K. Brand

Space Telescope Science Institute, Baltimore, Maryland, United States

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Publications (56)193.63 Total impact

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    ABSTRACT: Optically bright stellar sources (V 1 mJy are identified in a survey with the Multiband Imaging Photometer on Spitzer (MIPS) within 8.2 deg2 of the NOAO Deep Wide-Field Survey region in Boötes (NDWFS) and within 5.5 deg2 for the Spitzer Extra-Galactic First Look Survey (FLS). 128 stars are identified in Bootes and 140 in the FLS, and their photometry is given. Stars are classified using spectra from the Digitized First Byurakan Survey (DFBS) and (K-[24]) colors are determined using K magnitudes from the 2MASS survey to search for 24 mum excesses. In the combined sample of 268 stars, 26 have excesses with (K-[24]) > 0.2. The star with the greatest (K-[24]) excess is a variable star; the remaining 25 stars have 0.2 < (K-[24]) 0.2 criterion for defining an excess, 5 of 69 bright, main sequence FGK field stars in a previous Spitzer survey show excesses.
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    ABSTRACT: We use the multi-epoch, mid-infrared Spitzer Deep Wide-Field Survey to investigate the variability of objects in 8.1 deg2 of the NOAO Deep Wide Field Survey Boötes field. We perform a Difference Image Analysis of the four available epochs between 2004 and 2008, focusing on the deeper 3.6 and 4.5 μm bands. Out of 474, 179 analyzed sources, 1.1% meet our standard variability selection criteria that the two light curves are strongly correlated (r>0.8) and that their joint variance (σ12) exceeds that for all sources with the same magnitude by 2σ. We then examine the mid-IR colors of the variable sources and match them with X-ray sources from the XBoötes survey, radio catalogs, 24 μm selected active galactic nucleus (AGN) candidates, and spectroscopically identified AGNs from the AGN and Galaxy Evolution Survey (AGES). Based on their mid-IR colors, most of the variable sources are AGNs (76%), with smaller contributions from stars (11%), galaxies (6%), and unclassified objects, although most of the stellar, galaxy, and unclassified sources are false positives. For our standard selection criteria, 11%-12% of the mid-IR counterparts to X-ray sources, 24 μm AGN candidates, and spectroscopically identified AGNs show variability. The exact fractions depend on both the search depth and the selection criteria. For example, 12% of the 1131 known z>1 AGNs in the field and 14%-17% of the known AGNs with well-measured fluxes in all four Infrared Array Camera bands meet our standard selection criteria. The mid-IR AGN variability can be well described by a single power-law structure function with an index of γ 0.5 at both 3.6 and 4.5 μm, and an amplitude of S 0 0.1 mag on rest-frame timescales of 2 yr. The variability amplitude is higher for shorter rest-frame wavelengths and lower luminosities.
    The Astrophysical Journal 05/2010; 716(1):530. DOI:10.1088/0004-637X/716/1/530 · 6.28 Impact Factor
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    ABSTRACT: The Spitzer Deep, Wide-Field Survey (SDWFS) is a four-epoch infrared survey of 10 deg2 in the Boötes field of the NOAO Deep Wide-Field Survey using the IRAC instrument on the Spitzer Space Telescope. SDWFS, a Spitzer Cycle 4 Legacy project, occupies a unique position in the area-depth survey space defined by other Spitzer surveys. The four epochs that make up SDWFS permit—for the first time—the selection of infrared-variable and high proper motion objects over a wide field on timescales of years. Because of its large survey volume, SDWFS is sensitive to galaxies out to z ~ 3 with relatively little impact from cosmic variance for all but the richest systems. The SDWFS data sets will thus be especially useful for characterizing galaxy evolution beyond z ~ 1.5. This paper explains the SDWFS observing strategy and data processing, presents the SDWFS mosaics and source catalogs, and discusses some early scientific findings. The publicly released, full-depth catalogs contain 6.78, 5.23, 1.20, and 0.96 × 105 distinct sources detected to the average 5σ, 4''-diameter, aperture-corrected limits of 19.77, 18.83, 16.50, and 15.82 Vega mag at 3.6, 4.5, 5.8, and 8.0 μm, respectively. The SDWFS number counts and color-color distribution are consistent with other, earlier Spitzer surveys. At the 6 minute integration time of the SDWFS IRAC imaging, >50% of isolated Faint Images of the Radio Sky at Twenty cm radio sources and >80% of on-axis XBoötes sources are detected out to 8.0 μm. Finally, we present the four highest proper motion IRAC-selected sources identified from the multi-epoch imaging, two of which are likely field brown dwarfs of mid-T spectral class.
