arXiv subjects

James W. Colbert

Publications and source records attributed to James W. Colbert.

22 records · Page 2Linked to original sources

arXiv2005

The Bright Ages Survey. I. Imaging Data

This is the first paper in a series presenting and analyzing data for a K-selected sample of galaxies collected in order to identify and study galaxies at moderate to high redshift in rest-wavelength optical light. The sample contains 842 objects over 6 separate fields covering 75.6 arcmin^2 down to K=20-20.5. We combine the K-band with UBVRIzJH multi-band imaging, reaching depths of R~26. Two of the fields studied also have deep HST WFPC2 imaging, totaling more than 60 hours in the F300W, F450W, F606W, and F814W filters. Using artificial galaxy modeling and extraction we measure 85% completeness limits down to K=19.5-20, depending on the field examined. The derived K-band number counts are in good agreement with previous studies. We find a density for Extremely Red Objects(EROs; R-K>5) of 1.55+/-0.16 arcmin^{-2} for K<19.7, dominated by the 1714+5015 field (centered on 53w002), with an ERO number density more than 3 times that of the other sample fields. If we exclude the counts for 1714+5015, our density is 0.95+/- 0.14 arcmin. Both ERO densities are consistent with previous measurements due to the significant known cosmic variance of these red sources.Keck spectroscopic redshifts were obtained for 18 of the EROs, all but one of which are emission galaxies. None of the EROs in the 1714+5015 field for which we obtained spectroscopic redshifts are associated with the known z=2.39 over-density, although there are three different galaxy redshift pairs (z=0.90, z=1.03, z=1.22).

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arXiv2004

Near-Infrared Properties of Faint X-rays Sources from NICMOS Imaging in the Chandra Deep Fields

We measure the near-infrared properties of 42 X-ray detected sources from the Chandra Deep Fields North and South, the majority of which lie within the NICMOS Hubble Deep Field North and Ultra Deep Field. We detect all 42 Chandra sources with NICMOS, with 95% brighter than H = 24.5. We find that X-ray sources are most often in the brightest and most massive galaxies. Neither the X-ray fluxes nor hardness ratios of the sample show any correlation with near-infrared flux, color or morphology. This lack of correlation indicates there is little connection between the two emission mechanisms and is consistent with the near-infrared emission being dominated by starlight rather than a Seyfert non-stellar continuum. Near-infrared X-ray sources make up roughly half of all extremely red (J-H > 1.4) objects brighter than H > 24.5. These red X-ray sources have a range of hardness ratios similar to the rest of the sample, decreasing the likelihood of dust-obscured AGN activity as the sole explanation for their red color. Using a combination of spectroscopic and photometric redshifts, we find the red J-H objects are at high redshifts (z > 1.5), which we propose as the primary explanation for their extreme J-H color. Measurement of rest-wavelength absolute B magnitudes shows that X-ray sources are the brightest optical objects at all redshifts, which explains their dominance of the bright end of the red J-H population.

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arXiv2001

NICMOS Snapshot Survey of Damped Lyman Alpha Quasars

We image 19 quasars with 22 damped Lyman alpha (DLA) systems using the F160W filter and the Near-Infrared Camera and Multiobject Spectrograph aboard the Hubble Space Telescope, in both direct and coronagraphic modes. We reach 5 sigma detection limits of ~H=22 in the majority of our images. We compare our observations to the observed Lyman-break population of high-redshift galaxies, as well as Bruzual & Charlot evolutionary models of present-day galaxies redshifted to the distances of the absorption systems. We predict H magnitudes for our DLAs, assuming they are producing stars like an L* Lyman-break galaxy (LBG) at their redshift. Comparing these predictions to our sensitivity, we find that we should be able to detect a galaxy around 0.5-1.0 L* (LBG) for most of our observations. We find only one new possible candidate, that near LBQS0010-0012. This scarcity of candidates leads us to the conclusion that most DLA systems are not drawn from a normal LBG luminosity function nor a local galaxy luminosity function placed at these high redshifts.

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arXiv2000

The Optical and Near-Infrared Morphologies of Isolated Early Type Galaxies

In order to study early type galaxies in their simplest environments, we have constructed a well-defined sample of 30 isolated galaxies. The sample contains all RC3 early-type galaxies with no other cataloged galaxy with known redshift lying within a projected radius of 1 (h_100)^{-1} Mpc and =/- 1000 km/s (where we use the recessional velocities in the RC3). We have obtained optical and near-infrared images of 23 of the galaxies and of a comparison sample of 13 early-type galaxies in X-ray detected poor groups of galaxies. We have applied the techniques of unsharp masking, galaxy model division, and color maps to search for morphological features that might provide clues to the evolution of these galaxies. Evidence for dust features is found in approximately 75% of both the isolated and group galaxies (17 of 22 and 9 of 12, respectively). However, shells or tidal features are much more prevalent in our isolated sample than in our group sample (9 of 22 or 41% versus 1 of 12 or 8%, respectively). The isolation and colors of these shell galaxies make it unlikely that tidal interactions or asymmetric star formation are the causes of such features. One model that is not ruled out is that mergers produce the shells. If shells and dust are both merger signatures, the absence of shells in group ellipticals implies that shells: 1) form more easily, 2) are younger, and/or 3) are longer-lived in isolated environments.

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