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Browsing by Author "Windhorst, RA"

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    Active nuclei and star-forming objects at z > 2: Metallicities, winds, and formation histories
    (University of Chicago Press, 2002-06) Keel, WC; Wu, WT; Waddington, I; Windhorst, RA; Pascarelle, SM; University of Alabama Tuscaloosa; Arizona State University; Arizona State University-Tempe
    We present near-infrared observations of the active galactic nuclei (AGNs) and star-forming objects in the field of the radio galaxy 53W002 at z = 2.39. The star-forming objects are of special interest as candidate protogalactic objects. The 1.1-2.2 mum passbands sample the emitted-optical range at this redshift, providing new diagnostics of the structure, metal abundance, and age of the members of this grouping originally selected through Lyalpha emission. The star-forming objects are uniformly very blue in continuum slope, which fits with the strong Lyalpha emission in indicating metal abundances that are less than half solar; some are as blue as the most metal-poor local objects. They fall in a range of luminosity and metallicity that is not populated by local objects, indicating a shorter star-forming history at this early epoch. The best local analogs, such as Mrk 66 and 357, either have several times lower luminosity at comparable [O/H] or significantly higher [O/H] for comparable luminosity. Spectroscopy from the Near-Infrared Camera and Multi-Object Spectrometer yields detections of [O III] emission for two objects and interesting [O III] and Hbeta limits for the rest, augmented by Halpha limits from Infrared Telescope Facility imaging. These data are satisfied by model stellar populations that have been forming stars for the last 2-5 x 10(6) yr before z = 2.39. We do not see evidence for older preexisting stellar populations, either in the broadband colors or as redder halos in which the star-forming regions are imbedded. These results suggest that the compact star-forming objects we see at z = 2.0-2.5 are indeed early stages in the building of galaxies rather than transient star-forming events in larger pre-existing dynamical systems. The results also allow an alternative scheme, in which these are low-mass systems that are blowing winds rather than self-enriching, in which case they should fade rapidly with cosmic epoch. For the three prominent AGNs at z = 2.39, Halpha and [O III] emission were measured. Unlike the fainter star-forming objects, their line ratios (specifically Lyalpha/Halpha) show metallicities just as high as in nearby systems. If the AGNs occur in those systems that started with the highest density and began active star formation before the less massive surrounding objects, they will have higher metallicity ( as we see in their emitted-ultraviolet line ratios). The "ionization cones" seen prominently in Lyalpha also appear in [O III] and Halpha, with a role for continuum reflection in some cases as well. The contrast between the AGNs and fainter star-forming objects can be broadly accommodated in a hierarchical formation picture, although there are still important unknowns as to the fate of the star-forming objects.
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    A deep radio survey of Abell 2125. I. Radio, optical, and near-infrared observations
    (University of Chicago Press, 2005-01) Owen, FN; Keel, WC; Ledlow, MJ; Morrison, GE; Windhorst, RA; National Radio Astronomy Observatory (NRAO); University of Alabama Tuscaloosa; National Optical Astronomy Observatory; Arizona State University; Arizona State University-Tempe
    We present a description of deep radio, optical, and near-IR observations taken with the VLA, the KPNO 2 m telescope, and the KPNO 4 m telescope of the region containing the rich cluster of galaxies Abell 2125. The reduction of each data set is described. A catalog of radio sources apparently not associated with members of A2125 and the associated R-band magnitudes is presented.
