Mid-infrared Galaxy Morphology from the Spitzer Survey of Stellar Structure in Galaxies (S^4G): The Imprint of the De Vaucouleurs Revised Hubble-Sandage Classification System at 3.6 μm

Spitzer Space Telescope Infrared Array Camera imaging provides an opportunity to study all known morphological types of galaxies in the mid-IR at a depth significantly better than ground-based near-infrared and optical images. The goal of this study is to examine the imprint of the de Vaucouleurs classification volume in the 3.6 μm band, which is the best Spitzer waveband for galactic stellar mass morphology owing to its depth and its reddening-free sensitivity mainly to older stars. For this purpose, we have prepared classification images for 207 galaxies from the Spitzer archive, most of which are formally part of the Spitzer Survey of Stellar Structure in Galaxies (S^4G), a Spitzer post-cryogenic ("warm") mission Exploration Science Legacy Program survey of 2331 galaxies closer than 40 Mpc. For the purposes of morphology, the galaxies are interpreted as if the images are blue light, the historical waveband for classical galaxy classification studies. We find that 3.6 μm classifications are well correlated with blue-light classifications, to the point where the essential features of many galaxies look very similar in the two very different wavelength regimes. Drastic differences are found only for the most dusty galaxies. Consistent with a previous study by Eskridge et al., the main difference between blue-light and mid-IR types is an ≈1 stage interval difference for S0/a to Sbc or Sc galaxies, which tend to appear "earlier" in type at 3.6 μm due to the slightly increased prominence of the bulge, the reduced effects of extinction, and the reduced (but not completely eliminated) effect of the extreme population I stellar component. We present an atlas of all of the 207 galaxies analyzed here and bring attention to special features or galaxy types, such as nuclear rings, pseudobulges, flocculent spiral galaxies, I0 galaxies, double-stage and double-variety galaxies, and outer rings, that are particularly distinctive in the mid-IR

The Composite Nature of Dust-obscured Galaxies (DOGs) at z ∼ 2–3 in the COSMOS Field. II. The AGN Fraction

We present the X-ray properties of 108 Dust-Obscured Galaxies (DOGs; F 24 μm/F R > 1000) in the COSMOS field, all of which are detected in at least three far-infrared bands with the Herschel Observatory. Out of the entire sample, 22 are individually detected in the hard 2–8 keV X-ray band by the Chandra COSMOS Legacy survey, allowing us to classify them as AGN. Six (27%) of them are Compton-thick AGN candidates with column densities N H > 1024 cm−2, while 15 are moderately obscured AGNs with 1022 < N H < 1024 cm−2. Additionally, we estimate AGN contributions to the IR luminosity (8–1000 μm rest-frame) greater than 20% for 19 DOGs based on SED decomposition using Spitzer/MIPS 24 μm and the five Herschel bands (100–500 μm). Only 7 of these are detected in X-rays individually. We performed an X-ray stacking analysis for the 86 undetected DOGs. We find that the AGN fraction in DOGs increases with 24 μm flux and that it is higher than that of the general 24 μm population. However, no significant difference is found when considering only X-ray detections. This strongly motivates the combined use of X-ray and far-IR surveys to successfully probe a wider population of AGNs, particularly for the most obscured ones