Publications (7)30.12 Total impact
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Article: Constraining Halo Occupation Properties of X-Ray Active Galactic Nuclei Using Clustering of Chandra Sources in the Boötes Survey Region
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ABSTRACT: We present one of the most precise measurements to date of the spatial clustering of X-ray-selected active galactic nuclei (AGNs) using a sample derived from the Chandra X-ray Observatory survey in the Boötes field. The real-space two-point correlation function over a redshift interval from z = 0.17 to z ~ 3 is well described by the power law, ξ(r) = (r/r 0)–γ, for comoving separations r 20 h –1 Mpc. We find γ = 1.84 ± 0.12 and r 0 consistent with no redshift trend within the sample (varying between r 0 = 5.5 ± 0.6 h –1 Mpc for z = 0.37 and r 0 = 6.9 ± 1.0 h –1 Mpc for z = 1.28). Furthermore, we are able to measure the projections of the two-point correlation function both on the sky plane and in the line of sight. We use these measurements to show that the Chandra/Boötes AGNs are predominantly located at the centers of dark matter halos with circular velocity v max > 320 km s–1 or M 180 > 4.1 × 1012 h –1 M ☉, and tend to avoid satellite galaxies in halos of this or higher mass. The halo occupation properties inferred from the clustering properties of Chandra/Boötes AGNs—the mass scale of the parent dark matter halos, the lack of significant redshift evolution of the clustering length, and the low satellite fraction—are broadly consistent with the Hopkins et al. scenario of quasar activity triggered by mergers of similarly sized galaxies.The Astrophysical Journal 10/2011; 741(1):15. · 6.02 Impact Factor -
Article: The Mid-IR- and X-ray-Selected QSO Luminosity Function
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ABSTRACT: We present the J-band luminosity function (LF) of 1838 mid-infrared and X-ray-selected active galactic nuclei (AGNs) in the redshift range 0 < z < 5.85. These LFs are constructed by combining the deep multi-wavelength broadband observations from the UV to the mid-IR of the NDWFS Boötes field with the X-ray observations of the XBoötes survey and the spectroscopic observations of the same field by AGES. Our sample is primarily composed of IRAC-selected AGNs, targeted using modifications of the Stern et al. criteria, complemented by MIPS 24 μm and X-ray-selected AGNs to alleviate the biases of IRAC mid-IR selection against z ~ 4.5 quasars and AGNs faint with respect to their hosts. This sample provides an accurate link between low- and high-redshift AGN LFs and does not suffer from the usual incompleteness of optical samples at z ~ 3. We use a set of low-resolution spectral energy distribution templates for AGNs and galaxies presented in a previous paper by Assef et al. to model the selection function of these sources and apply host and reddening corrections. We find that the space density of the brightest quasars strongly decreases from z = 3 to z = 0, while the space density of faint quasars is at least flat, and possibly increasing, over the same redshift range. At z>3, we observe a decrease in the space density of quasars of all brightnesses. We model the LF by a double power law and find that its evolution cannot be described by either pure luminosity or pure density evolution, but must be a combination of both. We used the bright-end slope determined by Croom et al. (2QZ) as a prior to fit the data in order to minimize the effects of our small survey area. The bright-end power-law index of our best-fit model remains consistent with the prior, while the best-fit faint-end index is consistent with the low-redshift measurements based on the 2QZ and 2SLAQ surveys. Our best-fit model generally agrees with the number of bright quasars predicted by other LFs at all redshifts. If we construct the QSO luminosity function using only the IRAC-selected AGNs, we find that the biases inherent to this selection method significantly modify the behavior of the characteristic density *(z) only for z < 1 and have no significant impact upon the characteristic magnitude M *,J (z).The Astrophysical Journal 01/2011; 728(1):56. · 6.02 Impact Factor -
Article: Constraining halo occupation properties of X-ray AGNs using clustering of Chandra sources in the Bootes survey region
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ABSTRACT: We present one of the most precise measurement to date of the spatial clustering of X-ray selected AGNs using a sample derived from the Chandra X-ray Observatory survey in the Bootes field. The real-space two-point correlation function over a redshift interval from z=0.17 to z~3 is well described by the power law, xi(r)=(r/r0)^-gamma, for comoving separations r<~20h^-1 Mpc. We find gamma=1.84+-0.12 and r0 consistent with no redshift trend within the sample (varying between r0=5.5+-0.6 h^-1 Mpc for =0.37 and r0=6.9+-1.0 h^-1 Mpc for =1.28). Further, we are able to measure the projections of the two-point correlation function both on the sky plane and in the line of sight. We use these measurements to show that the Chandra/Bootes AGNs are predominantly located at the centers of dark matter halos with the circular velocity Vmax>320 km/s or M_200 > 4.1e12 h^-1 Msun, and tend to avoid satellite galaxies in halos of this or higher mass. The halo occupation properties inferred from the clustering properties of Chandra/Bootes AGNs --- the mass scale of the parent dark matter halos, the lack of significant redshift evolution of the clustering length, and the low satellite fraction --- are broadly consistent with the Hopkins et al. scenario of quasar activity triggered by mergers of similarly-sized galaxies.10/2010; -
