Obscured and Unobscured Active Galactic Nuclei in the Spitzer Space Telescope First Look Survey

Selection of active galactic nuclei (AGNs) in the infrared facilitates the discovery of AGNs whose optical emission is extinguished by dust. In this paper, we use the Spitzer Space Telescope First Look Survey (FLS) to assess the fraction of AGNs with mid-infrared (MIR) luminosities that are comparable to quasars and that are missed in optical quasar surveys because of dust obscuration. We begin by using the Sloan Digital Sky Survey (SDSS) database to identify 54 quasars within the 4 deg² extragalactic FLS. These quasars occupy a distinct region in MIR color space by virtue of their strong, red continua. This has allowed us to define an MIR color criterion for selecting AGN candidates. About 2000 FLS objects have colors that are consistent with them being AGNs, but most are much fainter in the MIR than the SDSS quasars, which typically have 8 μm flux densities S_8.0 ∼ 1 mJy. We have investigated the properties of 43 objects with S_8.0 ≥ 1 mJy that satisfy our AGN color selection. This sample should contain both unobscured quasars as well as AGNs that are absent from the SDSS survey because of extinction in the optical. After removing 16 known quasars, three probable normal quasars, and eight spurious or confused objects from the initial sample of 43, we are left with 16 objects that are likely to be obscured quasars or luminous Seyfert 2 galaxies. This suggests that the numbers of obscured and unobscured AGNs are similar in samples selected in the MIR at S_8.0 ∼ 1 mJy.