    The Astrophysical Journal 07/2009; 701(1):428. DOI:10.1088/0004-637X/701/1/428 · 6.28 Impact Factor
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    ABSTRACT: Using the Infrared Spectrograph on board the Spitzer Space Telescope, we present low-resolution (64 < λ/δλ < 124), mid-infrared (20-38 μm) spectra of 23 high-redshift ULIRGs detected in the Boötes field of the NOAO Deep Wide-Field Survey. All of the sources were selected to have (1) f ν(24 μm)>0.5 mJy; (2) R – [24]>14 Vega mag; and (3) a prominent rest frame 1.6 μm stellar photospheric feature redshifted into Spitzer's 3-8 μm IRAC bands. Of these, 20 show emission from polycyclic aromatic hydrocarbons (PAHs), usually interpreted as signatures of star formation. The PAH features indicate redshifts in the range 1.5 < z < 3.0, with a mean of z = 1.96 and a dispersion of 0.30. Based on local templates, these sources have extremely large infrared luminosities, comparable to that of submillimeter galaxies. Our results confirm previous indications that the rest-frame 1.6 μm stellar bump can be efficiently used to select highly obscured star-forming galaxies at z 2, and that the fraction of starburst-dominated ULIRGs increases to faint 24 μm flux densities. Using local templates, we find that the observed narrow redshift distribution is due to the fact that the 24 μm detectability of PAH-rich sources peaks sharply at z = 1.9. We can analogously use observed spectral energy distributions to explain the broader redshift distribution of Spitzer-detected ULIRGs that are dominated by an active galactic nucleus (AGN). Finally, we conclude that z 2 sources with a detectable 1.6 μm stellar opacity feature lack sufficient AGN emission to veil the 7.7 μm PAH band.
    The Astrophysical Journal 07/2009; 700(2):1190. DOI:10.1088/0004-637X/700/2/1190 · 6.28 Impact Factor
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    ABSTRACT: Optically bright Galactic stars (V lsim 13 mag) having f nu(24 mum) > 1 mJy are identified in Spitzer mid-infrared surveys within 8.2 deg2 for the Boötes field of the NOAO Deep Wide-Field Survey and within 5.5 deg2 for the First Look Survey (FLS). One hundred and twenty-eight stars are identified in Boötes and 140 in the FLS, and their photometry is given. (K - [24]) colors are determined using K magnitudes from the Two Micron All Sky Survey for all stars in order to search for excess 24 mum luminosity compared to that arising from the stellar photosphere. Of the combined sample of 268 stars, 141 are of spectral types F, G, or K, and 17 of these 141 stars have 24 mum excesses with (K - [24]) > 0.2 mag. Using limits on absolute magnitude derived from proper motions, at least eight of the FGK stars with excesses are main-sequence stars, and estimates derived from the distribution of apparent magnitudes indicate that all 17 are main-sequence stars. These estimates lead to the conclusion that between 9% and 17% of the main-sequence FGK field stars in these samples have 24 mum infrared excesses. This result is statistically similar to the fraction of stars with debris disks found among previous Spitzer targeted observations of much brighter, main-sequence field stars.
