Optical Spectroscopy and X-Ray Detections of a Sample of Quasars and Active Galactic Nuclei Selected in the Mid-Infrared from TwoSpitzer Space TelescopeWide-Area Surveys

Abstract

We present optical spectroscopy of a sample of 77 luminous AGN and quasars selected on the basis of their mid-infrared colors. Our objects are selected from the Spitzer Extragalactic First Look Survey and SWIRE XMM-LSS fields, with a typical 24µm flux density of 5mJy. The median redshift is 0.6, with a range of ∼ 0.05 − 4. Only 33% (25/77) of these objects are normal type-1 quasars, with no obscuration. 44% (34/77) are type-2 objects, with highionization, narrow emission lines, 14% (11/77) are dust-reddened type-1 quasars, showing broad lines but a dust-reddened or unusually weak quasar continuum. 9% (7/77) show no sign of an AGN in the optical spectrum, having either starburst spectra or spectra which could be of either starburst or LINER types. These latter objects are analogous to the X-ray detected population of AGN with weak or non-existent optical AGN emission (the "XBONGs"). 21 of our objects from the SWIRE field fall within moderately-deep XMM exposures. All the unobscured quasars, and about half the obscured quasars are detected in these exposures. This sample, when taken together with other samples of Spitzer selected AGN and quasars, and results from X-ray studies, confirms that obscured AGN dominate the AGN and quasar number counts of all rapidly-accreting supermassive black hole systems, at least for z < ∼ 4. This implies a high radiative efficiency for the black hole accretion process.

The eMERGE Survey – I: Very Large Array 5.5 GHz observations of the GOODS-North Field

“…Lacy et al 2004;Stern et al 2005;Lacy et al 2007;Donley et al 2012). These diagnostics are based on the evidence of a prominent dip in the SED of SFGs between the 1.6-µm stellar bump and the emission from star-formation-heated dust at longer wavelengths.…”

“…This is essential for a sample of sources spanning a wide range of redshifts in order to properly classify high redshift objects. Donley et al (2012) redefined, in a more restricted way, the IRAC colours-based AGN selection criteria previously developed by Lacy et al (2004Lacy et al ( , 2007 obtaining a highly-reliable classification for deep IRAC surveys (referred to as the "Donley wedge" hereafter). Messias et al (2012) developed their own IR colour-colour diagram, using Ks and IRAC bands (the KI diagram) and, finally, Kirkpatrick et al (2012Kirkpatrick et al ( , 2013 presented two different combinations of colours that combine MIR and far-IR (FIR) photometry (Spitzer IRAC/MIPS and Herschel PACS/SPIRE) to classify high redshift (z = 0.5 − 4) galaxies selected at 24 µm with Spitzer IRS spectroscopy.…”

The Spitzer Extragalactic Representative Volume Survey (SERVS): Survey Definition and Goals*

“…In addition, spectra of active galactic nuclei (AGNs) and quasars, now totaling several hundred objects selected by various techniques, have been obtained in the SERVS fields by Lacy et al (2007Lacy et al ( , 2012 and Trichas et al (2010).…”

The impact of dust in host galaxies on quasar luminosity functions

“…There have been many observational studies of AGN luminosity functions in several bands (e.g., Croom et al 2009Croom et al , 2001Ueda et al 2014). Effects of dust attenuation have also been studied for various classes of AGNs, including quasars (e.g., Sanders et al 1988;Gaskell & Benker 2007;Lacy et al 2007). These studies suggest the existence of obscured quasars, which are ⋆ E-mail: shirakata@astro1.sci.hokudai.ac.jp hidden by the surrounding dust and can be observed only in X-ray.…”