Article: Mid-infrared Variability from the Spitzer Deep Wide-field Survey
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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. · 6.02 Impact Factor -
Article: Low-Resolution Spectral Templates for Active Galactic Nuclei and Galaxies from 0.03 to 30 μm
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ABSTRACT: We present a set of low-resolution empirical spectral energy distribution (SED) templates for active galactic nuclei (AGNs) and galaxies in the wavelength range from 0.03 μm to 30 μm based on the multi-wavelength photometric observations of the NOAO Deep-Wide Field Survey Boötes field and the spectroscopic observations of the AGN and Galaxy Evolution Survey. Our training sample is comprised of 14,448 galaxies in the redshift range 0 z 1 and 5347 likely AGNs in the range 0 z 5.58. The galaxy templates correspond to the SED templates presented in 2008 by Assef et al. extended into the UV and mid-IR by the addition of FUV and NUV GALEX and MIPS 24 μm data for the field. We use our templates to determine photometric redshifts for galaxies and AGNs. While they are relatively accurate for galaxies (σ z /(1 + z) = 0.04, with 5% outlier rejection), their accuracies for AGNs are a strong function of the luminosity ratio between the AGN and galaxy components. Somewhat surprisingly, the relative luminosities of the AGN and its host are well determined even when the photometric redshift is significantly in error. We also use our templates to study the mid-IR AGN selection criteria developed by Stern et al. in 2005 and Lacy et al. in 2004. We find that the Stern et al. criterion suffers from significant incompleteness when there is a strong host galaxy component and at z 4.5, when the broad Hα emission line is redshifted into the [3.6] band, but that it is little contaminated by low- and intermediate-redshift galaxies. The Lacy et al. criterion is not affected by incompleteness at z 4.5 and is somewhat less affected by strong galaxy host components, but is heavily contaminated by low-redshift star-forming galaxies. Finally, we use our templates to predict the color-color distribution of sources in the upcoming Wide-Field Infrared Survey Explorer (WISE) mission and define a color criterion to select AGNs analogous to those developed for IRAC photometry. We estimate that in between 640,000 and 1,700,000 AGNs will be identified by these criteria, but without additional information, WISE-selected quasars will have serious completeness problems for z 3.4.The Astrophysical Journal 03/2010; 713(2):970. · 6.02 Impact Factor -
Article: Low Resolution Spectral Templates For AGNs and Galaxies From 0.03 -- 30 microns
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ABSTRACT: We present a set of low resolution empirical SED templates for AGNs and galaxies in the wavelength range from 0.03 to 30 microns based on the multi-wavelength photometric observations of the NOAO Deep-Wide Field Survey Bootes field and the spectroscopic observations of the AGN and Galaxy Evolution Survey. Our training sample is comprised of 14448 galaxies in the redshift range 0<~z<~1 and 5347 likely AGNs in the range 0<~z<~5.58. We use our templates to determine photometric redshifts for galaxies and AGNs. While they are relatively accurate for galaxies, their accuracies for AGNs are a strong function of the luminosity ratio between the AGN and galaxy components. Somewhat surprisingly, the relative luminosities of the AGN and its host are well determined even when the photometric redshift is significantly in error. We also use our templates to study the mid-IR AGN selection criteria developed by Stern et al.(2005) and Lacy et al.(2004). We find that the Stern et al.(2005) criteria suffers from significant incompleteness when there is a strong host galaxy component and at z =~ 4.5, when the broad Halpha emission line is redshifted into the [3.6] band, but that it is little contaminated by low and intermediate redshift galaxies. The Lacy et al.(2004) criterion is not affected by incompleteness at z =~ 4.5 and is somewhat less affected by strong galaxy host components, but is heavily contaminated by low redshift star forming galaxies. Finally, we use our templates to predict the color-color distribution of sources in the upcoming WISE mission and define a color criterion to select AGNs analogous to those developed for IRAC photometry. We estimate that in between 640,000 and 1,700,000 AGNs will be identified by these criteria, but will have serious completeness problems for z >~ 3.4.09/2009; -
Article: The Spitzer Deep, Wide-field Survey
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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. · 6.02 Impact Factor
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Institutions
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2009–2011
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Princeton University
- Department of Astrophysical Sciences
Princeton, NJ, USA
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