    The Astronomical Journal 07/2009; 138:251-261. DOI:10.1088/0004-6256/138/1/251 · 4.05 Impact Factor
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    ABSTRACT: Using the Infrared Spectrograph on board the Spitzer Space Telescope, we present low-resolution (64 < lambda / dlambda < 124), mid-infrared (20-38 micron) spectra of 23 high-redshift ULIRGs detected in the Bootes field of the NOAO Deep Wide-Field Survey. All of the sources were selected to have 1) fnu(24 micron) > 0.5 mJy; 2) R-[24] > 14 Vega mag; and 3) a prominent rest-frame 1.6 micron stellar photospheric feature redshifted into Spitzer's 3-8 micron IRAC bands. Of these, 20 show emission from polycyclic aromatic hydrocarbons (PAHs), usually interpreted as signatures of star formation. The PAH features indicate redshifts in the range 1.5 < z < 3.0, with a mean of =1.96 and a dispersion of 0.30. Based on local templates, these sources have extremely large infrared luminosities, comparable to that of submillimeter galaxies. Our results confirm previous indications that the rest-frame 1.6 micron stellar bump can be efficiently used to select highly obscured starforming galaxies at z~2, and that the fraction of starburst-dominated ULIRGs increases to faint 24 micron flux densities. Using local templates, we find that the observed narrow redshift distribution is due to the fact that the 24 micron detectability of PAH-rich sources peaks sharply at z = 1.9. We can analogously explain the broader redshift distribution of Spitzer-detected AGN-dominated ULIRGs based on the shapes of their SEDs. Finally, we conclude that z~2 sources with a detectable 1.6 micron stellar opacity feature lack sufficient AGN emission to veil the 7.7 micron PAH band. Comment: accepted for publication in ApJ; references corrected in Section 3.2 and Figure 8
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    ABSTRACT: A simple optical to mid-IR color selection, R – [24]>14, i.e., f ν(24 μm)/f ν(R) 1000, identifies highly dust obscured galaxies (DOGs) with typical redshifts of z ~ 2 ± 0.5. Extreme mid-IR luminosities (L IR > 1012-14) suggest that DOGs are powered by a combination of active galactic nuclei (AGNs) and star formation, possibly driven by mergers. In an effort to compare their photometric properties with their rest-frame optical morphologies, we obtained high-spatial resolution (005-01) Keck Adaptive Optics K'-band images of 15 DOGs. The images reveal a wide range of morphologies, including small exponential disks (eight of 15), small ellipticals (four of 15), and unresolved sources (two of 15). One particularly diffuse source could not be classified because of low signal-to-noise ratio. We find a statistically significant correlation between galaxy concentration and mid-IR luminosity, with the most luminous DOGs exhibiting higher concentration and smaller physical size. DOGs with high concentration also tend to have spectral energy distributions (SEDs) suggestive of AGN activity. Thus, central AGN light may be biasing the morphologies of the more luminous DOGs to higher concentration. Conversely, more diffuse DOGs tend to show an SED shape suggestive of star formation. Two of 15 in the sample show multiple resolved components with separations of ~1 kpc, circumstantial evidence for ongoing mergers.
    The Astronomical Journal 04/2009; 137(6):4854. DOI:10.1088/0004-6256/137/6/4854 · 4.05 Impact Factor
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    ABSTRACT: We combine IR, optical, and X-ray data from the overlapping, 9.3 deg2 NOAO Deep Wide-Field Survey, AGN and Galaxy Evolution Survey (AGES), and XBoötes Survey to measure the X-ray evolution of 6146 normal galaxies as a function of absolute optical luminosity, redshift, and spectral type over the largely unexplored redshift range 0.1 z 0.5. Because only the closest or brightest of the galaxies are individually detected in X-rays, we use a stacking analysis to determine the mean properties of the sample. Our results suggest that X-ray emission from spectroscopically late-type galaxies is dominated by star formation, while that from early-type galaxies is dominated by a combination of hot gas and active galactic nucleus (AGN) emission. We find that the mean star formation and supermassive black hole accretion rate densities evolve like ~(1 + z)3±1, in agreement with the trends found for samples of bright, individually detectable starburst galaxies and AGN. Our work also corroborates the results of many previous stacking analyses of faint source populations, with improved statistics.
