[show abstract][hide abstract] ABSTRACT: 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 deg-2) with log ≥ 0.3 and log ≥ 1.0 are probably infrared luminous starburst galaxies with LIR ≥ 3 × 1011 L at z ≥ 1. In particular, 13% of the sample (660 deg-2) 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 deg-2) have extremely red mid-infrared-to-optical color (log ≥ 1.5) and fairly moderate 24/8 μm color (log ~ 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.
The Astrophysical Journal Supplement Series 12/2008; 154(1):60. · 16.24 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present the first results from the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) Grism Parallel Survey, a large program obtaining deep, slitless ACS grism spectroscopy of high-latitude HST parallel fields. We report on 11 high Galactic latitude fields here, each with grism integration times of more than 12 ks. We identify 601 compact emission-line galaxies at z ≤ 1.6, reaching emission lines to a flux limit of 5 × 10-18 ergs cm-2 s-1 (3 σ). We determine redshifts by cross-correlation of the target spectra with template spectra, followed by visual inspection. We measure star formation rates from the observed [O II] λ3727, [O III] λ5007, and Hα line fluxes. Follow-up observations with the Keck telescope of one of the survey fields confirms our classification and redshifts with σ(z) 0.02. This is one of the deepest emission-line surveys to date, covering a total area of 121 arcmin2. The rough estimate of the comoving number density of emission-line galaxies in our survey at 0.3 < z < 1.3 is ~4.5 × 10-3 h Mpc-3. We reach deeper into the emission-line luminosity function than either the STIS or NICMOS grism parallel surveys, finding an apparent space density of emission-line galaxies several times higher than those surveys. Because of the ACS high spatial resolution, our survey is very sensitive to faint, compact galaxies with strong emission lines and weak continua. The ACS grism survey provides the comoving star formation density at z ≤ 1.6 at a high level of completeness.
The Astronomical Journal 12/2007; 130(4):1324. · 4.97 Impact Factor