2004ApJS..154...60Y

In this paper, we present the initial characterization of extragalactic 24 µm sources in the Spitzer First Look Survey by examining their counterparts at the 8 µm and R bands. The color-color diagram of 24/8 µm versus 24/0.7 µm is populated with 18,734 sources brighter than the 3 σ flux limit of 110 µJy. The data cover a total area of 3.7 deg². The 24/0.7 µm colors of these sources span almost 4 orders of magnitudes, while the 24/8 µm colors are distributed over at least 2 orders of magnitude. In addition to identifying ∼30% of the total sample with infrared-quiescent, mostly low-redshift galaxies, we also found that (1) 23% of the 24 µm sources (∼1200/deg2) with logνf_ν(24µm)/νf_ν(8µm)≥0.3 and logνf_ν(24µm)/νf_ν(0.7µm)≥1.0 are probably infrared luminous starburst galaxies with L_IR≥3x10¹¹L_☉at z≥1. In particular, 13% of the sample (660/deg2) are detected only at 24 µm, with no detectable emission in either the 8 µm band or the R band. With such extremely red IR/visible and mid-IR colors, these sources are good candidates for being ultraluminous infrared galaxies at z≥2. (2) 2% of the sample (85/deg2) have extremely red mid-infrared-to-optical color (logνf_ν(24µm)/νf_ν(0.7µm)≥1.5) and fairly moderate 24/8 µm color (logνf_ν(24µm)/νf_ν(8µm)∼0.5), and they are likely candidates for being dust-reddened active galactic nuclei, like Mrk 231 at z∼0.6-3. (3) We anticipate that some of these sources with extremely red colors may be new types of sources, since they cannot be modeled with any familiar type of spectral energy distribution. We find that close to 38% of the 24 µm sources have optical R fainter than 23.0 Vega magnitudes, and 17% of these have no detectable optical counterparts brighter than the R limit of 25.5 mag. Optical spectroscopy of these extremely faint 24 µm sources is very difficult, and therefore mid-infrared spectroscopy from the Spitzer is critical for understanding their physical nature.