes at z>0.65 with chance
probabilities <4xE-4$, some of which are clearly related to protocluster
structures of sizes >10 Mpc. An object-to-object comparison with a multitude of
other spectroscopic samples in the same field shows that our DEIMOS sample is
among the best in terms of fraction of spectroscopic failures and relative
redshift accuracy. We have determined the fraction of spectroscopic blends to
about 0.8% in our sample. This is likely a lower limit and at any rate well
below the most pessimistic expectations. Interestingly, we find evidence for
strong lensing of Ly-alpha background emitters within the slits of 12 of our
target galaxies, increasing their apparent density by about a factor of 4.Comment: 28 pages, 11 figures and 5 tables. The full catalogue table is
available on http://cosmos.astro.caltech.edu. Accepted for publication in the
Astrophysical Journa
The Nature of the faint far-infrared extragalactic source population: Optical/NIR and radio follow-up observations of ISOPHOT deep-field sources using Keck, Subaru, and VLA telescopes
We report on optical and near-infrared (NIR) follow-up spectroscopy of faint
far-infrared (FIR) sources found in our deep FIR survey by Kawara et al.Comment: 2 pages, 1 figure. To appear in the proceedings of "AGN Surveys" (IAU
Colloquium 184
Unveiling a population of galaxies harboring low-mass black holes with X-rays
We report the discovery of three low-mass black hole (BH) candidates residing in the centers of low-mass galaxies at z < 0.3 in the Chandra Deep Field-South Survey. These BHs are initially identified as candidate active galactic nuclei based on their X-ray emission in deep Chandra observations. Multi-wavelength observations are used to strengthen our claim that such emission is powered by an accreting supermassive BH. While the X-ray luminosities are low at L_X ~ 10^(40) erg s^(â1) (and variable in one case), we argue that they are unlikely to be attributed to star formation based on Hα or UV fluxes. Optical spectroscopy from Keck and the VLT allows us to (1) measure accurate redshifts, (2) confirm their low stellar host mass, (3) investigate the source(s) of photo-ionization, and (4) estimate extinction. With stellar masses of M_* < 3 Ă 10^9 M_â determined from Hubble Space Telescope/Advanced Camera for Surveys imaging, the host galaxies are among the lowest mass systems known to host actively accreting BHs. We estimate BH masses M_(BH) ~ 2 Ă 10^5 M_â based on scaling relations between BH mass and host properties for more luminous systems. In one case, a broad component of the Hα emission-line profile is detected, thus providing a virial mass estimate. BHs in such low-mass galaxies are of considerable interest as the low-redshift analogs to the seeds of the most massive BHs at high redshift which have remained largely elusive to date. Our study highlights the power of deep X-ray surveys to uncover such low-mass systems
The Discovery of a Very Narrow-Line Star Forming Obat a Redshift of 5.66ject
We report on the discovery of a very narrow-line star forming object beyond
redshift of 5. Using the prime-focus camera, Suprime-Cam, on the 8.2 m Subaru
telescope together with a narrow-passband filter centered at
= 8150 \AA with passband of = 120 \AA, we have obtained a very
deep image of the field surrounding the quasar SDSSp J104433.04012502.2 at a
redshift of 5.74. Comparing this image with optical broad-band images, we have
found an object with a very strong emission line. Our follow-up optical
spectroscopy has revealed that this source is at a redshift of
, forming stars at a rate
yr. Remarkably, the velocity dispersion of Ly-emitting gas is
only 22 km s. Since a blue half of the Ly emission could be
absorbed by neutral hydrogen gas, perhaps in the system, a modest estimate of
the velocity dispersion may be 44 km s. Together with a linear
size of 7.7 kpc, we estimate a lower limit of the dynamical mass
of this object to be . It is thus suggested that
LAE J10440123 is a star-forming dwarf galaxy (i.e., a subgalactic object or
a building block) beyond redshift 5 although we cannot exclude a possibility
that most Ly emission is absorbed by the red damping wing of neutral
intergalactic matter.Comment: 6 pages, 2 figures. ApJ Letters, in pres
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