    The Astrophysical Journal 04/2009; 696(2):2206. DOI:10.1088/0004-637X/696/2/2206 · 6.28 Impact Factor
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    ABSTRACT: We present high-spatial resolution optical and near-infrared imaging obtained using the ACS, WFPC2, and NICMOS cameras aboard the Hubble Space Telescope of 31 24 μm bright z 2 Dust Obscured Galaxies (DOGs) identified in the Boötes Field of the NOAO Deep Wide-Field Survey. Although this subset of DOGs have mid-IR spectral energy distributions dominated by a power-law component suggestive of an AGN, all but one of the galaxies are spatially extended and not dominated by an unresolved component at rest-frame UV or optical wavelengths. The observed V – H and I-H colors of the extended components are 0.2-3 magnitudes redder than normal star-forming galaxies. All but one have axial ratios >0.3, making it unlikely that DOGs are composed of an edge-on star-forming disk. We model the spatially extended component of the surface brightness distributions of the DOGs with a Sérsic profile and find effective radii of 1-6 kpc. This sample of DOGs is smaller than most submillimeter galaxies (SMGs), but larger than quiescent high-redshift galaxies. Nonparametric measures (Gini and M20) of DOG morphologies suggest that these galaxies are more dynamically relaxed than local ULIRGs. We estimate lower limits to the stellar masses of DOGs based on the rest-frame optical photometry and find that these range from ~109-1011 M ☉. If major mergers are the progenitors of DOGs, then these observations suggest that DOGs may represent a postmerger evolutionary stage.
    The Astrophysical Journal 03/2009; 693(1):750. DOI:10.1088/0004-637X/693/1/750 · 6.28 Impact Factor
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    ABSTRACT: Observations with the Spitzer Space Telescope have revealed a population of red sequence galaxies with a significant excess in their 24 μm emission compared to what is expected from an old stellar population. We identify ~900 red galaxies with 0.15 ≤ z ≤ 0.3 from the AGN and Galaxy Evolution Survey (AGES) selected from the NOAO Deep Wide-Field Survey Boötes field. Using Spitzer MIPS, we classify 89 (~10%) with 24 μm infrared excess (f 24 ≥ 0.3 mJy). We determine the prevalence of active galactic nucleus (AGN) and star-formation activity in all the AGES galaxies using optical line diagnostics and mid-IR color-color criteria. Using the IRAC color-color diagram from the Spitzer Shallow Survey, we find that 64% of the 24 μm excess red galaxies are likely to have strong polycyclic aromatic hydrocarbon (PAH) emission features in the 8 μm IRAC band. This fraction is significantly larger than the 5% of red galaxies with f 24< 0.3 mJy that are estimated to have strong PAH emission, suggesting that the infrared emission is largely due to star-formation processes. Only 15% of the 24 μm excess red galaxies have optical line diagnostics characteristic of star formation (64% are classified as AGN and 21% are unclassifiable). The difference between the optical and infrared results suggests that both AGN and star-formation activity are occurring simultaneously in many of the 24 μm excess red galaxies. These results should serve as a warning to studies that exclusively use optical line diagnostics to determine the dominant emission mechanism in the infrared and other bands. We find that ~40% of the 24 μm excess red galaxies are edge-on spiral galaxies with high optical extinctions. The remaining sources are likely to be red galaxies whose 24 μm emission comes from a combination of obscured AGN and star-formation activity.
    The Astrophysical Journal 03/2009; 693(1):340. DOI:10.1088/0004-637X/693/1/340 · 6.28 Impact Factor
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    ABSTRACT: We present new 70 and 160 μm observations of a sample of extremely red (R – [24] 15 mag), mid-infrared bright, high-redshift (1.7 z 2.8) galaxies. All targets detected in the far-infrared exhibit rising spectral energy distributions (SEDs) consistent with dust emission from obscured active galactic nuclei (AGNs) and/or star-forming regions in luminous IR galaxies (LIRGs). We find that the SEDs of the high-redshift sources are more similar to canonical AGN-dominated local ultraluminous IR galaxies (ULIRGs) with significant warm dust components than to typical local star-forming ULIRGs. The inferred IR (8-1000 μm) bolometric luminosities are found to be L bol ~ 4 × 1012 L ☉ to ~3 × 1013 L ☉ (ULIRGs/hyper-luminous IR galaxies (HyLIRGs)), representing the first robust constraints on L bol for this class of object.
