Katherine J. Brand

A Large Population of Mid-Infrared-selected, Obscured Active Galaxies in the Boötes Field

We identify a population of 640 obscured and 839 unobscured AGNs at redshifts 0.7 < z 3 using multiwavelength observations of the 9 deg2 NOAO Deep Wide-Field Survey (NDWFS) region in Bootes. We select AGNs on the basis of Spitzer IRAC colors obtained by the IRAC Shallow Survey. Redshifts are obtained from optical spectroscopy or photometric redshift estimators. We classify the IR-selected AGNs as IRAGN 1 (unobscured) and IRAGN 2 (obscured) using a simple criterion based on the observed optical to mid-IR color, with a selection boundary of R − [ 4.5] = 6.1, where R and [4.5] are the Vega magnitudes in the R and IRAC 4.5 μm bands, respectively. We verify this selection using X-ray stacking analyses with data from the Chandra XBootes survey, as well as optical photometry from NDWFS and spectroscopy from MMT/AGES. We show that (1) these sources are indeed AGNs, and (2) the optical/IR color selection separates obscured sources (with average NH ~ 3 × 1022 cm −2 obtained from X-ray hardness ratios, and optical colors and morphologies typical of galaxies) and unobscured sources (with no X-ray absorption, and quasar colors and morphologies), with a reliability of 80%. The observed numbers of IRAGNs are comparable to predictions from previous X-ray, optical, and IR luminosity functions, for the given redshifts and IRAC flux limits. We observe a bimodal distribution in R − [ 4.5] color, suggesting that luminous IR-selected AGNs have either low or significant dust extinction, which may have implications for models of AGN obscuration.

The active galactic nuclei contribution to the mid-infrared emission of luminous infrared galaxies

We determine the contribution of AGN to the mid-IR emission of luminous infrared galaxies (LIRGs) at z > 0.6 by measuring the mid-IR dust continuum slope of 20,039 mid-IR sources. The 24 μm sources are selected from a Spitzer MIPS survey of the NOAO Deep Wide-Field Survey Bootes field and have corresponding 8 μm data from the IRAC Shallow Survey. There is a clear bimodal distribution in the 24 to 8 μm flux ratio. The X-ray-detected sources fall within the peak corresponding to a flat spectrum in νfν, implying that it is populated by AGN-dominated LIRGs, whereas the peak corresponding to a higher 24 to 8 μm flux ratio is likely due to LIRGs whose IR emission is powered by starbursts. The 24 μm emission is increasingly dominated by AGN at higher 24 μm flux densities (f24): the AGN fraction of the z > 0.6 sources increases from 9% at f24 ≈ 0.35 mJy to 74% ± 20% at f24 ≈ 3 mJy, in good agreement with model predictions. Deep 24 μm, small-area surveys, like GOODS, will be strongly dominated by starburst galaxies. AGN are responsible for ~3%-7% of the total 24 μm background.

XBOOTES: AN X-RAY SURVEY OF THE NDWFS BOOTES FIELD. II. THE X-RAY SOURCE CATALOG

We present results from a Chandra survey of the 9 deg 2 Bootes field of the NOAO Deep Wide-Field Survey (NDWFS). This XBootes survey consists of 126 separate contiguous ACIS-I observations each of approximately 5000 s in duration. These unique Chandra observations allow us to search for large-scale structure and to calculate X-raysource statistics overawide,contiguousfieldofviewwitharcsecondangularresolutionanduniformcoverage. Opticalspectroscopicfollow-upobservationsandtherichNDWFSdatasetwillallowustoidentifyandclassifythese X-ray‐selected sources. Using wavelet decomposition, we detect 4642 point sources with n � 2 counts. In order to keep our detections � 99% reliable, we limit our list to sources with n � 4 counts. For a 5000 s observation and assuming a canonical unabsorbed active galactic nucleus (AGN) type X-ray spectrum, a 4 count on-axis source correspondsto afluxof4:7 ;10 � 15 ergscm � 2 s � 1 inthe soft (0.5‐2keV) band, 1:5 ; 10 � 14 ergscm � 2 s � 1 in thehard (2‐7 keV) band, and 7:8 ; 10 � 15 ergs cm � 2 s � 1 in the full (0.5‐7 keV) band. The full 0.5‐7 keV band n � 4 count list has 3293 point sources. In addition to the point sources, 43 extended sources have been detected, consistent with the depth of these observations and the number counts of clusters. We present here the X-ray catalog for the XBootes survey, including source positions, X-ray fluxes, hardness ratios, and their uncertainties. We calculate and present the differential number of sources per flux density interval, N(S), for the point sources. In the soft (0.5‐2 keV) band, N(S) is well fitted by a broken power law with slope of 2:60 þ0:11 � 0:12 at bright fluxes and 1:74 þ0:28 � 0:22 for faint fluxes. The hard source N(S) is well described by a single power law with an index of � 2:93 þ0:09 � 0:09.

Optical Line Diagnostics of z ≈ 2 Optically Faint Ultraluminous Infrared Galaxies in the Spitzer Boötes Survey

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 Bootes 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.

Hubble Space Telescope Morphologies of z ~ 2 Dust Obscured Galaxies. I. Power-Law Sources

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 Bootes 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 Sersic 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 M_20) 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 ~10^9-10^(11) M☉ . If major mergers are the progenitors of DOGs, then these observations suggest that DOGs may represent a postmerger evolutionary stage.

The Mid-Infrared Properties of X-Ray Sources

We combine the results of the Spitzer IRAC Shallow Survey and the Chandra XBootes Survey of the 8.5 deg2 Bootes field of the NOAO Deep Wide-Field Survey to produce the largest comparison of mid-IR and X-ray sources to date. The comparison is limited to sources with X-ray fluxes >8 × 10−15 ergs cm−2 s−1 in the 0.5-7.0 keV range and mid-IR sources with 3.6 μm fluxes brighter than 18.4 mag (12.3 μJy). In this most sensitive IRAC band, 85% of the 3086 X-ray sources have mid-IR counterparts at an 80% confidence level based on a Bayesian matching technique. Only 2.5% of the sample have no IRAC counterpart at all based on visual inspection. Even for a smaller but a significantly deeper Chandra survey in the same field, the IRAC Shallow Survey recovers most of the X-ray sources. A majority (65%) of the Chandra sources detected in all four IRAC bands occupy a well-defined region of IRAC [3.6] − [4.5] versus [5.8] − [8.0] color-color space. These X-ray sources are likely infrared-luminous, unobscured type I AGNs with little mid-infrared flux contributed by the AGN host galaxy. Of the remaining Chandra sources, most are lower luminosity type I and type II AGNs whose mid-IR emission is dominated by the host galaxy, while approximately 5% are either Galactic stars or very local galaxies.

MIPS J142824.0+352619: A hyperluminous starburst galaxy at z = 1.325

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.

MID-INFRARED VARIABILITY FROM THE SPITZER DEEP WIDE-FIELD SURVEY

We use the multi-epoch, mid-infrared Spitzer Deep Wide-Field Survey to investigate the variability of objects in 8.1 deg^2 of the NOAO Deep Wide Field Survey Bootes 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 (σ_(12)) 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 XBootes 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.