Kate Brand

The Evolving Luminosity Function of Red Galaxies

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 todays 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.

A Significant Population of Very Luminous Dust-Obscured Galaxies at Redshift z ~ 2

The Spitzer Space Telescope has revealed a significant population of high-redshift (z ~ 2) dust-obscured galaxies with large mid-infrared to ultraviolet luminosity ratios. Due to their optical faintness, these galaxies have been previously missed in traditional optical studies of the distant universe. We present a simple method for selecting this high-redshift population based solely on the ratio of the observed mid-infrared 24 μm to optical R-band flux density. We apply this method to observations of the ≈8.6 deg^2 NOAO Deep Wide-Field Survey Bootes field, and uncover ≈2600 dust-obscured galaxy candidates [i.e., 0.089 arcmin^−2) with 24 μm flux densities F24 μm ≥ 0.3 mJy and (R − [24]) ≥ 14 (i.e., Fν(24 μm)/Fν(R)≳1000]. These galaxies have no counterparts in the local universe. They represent 7% ± 0.6% of the 24 μm source population at F24 μm ≥ 1 mJy but increase to ≈13% ± 1% of the population at ≈0.3 mJy. These galaxies exhibit evidence of both star formation and AGN activity, with the brighter 24 μm sources being more AGN-dominated. We have measured spectroscopic redshifts for 86 of these galaxies, and find a broad redshift distribution centered at z ≈ 1.99±0.05. The space density of this population is ΣDOG(F24μ m ≥ 0.3 mJy) = (2.82 ± 0.05) × 10^−5^h3 70 Mpc^−3, similar to that of bright submillimeter-selected galaxies at comparable redshifts. These redshifts imply large luminosities, with median νLν(8 μm)≈4 × 10^11 L⊙. nThe infrared luminosity density contributed by this relatively rare dust-obscured galaxy population is log (IRLD) ≈8.23^+0.18 −0.30. This is ≈60^+40 −15% of that contributed by z ~ 2 ultraluminous infrared galaxies (ULIRGs, with LIR > 10^12 L⊙); our simple selection thus identifies a significant fraction of z ~ 2 ULIRGs. This IRLD is ≈26% ± 14% of the total contributed by all z ~ 2 galaxies. We suggest that these dust-obscured galaxies are the progenitors of luminous (~4L*) present-day galaxies, seen undergoing an extremely luminous, short-lived phase of both bulge and black hole growth. They may represent a brief evolutionary phase between submillimeter-selected galaxies and less obscured quasars or galaxies.

Red Galaxy Growth and the Halo Occupation Distribution

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 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%.

Photometric redshifts in the IRAC shallow survey

Accurate photometric redshifts are calculated for nearly 200,000 galaxies to a 4.5 micron flux limit of {approx} 13 {micro}Jy in the 8.5 deg{sup 2} Spitzer/IRAC Shallow survey. Using a hybrid photometric redshift algorithm incorporating both neural-net and template-fitting techniques, calibrated with over 15,000 spectroscopic redshifts, a redshift accuracy of {sigma} = 0.06 (1+z) is achieved for 95% of galaxies at 0 1) galaxy clusters. We present one such spectroscopically confirmed cluster at = 1.24, ISCS J1434.2+3426. Finally, we present a measurement of the 4.5 {micro}m-selected galaxy redshift distribution.

Discovery of a large 200 kpc gaseous nebula at z=2.7 with the Spitzer Space Telescope

We report the discovery of a very large, spatially extended Lyα-emitting nebula at z = 2.656 associated with a luminous mid-infrared source. The bright mid-infrared source (F24 μm = 0.86 mJy) was first detected in observations made using the Spitzer Space Telescope. Existing broadband imaging data from the NOAO Deep Wide-Field Survey revealed the mid-infrared source to be associated with a diffuse, spatially extended, optical counterpart in the BW band. Spectroscopy and further imaging of this target reveals that the optical source is an almost purely line-emitting nebula with little, if any, detectable diffuse continuum emission. The Lyα nebula has a luminosity of LLyα ≈ 1.7 × 1044xa0ergs s-1 and an extent of at least 20 (160xa0kpc). Its central ≈8 shows an ordered, monotonic velocity profile; interpreted as rotation, this region encloses a mass M ≈ 6 × 1012 M☉. Several sources lie within the nebula. The central region of the nebula shows narrow (≈365xa0kmxa0s-1) emission lines of C IV and He II. The mid-infrared source is a compact object lying within the nebula but offset from the center by a projected distance of ≈25 (20 kpc), and likely to be an enshrouded AGN. A young star-forming galaxy lies near the northern end of the nebula. We suggest that the nebula is a site of recent multiple galaxy and AGN formation, with the spatial distribution of galaxies within the nebula perhaps tracking the formation history of the system.