    The Astrophysical Journal 02/2009; 691(2):1846. DOI:10.1088/0004-637X/691/2/1846 · 6.28 Impact Factor
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    ABSTRACT: The shape and evolution of the quasar luminosity function (QLF) is important in constraining the details of galaxy evolution models and quasar ignition and evolution. Accurate determination of the bolometric QLF, requires samples of the quasar population in as many different wavelength bins as possible since selection at any one wavelength is biased. We measure the infrared QLF for sources with redshift range 1 < z < 5 selected from 9 sq. deg. of the NOAO Deep Wide Field Survey (NDWFS) Bootes field. Our sample of 712 sources is selected to have a Spitzer Space Telescope 24 micron flux limit of 0.5 mJy and an optical R-band magniturd < 17.0 and doubles the size of earlier work in this field. 396 of these sources have spectral classifications and redshifts from the AGN and Galaxy Evolution Survey (AGES). We compare results to other measures of the QLF with different selection criteria.
    American Astronomical Society, AAS Meeting #213; 01/2009
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    ABSTRACT: We have traced the past 7 Gyr of red galaxy stellar mass growth within dark matter halos. We have determined the halo occupation distribution, which describes how galaxies reside within dark matter halos, using the observed luminosity function and clustering of 40,696 0.2 < z < 1.0 red galaxies in Boötes. Half of 1011.9 h−1 M☉ halos host a red central galaxy, and this fraction increases with increasing halo mass. We do not observe any evolution of the relationship between red galaxy stellar mass and host halo mass, although we expect both galaxy stellar masses and halo masses to evolve over cosmic time. We find that the stellar mass contained within the red population has doubled since z = 1, with the stellar mass within red satellite galaxies tripling over this redshift range. In cluster mass halos (>1014 h−1 M☉) most of the stellar mass resides within satellite galaxies and the intracluster light, with a minority of the stellar mass residing within central galaxies. The stellar masses of the most luminous red central galaxies are proportional to halo mass to the power of 0.35. We thus conclude that halo mergers do not always lead to rapid growth of central galaxies. While very massive halos often double in mass over the past 7 Gyr, the stellar masses of their central galaxies typically grow by only 30%.
    The Astrophysical Journal 12/2008; 682(2):937. DOI:10.1086/589538 · 6.28 Impact Factor
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    ABSTRACT: We present near-infrared spectroscopic observations for a sample of 10 optically faint luminous infrared galaxies (R - [24] ≥ 14) using Keck NIRSPEC and Gemini NIRI. The sample is selected from a 24 μm Spitzer MIPS imaging survey of the NDWFS Boötes field. We measure accurate redshifts in the range 1.3 z 3.4. Based on either emission-line widths or line diagnostics, we find that all 10 galaxies harbor luminous active galactic nuclei (AGNs). Seven sources are type I AGNs, exhibiting broad (>1900 km s-1) Hα or Hβ emission lines; the remaining three are type II AGNs. Given their large mid-IR luminosities and faint optical magnitudes, we might expect these sources to be heavily extincted quasars, and therefore only visible as type II AGNs. The visibility of broad lines in 70% of the sources suggests that it is unlikely that these AGNs are being viewed through the midplane of a dusty torus. For four of the sources we constrain the Hα/Hβ Balmer decrement and estimate the extinction to the emission-line region to be large for both type I and type II AGNs, with AHα 2.4-5 mag. Since the narrow-line region is also extincted and the UV continuum emission from the host galaxies is extremely faint, this suggests that much of the obscuration is contributed by dust on large (~kiloparsec) scales within the host galaxies. These sources may be examples of "host-obscured" AGNs, which could have space densities comparable to or greater than that of optically luminous type I AGNs with similar bolometric luminosities.
