arXiv subjects

J. Silverman

Publications and source records attributed to J. Silverman.

78 records · Page 5Linked to original sources

arXiv2006

zCOSMOS: A Large VLT/VIMOS redshift survey covering 0 < z < 3 in the COSMOS field

zCOSMOS is a large redshift survey that is being undertaken in the COSMOS field using 600 hours of observation with the VIMOS spectrograph on the 8-m VLT. The survey is designed to characterise the environments of COSMOS galaxies from the 100 kpc scales of galaxy groups up to the 100 Mpc scale of the cosmic web and to produce diagnostic information on galaxies and active galactic nuclei. The zCOSMOS survey consists of two parts: (a) zCOSMOS-bright, a magnitude-limited I-band IAB < 22.5 sample of about 20,000 galaxies with 0.1 < z < 1.2 covering the whole 1.7 deg2 COSMOS ACS field and designed to mimic the parameters of the 2dfGRS; and (b) zCOSMOS-deep, a survey of approximately 10,000 galaxies selected through colour-selection criteria to have 1.4 < z < 3.0, within the central 1 deg2. This paper describes the survey design and the construction of the target catalogues, and briefly outlines the observational program and the data pipeline. In the first observing season, spectra of 1303 zCOSMOS-bright targets and of 977 zCOSMOS-deep targets have been obtained. These are briefly analysed to demonstrate the characteristics that may be expected from zCOSMOS, and particularly zCOSMOS-bright, when it is finally completed between 2008-2009. The power of combining spectroscopic and photometric redshifts is demonstrated, especially in correctly identifying the emission line in single-line spectra and in determining which of the less reliable spectroscopic redshifts are correct and which are incorrect. Our zCOSMOS-deep spectra demonstrate the effectiveness of our selection techniques to isolate high redshift galaxies at 1.4 < z < 3.0 and of VIMOS to measure their redshifts using ultraviolet absorption lines.

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arXiv2006

The Zurich Extragalactic Bayesian Redshift Analyzer (ZEBRA) and its first application: COSMOS

We present ZEBRA, the Zurich Extragalactic Bayesian Redshift Analyzer. The current version of ZEBRA combines and extends several of the classical approaches to produce accurate photometric redshifts down to faint magnitudes. In particular, ZEBRA uses the template-fitting approach to produce Maximum Likelihood and Bayesian redshift estimates based on: (1.) An automatic iterative technique to correct the original set of galaxy templates to best represent the SEDs of real galaxies at different redshifts; (2.) A training set of spectroscopic redshifts for a small fraction of the photometric sample; and (3.) An iterative technique for Bayesian redshift estimates, which extracts the full two-dimensional redshift and template probability function for each galaxy. We demonstrate the performance of ZEBRA by applying it to a sample of 866 I_AB <= 22.5 COSMOS galaxies with available u*, B, V, g', r', i', z' and K_s photometry and zCOSMOS spectroscopic redshifts in the range 0 < z < 1.3. Adopting a 5-sigma-clipping that excludes less than 10 galaxies, both the Maximum Likelihood and Bayesian estimates for this sample have an accuracy dz/1+z better than 0.03. Similar accuracies are recovered using mock galaxies.

