41 research outputs found
Evidence for a large fraction of Compton-thick quasars at high redshift
Using mid-infrared and radio selection criteria, we pre-select a sample of
candidate high-redshift type-2 quasars in the Subaru XMM-Newton Deep Field
(SXDF). To filter out starburst contaminants, we use a bayesian method to fit
the spectral energy distributions (SEDs) between 24-microns and B-band, obtain
photometric redshifts, and identify the best candidates for high-z type-2
quasars. This leaves us with 12 z_phot >= 1.7 type-2 quasar candidates in an
area ~0.8 deg^2, of which only two have secure X-ray detections. The two
detected sources have estimated column densities N_H~2 & 3x10^27 m^-2, i.e.
heavily obscured but Compton-thin quasars. Given the large bolometric
luminosities and redshifts of the undetected objects, the lack of X-ray
detections suggests extreme absorbing columns N_H >= 10^28 m^-2 are typical. We
have found evidence for a population of ``Compton-thick'' high-redshift type-2
quasars, at least comparable to, and probably larger than the type-1 quasar
population, although spectroscopic confirmation of their AGN nature is
important.Comment: 6 pages, 2 colour figures. Accepted by MNRAS. Full resolution version
and supplementary figures can be found at:
http://www.mpia.de/homes/martinez/publications.htm
Star formation in the cluster CLG0218.3-0510 at z=1.62 and its large-scale environment: the infrared perspective
The galaxy cluster CLG0218.3-0510 at z=1.62 is one of the most distant galaxy
clusters known, with a rich muti-wavelength data set that confirms a mature
galaxy population already in place. Using very deep, wide area (20x20 Mpc)
imaging by Spitzer/MIPS at 24um, in conjunction with Herschel 5-band imaging
from 100-500um, we investigate the dust-obscured, star-formation properties in
the cluster and its associated large scale environment. Our galaxy sample of
693 galaxies at z=1.62 detected at 24um (10 spectroscopic and 683 photo-z)
includes both cluster galaxies (i.e. within r <1 Mpc projected clustercentric
radius) and field galaxies, defined as the region beyond a radius of 3 Mpc. The
star-formation rates (SFRs) derived from the measured infrared luminosity range
from 18 to 2500 Ms/yr, with a median of 55 Ms/yr, over the entire radial range
(10 Mpc). The cluster brightest FIR galaxy, taken as the centre of the galaxy
system, is vigorously forming stars at a rate of 25670 Ms/yr, and the
total cluster SFR enclosed in a circle of 1 Mpc is 116196 Ms/yr. We
estimate a dust extinction of about 3 magnitudes by comparing the SFRs derived
from [OII] luminosity with the ones computed from the 24um fluxes. We find that
the in-falling region (1-3 Mpc) is special: there is a significant decrement
(3.5x) of passive relative to star-forming galaxies in this region, and the
total SFR of the galaxies located in this region is lower (130 Ms/yr/Mpc2) than
anywhere in the cluster or field, regardless of their stellar mass. In a
complementary approach we compute the local galaxy density, Sigma5, and find no
trend between SFR and Sigma5. However, we measure an excess of star-forming
galaxies in the cluster relative to the field by a factor 1.7, that lends
support to a reversal of the SF-density relation in CLG0218.Comment: accepted for publication in MNRAS. v2: minor correction
The mineralogy of newly formed dust in active galactic nuclei
The tori around active galactic nuclei (AGN) are potential formation sites
for large amounts of dust, and they may help resolve the so-called dust budget
crisis at high redshift. We investigate the dust composition in 53 of the 87
Palomar Green (PG) quasars showing the 9.7 micron silicate feature in emission.