    The Astrophysical Journal 12/2008; 663(1):204. DOI:10.1086/518119 · 6.28 Impact Factor
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    ABSTRACT: We present the redshift distribution of a complete, unbiased sample of 24 μm sources down to fν(24 μ m) = 300 μ Jy (5 σ). The sample consists of 591 sources detected in the Boötes field of the NOAO Deep Wide-Field Survey. We have obtained optical spectroscopic redshifts for 421 sources (71%). These have a redshift distribution peaking at z ~ 0.3, with a possible additional peak at z ~ 0.9, and objects detected out to z = 4.5. The spectra of the remaining 170 (29%) exhibit no strong emission lines from which to determine a redshift. We develop an algorithm to estimate the redshift distribution of these sources, based on the assumption that they have emission lines but that these lines are not observable due to the limited wavelength coverage of our spectroscopic observations. The redshift distribution derived from all 591 sources exhibits an additional peak of extremely luminous (L8–1000 μ m > 3 × 1012 L☉) objects at z ~ 2, consisting primarily of sources without observable emission lines. We use optical line diagnostics and IRAC colors to estimate that 55% of the sources within this peak are AGN-dominated. We compare our results to published models of the evolution of infrared-luminous galaxies. The models which best reproduce our observations predict a large population of star-formation-dominated ULIRGs at z > 1.5 rather than the AGN-dominated sources we observe.
    The Astrophysical Journal 12/2008; 679(2):1204. DOI:10.1086/587637 · 6.28 Impact Factor
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    ABSTRACT: The XBoötes Survey is a 5 ks Chandra survey of the Boötes Field of the NOAO Deep Wide-Field Survey (NDWFS). This survey is unique in that it is the largest (9.3 deg2) contiguous region imaged in X-ray with complementary deep optical and near-infrared (near-IR) observations. We present a catalog of the optical counterparts to the 3213 X-ray point sources detected in the XBoötes survey. Using a Bayesian identification scheme, we successfully identified optical counterparts for 98% of the X-ray point sources. The optical colors suggest that the optically detected galaxies are a combination of z < 1 massive early-type galaxies and bluer star-forming galaxies whose optical AGN emission is faint or obscured, whereas the majority of the optically detected point sources are likely quasars over a large redshift range. Our large-area, X-ray-bright, optically deep survey enables us to select a large subsample of sources (773) with high X-ray-to-optical flux ratios (fX/fo > 10). These objects are likely high-redshift and/or dust-obscured AGNs. These sources have generally harder X-ray spectra than sources with 0.1 < fX/fo < 10. Of the 73 X-ray sources with no optical counterpart in the NDWFS catalog, 47 are truly optically blank down to R ~ 25.5 (the average 50% completeness limit of the NDWFS R-band catalogs). These sources are also likely to be high-redshift and/or dust-obscured AGNs.
    The Astrophysical Journal 12/2008; 641(1):140. DOI:10.1086/500312 · 6.28 Impact Factor
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    ABSTRACT: We investigate the evolution of the hard X-ray luminosity of the red galaxy population using a large sample of 3316 red galaxies selected over a wide range in redshift (0.3 < z < 0.9) from a 1.4 deg2 region in the Boötes field of the NOAO Deep Wide-Field Survey (NDWFS). The red galaxies are early-type, bulge-dominated galaxies and are selected to have the same evolution-corrected, absolute R-band magnitude distribution as a function of redshift to ensure that we are tracing the evolution in the X-ray properties of a comparable optical population. Using a stacking analysis of 5 ks Chandra/ACIS observations within this field to study the X-ray emission from these red galaxies in three redshift bins, we find that the mean X-ray luminosity increases as a function of redshift. The large mean X-ray luminosity and the hardness of the mean X-ray spectrum suggest that the X-ray emission is largely dominated by active galactic nuclei (AGNs) rather than stellar sources. The hardness ratio can be reproduced by either an absorbed (NH ≈ 2 × 1022 cm-2) Γ = 1.7 power-law source, consistent with that of a population of moderately obscured Seyfert-like AGNs, or an unabsorbed Γ = 0.7 source, suggesting a radiatively inefficient accretion flow (e.g., an advection-dominated accretion flow). We also find that the emission from this sample of red galaxies constitutes at least 5% of the hard X-ray background. These results suggest a global decline in the mean AGN activity of normal early-type galaxies from z ~ 1 to the present, which indicates that we are witnessing the tailing off of the accretion activity onto supermassive black holes in early-type galaxies since the quasar epoch.
