Browsing by Author "Blain, A W"

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    Mid-infrared selection of active galactic nuclei with the wide-field infrared survey explorer. II. properties of wise -selected active galactic nuclei in the NDWFS Boötes field
    (2013) Assef, R J; Stern, D; Kochanek, C S; Blain, A W; Brodwin, M; Brown, M J I; Donoso, E; Eisenhardt, P R M; Jannuzi, B T; Jarrett, T H; Stanford, S A; Tsai, C W; Wu, J; Yan, L
    Stern et al. presented a study of Wide-field Infrared Survey Explorer (WISE) selection of active galactic nuclei (AGNs) in the 2 deg2 COSMOS field, finding that a simple criterion W1-W2 ≥ 0.8 provides a highly reliable and complete AGN sample for W2 < 15.05, where the W1 and W2 passbands are centered at 3.4 μm and 4.6 μm, respectively. Here we extend this study using the larger 9 deg2 NOAO Deep Wide-Field Survey Boötes field which also has considerably deeper WISE observations than the COSMOS field, and find that this simple color cut significantly loses reliability at fainter fluxes. We define a modified selection criterion combining the W1–W2 color and the W2 magnitude to provide highly reliable or highly complete AGN samples for fainter WISE sources. In particular, we define a color-magnitude cut that finds 130 ± 4 deg–2 AGN candidates for W2 < 17.11 with 90% reliability. Using the extensive UV through mid-IR broadband photometry available in this field, we study the spectral energy distributions of WISE AGN candidates. We find that, as expected, the WISE AGN selection can identify highly obscured AGNs, but that it is biased toward objects where the AGN dominates the bolometric luminosity output. We study the distribution of reddening in the AGN sample and discuss a formalism to account for sample incompleteness based on the step-wise maximum-likelihood method of Efstathiou et al. The resulting dust obscuration distributions depend strongly on AGN luminosity, consistent with the trend expected for a receding torus. At L AGN ~ 3 × 1044 erg s–1, 29% ± 7% of AGNs are observed as Type 1, while at ~4 × 1045 erg s–1 the fraction is 64% ± 13%. The distribution of obscuration values suggests that dust in the torus is present as both a diffuse medium and in optically thick clouds.
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    UV-BRIGHT NEARBY EARLY-TYPE GALAXIES OBSERVED IN THE MID-INFRARED: EVIDENCE FOR A MULTI-STAGE FORMATION HISTORY BY WAY OF WISE AND GALEX IMAGING
    (2013) Petty, S M; Neill, J D; Jarrett, T H; Blain, A W; Farrah, D G; Rich, R M; Tsai, C W; Benford, D J; Bridge, C R; Lake, S E; Masci, F J; Wright, E L
    In the local Universe, 10% of massive elliptical galaxies are observed to exhibit a peculiar property: a substantial excess of ultraviolet emission (UVX) over what is expected from their old, red stellar populations. Several origins for the UVX have been proposed, including a population of hot young stars, or a population of old, blue horizontal branch or extended horizontal branch (BHB or EHB) stars that have undergone substantial mass loss from their outer atmospheres. We explore the radial distribution of ultraviolet excess (UVX) in a selection of 49 nearby E/S0-type galaxies by measuring the extended photometry in the UV-midIR with GALEX, SDSS and WISE. We compare UV/optical and UV/mid-IR colors with the Flexible Stellar Population Synthesis with EHB models (Conroy & Gunn 2010). We find that combined WISE mid-IR and GALEX UV colors are more effective in distinguishing models than optical colors, and that the UV/mid-IR combination is sensitive to EHB fraction. There are strong color gradients with the outer radii bluer than the inner half-light radii by ~1 magnitude. This color difference is easily accounted for with a BHB fraction increase of 0.25 with radius. We estimated the average ages for the inner and outer radii are 7.0+/-0.3 Gyr, and 6.2+/-0.2 Gyr, respectively, with the implication that the outer regions are likely to have formed ~1 Gyr after the inner regions. Additionally, we find that metallicity gradients are likely not a significant factor in the color difference. The separation of color between the inner and outer regions, which agrees with a specific stellar population difference (e.g., higher EHB populations), and the ~0.5-2 Gyr age difference suggests multi-stage formation. Our results are best explained by inside-out formation: rapid star formation within the core at early epochs (>4 Gyr ago) and at least one later stage starburst event coinciding with z~1