XBootes: An X-Ray Survey of the NDWFS Bootes Field. I. Overview and Initial Results

We obtained a 5 ks deep Chandra X-ray Observatory ACIS-I map of the 9.3 deg2 Bootes field of the NOAO Deep Wide-Field Survey. Here we describe the data acquisition and analysis strategies leading to a catalog of 4642 (3293) point sources with 2 or more (4 or more) counts, corresponding to a limiting flux of roughly 4(8) ? 10-15 ergs cm-2 s-1 in the 0.5-7 keV band. These Chandra XBootes data are unique in that they constitute the widest contiguous X-ray field yet observed to such a faint flux limit. Because of the extraordinarily low background of the ACIS, we expect only 14% (0.7%) of the sources to be spurious. We also detected 43 extended sources in this survey. The distribution of the point sources among the 126 pointings (ACIS-I has a 16 ? 16 field of view) is consistent with Poisson fluctuations about the mean of 36.8 sources per pointing. While a smoothed image of the point source distribution is clumpy, there is no statistically significant evidence of large-scale filamentary structure. We do find however, that for ? > 1, the angular correlation function of these sources is consistent with previous measurements, following a power law in angle with slope ~-0.7. In a 1.4 deg2 sample of the survey, approximately 87% of the sources with 4 or more counts have an optical counterpart to R ~ 26 mag. As part of a larger program of optical spectroscopy of the NDWFS Bootes area, spectra have been obtained for ~900 of the X-ray sources, most of which are quasars or active galactic nuclei.

The 1 < z < 5 Infrared Luminosity Function of Type I Quasars

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 Chandra XBoötes survey. III. Optical and near-infrared counterparts

The XBootes Survey is a 5 ks Chandra survey of the Bootes 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 XBootes 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 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.

STRONG POLYCYCLIC AROMATIC HYDROCARBON EMISSION FROM z ≈ 2 ULIRGs ∗

Using the Infrared Spectrograph on board the Spitzer Space Telescope, we present low-resolution (64 0.5 mJy; (2) R – [24]>14 Vega mag; and (3) a prominent rest frame 1.6 μm stellar photospheric feature redshifted into Spitzers 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 = 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.Using the Infrared Spectrograph on board the Spitzer Space Telescope, we present low-resolution (64 0.5 mJy; 2) R-[24] > 14 Vega mag; and 3) a prominent rest-frame 1.6 micron stellar photospheric feature redshifted into Spitzers 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 =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.

MID-INFRARED SELECTION OF BROWN DWARFS AND HIGH-REDSHIFT QUASARS

We discuss color selection of rare objects in a wide-field multiband survey spanning from the optical to the mid-infrared. Simple color criteria simultaneously identify and distinguish two of the most sought after astrophysical sources: the coolest brown dwarfs and the most distant quasars. We present spectroscopically confirmed examples of each class identified in the IRAC Shallow Survey of the Bootes field of the NOAO Deep Wide-Field Survey. IRAC J142950.8+333011 is a T4.5 brown dwarf at a distance of approximately 30-40 pc, and IRAC J142738.5+331242 is a radio-loud quasar at redshift z = 6.12. Our selection criteria identify a total of four candidates over 8 deg^2 of the Bootes field. The other two candidates are both confirmed 5.5 < z < 6 quasars, previously reported by Cool et al. (2006). We discuss the implications of these discoveries and conclude that there are excellent prospects for extending such searches to cooler brown dwarfs and higher redshift quasars.