By simultaneously fitting the mid-infrared spectroscopic features and the
underlying continuum, we estimate the mass fraction in various amorphous and
crystalline dust species. We find that the dust consists predominantly of
alumina and amorphous silicates, with a small fraction in crystalline form. The
mean crystallinity is 8 +/- 6%, with more than half of the crystallinities
greater than 5%, well above the upper limit determined for the Galaxy. Higher
values of crystallinity are found for higher oxide fractions and for more
luminous sources.Comment: 24 pages, 9 figures, accepted for publication in Planetary and Space
Scienc
Near-Infrared Survey and Photometric Redshifts in the Extended GOODS-North field
We present deep and -band images in the extended Great Observatories
Origins Deep Survey-North (GOODS-N) field covering an area of 0.22
. The observations were taken using WIRCam on the 3.6-m Canada
France Hawaii Telescope (CFHT). Together with the reprocessed -band
image, the limiting AB magnitudes (in 2" diameter apertures) are
24.7, 24.2, and 24.4 AB mag in the , , and bands,
respectively. We also release a multi-band photometry and photometric redshift
catalog containing 93598 sources. For non-X-ray sources, we obtained a
photometric redshift accuracy with an outlier
fraction . For X-ray sources, which are mainly active galactic
nuclei (AGNs), we cross-matched our catalog with the updated 2M-CDFN X-ray
catalog from Xue et al. (2016) and found that 658 out of 683 X-ray sources have
counterparts. UV data are included in the photometric redshift
computation for the X-ray sources to give with
. Our approach yields more accurate photometric redshift estimates
compared to previous works in this field. In particular, by adopting AGN-galaxy
hybrid templates, our approach delivers photometric redshifts for the X-ray
counterparts with fewer outliers compared to the 3D-HST catalog, which fit
these sources with galaxy-only templates
The Pan-STARRS1 Medium-deep Survey: Star Formation Quenching in Group and Cluster Environments
We make use of a catalog of 1600 Pan-STARRS1 groups produced by the probability friends-of-friends algorithm to explore how the galaxy properties, i.e., the specific star formation rate (SSFR) and quiescent fraction, depend on stellar mass and group-centric radius. The work is the extension of Lin et al. In this work, powered by a stacking technique plus a background subtraction for contamination removal, a finer correction and more precise results are obtained than in our previous work. We find that while the quiescent fraction increases with decreasing group-centric radius, the median SSFRs of star-forming galaxies in groups at fixed stellar mass drop slightly from the field toward the group center. This suggests that the main quenching process in groups is likely a fast mechanism. On the other hand, a reduction in SSFRs by ~0.2 dex is seen inside clusters as opposed to the field galaxies. If the reduction is attributed to the slow quenching effect, the slow quenching process acts dominantly in clusters. In addition, we also examine the density–color relation, where the density is defined by using a sixth-nearest-neighbor approach. Comparing the quiescent fractions contributed from the density and radial effect, we find that the density effect dominates the massive group or cluster galaxies, and the radial effect becomes more effective in less massive galaxies. The results support mergers and/or starvation as the main quenching mechanisms in the group environment, while harassment and/or starvation dominate in clusters
Average Metallicity and Star Formation Rate of Lya Emitters Probed by a Triple Narrow-Band Survey
We present the average metallicity and star-formation rate of Lya emitters
(LAEs) measured from our large-area survey with three narrow-band (NB) filters
covering the Lya, [OII]3727, and Ha+[NII] lines of LAEs at z=2.2. We select 919
z=2.2 LAEs from Subaru/Suprime-Cam NB data in conjunction with Magellan/IMACS
spectroscopy. Of these LAEs, 561 and 105 are observed with KPNO/NEWFIRM
near-infrared NB filters whose central wavelengths are matched to redshifted
[OII] and Ha nebular lines, respectively. By stacking the near-infrared images
of the LAEs, we successfully obtain average nebular-line fluxes of LAEs, the
majority of which are too faint to be identified individually by narrow-band
imaging or deep spectroscopy. The stacked object has an Ha luminosity of
1.7x10^{42} erg s^{-1} corresponding to a star formation rate (SFR) of 14
M_{sun} yr^{-1}. We place, for the first time, a firm lower limit to the
average metallicity of LAEs of Z>~0.09 Z_{sun} (2sigma) based on the
[OII]/(Ha+[NII]) index together with photo-ionization models and empirical
relations. This lower limit of metallicity rules out the hypothesis that LAEs,
so far observed at z~2, are extremely metal poor (Z<2x10^{-2} Z_{sun}) young
galaxies at the 4sigma level. This limit is higher than a simple extrapolation
of the observed mass-metallicity relation of z~2 UV-selected galaxies toward
lower masses (5x10^{8} M_{sun}), but roughly consistent with a recently
proposed fundamental mass-metallicity relation when the LAEs' relatively low
SFR is taken into account. The Ha and Lya luminosities of our NB-selected LAEs
indicate that the escape fraction of Lya photons is ~12-30 %, much higher than
the values derived for other galaxy populations at z~2.Comment: 21 pages, 15 figures, 8 tables. Accepted for publication in Ap