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arXiv2006

ChaMP Serendipitous Galaxy Cluster Survey

We present a survey of serendipitous extended X-ray sources and optical cluster candidates from the Chandra Multi-wavelength Project (ChaMP). Our main goal is to make an unbiased comparison of X-ray and optical cluster detection methods. In 130 archival Chandra pointings covering 13 square degrees, we use a wavelet decomposition technique to detect 55 extended sources, of which 6 are nearby single galaxies. Our X-ray cluster catalog reaches a typical flux limit of about ~10^-14 erg s^-1 cm^-2, with a median cluster core radius of 21 arcsec. For 56 of the 130 X-ray fields, we use the ChaMP's deep NOAO/4m MOSAIC g, r, and i imaging to independently detect cluster candidates using a Voronoi tessellation and percolation (VTP) method. Red-sequence filtering decreases the galaxy fore/background contamination and provides photometric redshifts to z~0.7. From the overlapping 6.1 square degree X-ray/optical imaging, we find 115 optical clusters (of which 11% are in the X-ray catalog) and 28 X-ray clusters (of which 46% are in the optical VTP catalog). The median redshift of the 13 X-ray/optical clusters is 0.41, and their median X-ray luminosity (0.5-2 keV) is L_X=(2.65\pm0.19)x10^43 ergs s^-1. The clusters in our sample that are only detected in our optical data are poorer on average (~4sigma) than the X-ray/optically matched clusters, which may partially explain the difference in the detection fractions.

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arXiv2005

Extending the X-ray luminosity function of AGN to high redshift

X-ray surveys of the extragalactic universe are now able to detect significant numbers of AGN out to high redshift (z~5). We highlight some results from the Chandra Multiwavelength Project (ChaMP) to measure the X-ray luminosity function out to these early epochs. At z > 3, we show that the comoving space density of luminous (log Lx > 44.5) AGN has a behavior similar to the optical QSO luminosity function. With a newly compiled sample of AGN from ChaMP supplemented with those from additional surveys including the Chandra Deep fields, we present a preliminary measure of the luminosity function in the hard (2-8 keV) band. With 37 AGN at z > 3, we continue to see a decline in the space density at high redshift over a wider range in luminosity. We discuss the need to identify a larger sample of obscured AGN at high redshift to determine if an early epoch of hidden supermassive black hole growth occurred.

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arXiv2004

Hard X-ray emitting Active Galactic Nuclei selected by the Chandra Multi-wavelength Project

We present X-ray and optical analysis of 188 AGN identified from 497 hard X-ray (f (2.0-8.0 keV) > 2.7x10^-15 erg cm^-2 s^-1) sources in 20 Chandra fields (1.5 deg^2) forming part of the Chandra Multi-wavelength Project. These medium depth X-ray observations enable us to detect a representative subset of those sources responsible for the bulk of the 2-8 keV Cosmic X-ray Background. Brighter than our optical spectroscopic limit, we achieve a reasonable degree of completeness (77% of X-ray sources with counter-parts r'< 22.5 have been classified): broad emission line AGN (62%), narrow emission line galaxies (24%), absorption line galaxies (7%), stars (5%) or clusters (2%). We find that most X-ray unabsorbed AGN (NH<10^22 cm^-2) have optical properties characterized by broad emission lines and blue colors, similiar to optically-selected quasars from the Sloan Digital Sky Survey but with a slighly broader color distribution. However, we also find a significant population of redder (g'-i'>1.0) AGN with broad optical emission lines. Most of the X-ray absorbed AGN (10^22 10^22 cm^-2) and broad emission lines. Overall, we find that 81% of X-ray selected AGN can be easily interpreted in the context of current AGN unification models. Most of the deviations seem to be due to an optical contribution from the host galaxies of the low luminosity AGN.

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arXiv2004

Discovery of a Jet-Like Structure at the High Redshift QSO CXOMP J084128.3+131107

The Chandra Multiwavelength Project (ChaMP) has discovered a jet-like structure associated with a newly recognized QSO at redshift z=1.866. The system was 9.4 arcmin off-axis during an observation of 3C 207. Although significantly distorted by the mirror PSF, we use both a raytrace and a nearby bright point source to show that the X-ray image must arise from some combination of point and extended sources, or else from a minimum of three distinct point sources. We favor the former situation, as three unrelated sources would have a small probability of occurring by chance in such a close alignment. We show that interpretation as a jet emitting X-rays via inverse Compton (IC) scattering on the cosmic microwave background (CMB) is plausible. This would be a surprising and unique discovery of a radio-quiet QSO with an X-ray jet, since we have obtained upper limits of 100 microJy on the QSO emission at 8.46 GHz, and limits of 200 microJy for emission from the putative jet.

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