Characterization of extragalactic 24 micron sources in the Spitzer First Look Survey

In this paper, we present the initial characterization of extragalactic 24 mum sources in the Spitzer First Look Survey by examining their counterparts at the 8 mum and R bands. The color-color diagram of 24/8 mum versus 24/0.7 mum is populated with 18,734 sources brighter than the 3 sigma flux limit of 110 muJy. The data cover a total area of 3.7 deg(2). The 24/0.7 mum colors of these sources span almost 4 orders of magnitudes, while the 24/8 mum colors are distributed over at least 2 orders of magnitude. In addition to identifying similar to 30% of the total sample with infrared-quiescent, mostly low-redshift galaxies, we also found that ( 1) 23% of the 24 mum sources (similar to1200 deg(-2)) with log [nuf(nu)(24 mum)/nuf(nu)(8 mum) greater than or equal to 0.3 and log [nuf(nu)(24 mum)/nuf(nu)(0.7 mum)] greater than or equal to 1.0 are probably infrared luminous starburst galaxies with L-IR greater than or equal to 3 x 10(11) L-. at z greater than or equal to 1. In particular, 13% of the sample (660 deg(-2)) are detected only at 24 mum, with no detectable emission in either the 8 mum 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 greater than or equal to 2. (2) 2% of the sample (85 deg(-2)) have extremely red mid-infrared-to-optical color (log[nuf(nu)(24 mum)/nuf(nu)(0.7 mum)] greater than or equal to 1.5) and fairly moderate 24/8 mum color (log[nuf(nu)(24 mum)/nuf(nu)(8 mum)] similar to 0.5), and they are likely candidates for being dust-reddened active galactic nuclei, like Mrk 231 at z similar to 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 mum 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 mum sources is very difficult, and therefore mid-infrared spectroscopy from the Spitzer is critical for understanding their physical nature.