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    The infrared counterparts of the optically unidentified Chandra Deep Field-South 1 Ms sources
    (University of Chicago Press, 2003) Yan, HJ; Windhorst, RA; Rottgering, HJA; Cohen, SH; Odewahn, SC; Chapman, SC; Keel, WC; Arizona State University; Arizona State University-Tempe; Leiden University; Leiden University - Excl LUMC; California Institute of Technology; University of Alabama Tuscaloosa
    The Chandra Deep Field South (CDF-S) 1 Ms exposure produced a catalog of 346 X-ray sources, of which 59 were not visible on the Very Large Telescope ( VLT)/FORS1 and the ESO-MPI/WFI deep R-band images to a limit of R-vega=26.1-26.7 mag. Using the first release of the ESO VLT/ISAAC JHK(s) data on the CDF-S, we identified six of the 12 such objects that were within the coverage of these IR observations. The VLT/FORS1 I-band data further confirm that five of these six objects are undetected in the optical. The photometric properties of these six counterparts are compared against those of the optically brighter counterparts of Chandra sources in the same field. We found that the location of these optically brighter Chandra sources in the near-IR color space was bifurcated, with the color of one branch being consistent with that of E/S0 galaxies at 0less than or equal tozless than or equal to1.5, and the other branch being consistent with that of unreddened active galactic nuclei/quasi-stellar objects (AGNs/QSOs) at 0less than or equal tozless than or equal to3.5. The six counterparts that we identified seemed to lie on the E/S0 branch and its extension, suggesting that these X-ray source hosts are mostly luminous E/S0 galaxies (M-V similar to -20 mag in the AB system) at 1less than or equal tozless than or equal to2.5. On the other hand, some of them can also be explained by AGNs/QSOs over a wide redshift range (0less than or equal tozless than or equal to5), if a range of internal extinction (A(V)=0-1 mag) is allowed. However, the later interpretation requires fine-tuning extinction together with redshift for these objects individually. If they are indeed AGNs/QSOs, the most luminous of them is just barely qualified for being a QSO. Finally, we point out that neither high-redshift (z>5) star-forming galaxies nor irregular galaxies at lower redshift can be a viable explanation to the nature of these six counterparts.
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    Infrared Space Observatory observations of the 53W002 group at 6.7 microns: In search of the oldest stellar populations at z=2.4
    (IOP Publishing, 2004) Keel, WC; Wu, WT; van der Werf, PP; Windhorst, RA; Dunlop, JS; Eales, SA; Waddington, I; Holmes, M; University of Alabama Tuscaloosa; Leiden University; Leiden University - Excl LUMC; Arizona State University; Arizona State University-Tempe; University of Edinburgh; Cardiff University; University of Sussex; Vanderbilt University
    We present a deep Infrared Space Observatory (ISO) observation at 6.7 mum of the 53W002 group of galaxies and active galactic nuclei (AGNs) at z = 2.4. This approximately samples the emitted K band. Faint, blue star-forming objects are not detected, as expected from their very blue color across the emitted optical and UV spectrum. However, 53W002 itself is detected at the similar to3 sigma level, with an emitted color that is appropriate V - K for a population that formed starting at z = 3.6-7.0 most likely z = 4.7. This fits with shorter wavelength data, suggesting that the more massive members of this group, which may all host AGNs, began star formation earlier in deeper potential wells than the compact Lyalpha emission objects. Two foreground galaxies are detected, as are several stars. One additional 6.7 mum source closely coincides with an optically faint galaxy, potentially at z p. The overall source counts are consistent ( within the errors of such a small sample) with the other ISO deep 2 - 3 fields at 6.7 mum.
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    A simple prediction of the surface density of galaxies at z similar or equal to 6
    (University of Chicago Press, 2002-12-01) Yan, HJ; Windhorst, RA; Odewahn, SC; Cohen, SH; Rottgering, HJA; Keel, WC; Arizona State University; Arizona State University-Tempe; Leiden University; Leiden University - Excl LUMC; University of Alabama Tuscaloosa
    Systematic surveys are being proposed to discover a significant number of galaxies at z similar or equal to 6, which is now suggested as the epoch at which the reionization era of the universe ends. To plan such surveys, we need a reasonable expectation of the surface density of high-redshift galaxies at different flux limits. Here we present a simple prediction of the surface density of 5.5 less than or equal to z less than or equal to 6.5 galaxies in the optical regime, extrapolating from what is already known about galaxies at z similar or equal to 3. This prediction is consistent with the results of nearly all known searches for objects at z similar or equal to 6, giving confidence that we may use it to plan optimal combination of survey depth and sky coverage in searching for such objects. We suggest that the most efficient strategy with existing ground-based facilities is to do medium-depth (m(AB) similar or equal to 24.0-24.5 mag), wide-field (a couple of square degrees) survey using a wide-field camera at a 4 m class telescope. As the predicted surface density at this brightness level is very sensitive to the value of L-*, the result of such a survey can be easily used to constrain the luminosity evolution from z similar or equal to 3 to 6.