    The Astrophysical Journal 12/2008; 626(2):723. DOI:10.1086/430124 · 6.28 Impact Factor
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    ABSTRACT: We trace the assembly history of red galaxies since z = 1 by measuring their evolving space density with the B-band luminosity function. Our sample of 39,599 red galaxies, selected from 6.96 deg2 of imaging from the NOAO Deep Wide-Field Survey and the Spitzer IRAC Shallow Survey, is an order of magnitude larger, in size and volume, than comparable samples in the literature. We measure a higher space density of z ~ 0.9 red galaxies than some of the recent literature, in part because we account for the faint yet significant galaxy flux that falls outside of our photometric aperture. The B-band luminosity density of red galaxies, which effectively measures the evolution of ~L* galaxies, increases by only 36% ± 13% from z = 0 to z = 1. If red galaxy stellar populations have faded by 1.24 B-band magnitudes since z = 1, the stellar mass contained within the red galaxy population has roughly doubled over the past 8 Gyr. This is consistent with star-forming galaxies being transformed into L* red galaxies after a decline in their star formation rates. In contrast, the evolution of 4L* red galaxies differs only slightly from a model with negligible z < 1 star formation and no galaxy mergers. If this model approximates the luminosity evolution of red galaxy stellar populations, then 80% of the stellar mass contained within today's 4L* red galaxies was already in place at z = 0.7. While red galaxy mergers have been observed, such mergers do not produce rapid growth of 4L* red galaxy stellar masses between z = 1 and the present day.
    The Astrophysical Journal 12/2008; 654(2):858. DOI:10.1086/509652 · 6.28 Impact Factor
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    ABSTRACT: Using the SHARC-II camera at the Caltech Submillimeter Observatory to obtain 350 μm images of sources detected with the MIPS instrument on Spitzer, we have discovered a remarkable object at z = 1.325 ± 0.002 with an apparent far-infrared luminosity of 3.2(±0.7) × 1013 L☉. Unlike other z > 1 sources of comparable luminosity selected from mid-IR surveys, MIPS J142824.0+352619 lacks any trace of AGN activity, and is likely a luminous analog of galaxies selected locally by IRAS, or at high redshift in the submillimeter. This source appears to be lensed by a foreground elliptical galaxy at z = 1.034, although the amplification is likely modest (10). We argue that the contribution to the observed optical/near-IR emission from the foreground galaxy is small, and hence are able to present the rest-frame UV through radio spectral energy distribution of this galaxy. Due to its unusually high luminosity, MIPS J142824.0+352619 presents a unique chance to study a high-redshift dusty starburst galaxy in great detail.
    The Astrophysical Journal 12/2008; 636(1):134. DOI:10.1086/497983 · 6.28 Impact Factor
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    ABSTRACT: We determine the rest-frame 8 μm luminosity function of type I quasars over the redshift range 1 < z < 5. Our sample consists of 292 24 μm sources brighter than 1 mJy selected from 7.17 deg2 of the Spitzer Space Telescope MIPS survey of the NOAO Deep Wide-Field Survey Bootes field. The AGN and Galaxy Evolution Survey (AGES) has measured redshifts for 270 of the R < 21.7 sources, and we estimate that the contamination of the remaining 22 sources by stars and galaxies is low. We are able to select quasars missed by ultraviolet excess quasar surveys, including reddened type I quasars and 2.2 < z < 3.0 quasars with optical colors similar to main-sequence stars. We find that reddened type I quasars comprise ~20% of the type I quasar population. Nonetheless, the shape, normalization, and evolution of the rest-frame 8 μm luminosity function are comparable to those of quasars selected from optical surveys. The 8 μm luminosity function of type I quasars is well approximated by a power law with index -2.75 ± 0.14. We directly measure the peak of the quasar space density to be at z = 2.6 ± 0.3.
    The Astrophysical Journal 12/2008; 638(1):88. DOI:10.1086/498843 · 6.28 Impact Factor