Obscuration by gas and dust in luminous quasars

Abstract

We explore the connection between absorption by neutral gas and extinction by dust in mid-infrared (IR) selected luminous quasars. We use a sample of 33 quasars at redshifts 0.7 < z ≲ 3 in the 9 deg2 Boötes multiwavelength survey field that are selected using Spitzer Space Telescope Infrared Array Camera colors and are well-detected as luminous X-ray sources (with >150 counts) in Chandra observations. We divide the quasars into dust-obscured and unobscured samples based on their optical to mid-IR color, and measure the neutral hydrogen column density NH through fitting of the X-ray spectra. We find that all subsets of quasars have consistent power law photon indices Γ ≈ 1.9 that are uncorrelated with NH. We classify the quasars as gas-absorbed or gas-unabsorbed if NH > 1022 cm-2 or NH < 1022 cm -2, respectively. Of 24 dust-unobscured quasars in the sample, only one shows clear evidence for significant intrinsic NH, while 22 have column densities consistent with NH < 1022 cm -2. In contrast, of the nine dust-obscured quasars, six show evidence for intrinsic gas absorption, and three are consistent with NH < 1022 cm-2. We conclude that dust extinction in IR-selected quasars is strongly correlated with significant gas absorption as determined through X-ray spectral fitting. These results suggest that obscuring gas and dust in quasars are generally co-spatial, and confirm the reliability of simple mid-IR and optical photometric techniques for separating quasars based on obscuration. © 2014. The American Astronomical Society. All rights reserved.