Spitzer Mid-Infrared Spectroscopy of Infrared Luminous Galaxies at z ~ 2. I. The Spectra

April 2007 • 2007ApJ...658..778Y

Authors • Yan, Lin • Sajina, Anna • Fadda, Dario • Choi, Phil • Armus, Lee • Helou, George • Teplitz, Harry • Frayer, David • Surace, Jason

Abstract • We present the mid-infrared spectra obtained with the Spitzer IRS for 52 sources, selected as infrared luminous, z>~1 candidates in the Extragalactic First Look Survey. The sample selection criteria are f_24μm>~0.9 mJy, νf_ν(24 μm)/νf_ν(8 μm)>~3.16, and νf_ν(24 μm)/νf_ν(0.7 μm)>~10. Of the 52 spectra, 47 (90%) produced measurable redshifts based solely on the mid-IR spectral features, with 35/47 (74%) at 1.5<~z<~3.2. Keck spectroscopy of a subsample (17/47) agrees with the mid-IR redshift measurements. The observed spectra fall into three categories: (1) 33% (17/52) have strong PAH emission and are probably powered by star formation with total IR luminosity roughly a factor of 5 higher than the local starburst ULIRGs; (2) 33% (17/52) have only deep silicate absorption at 9.8 μm, indicatiing deeply embedded dusty systems-these data alone cannot determine the energetic nature of the heating sources in these systems; and (3) the remaining 34% are mid-IR continuum-dominated systems with weak PAH emission and/or silicate absorption. This subsample is probably AGNs. We derived monochromatic, rest-frame 5.8 μm, continuum luminosities (νL_ν), ranging from 10^10.3 to 10^12.6 L_solar. Our spectra have mid-IR slope α_5-15μm>~2.1, much redder than the median value of 1.3 for the optically selected PG quasars. From the silicate absorption feature, we estimate that roughly two-thirds of the sample have optical depth τ_9.8μm>1. Their L_1600Å and L_IR suggest that our sample is among the most luminous and most dust-enshrouded systems of its epoch. Our study has revealed a significant population of dust-enshrouded galaxies at z~2, whose enormous energy output, comparable to that of quasars, is generated by AGNs, as well as starbursts. This IR-luminous population has very little overlap with submillimeter and UV-selected populations.

Links

• PREPRINT http://arxiv.org/abs/astro-ph/0612297
• NED https://ned.ipac.caltech.edu/uri/NED::InRefcode/2007ApJ...658..778Y
• ELECTR https://doi.org/10.1086%2F511516
• SIMBAD http://simbad.u-strasbg.fr/simbo.pl?bibcode=2007ApJ...658..778Y
• PDF https://stacks.iop.org/0004-637X/658/778/pdf
• DATA http://cdsarc.cds.unistra.fr/viz-bin/cat/J/ApJ/658/778
• DATA https://irsa.ipac.caltech.edu/bibdata/2007/Y/2007ApJ...658..778Y.html
• SPIRES http://inspirehep.net/search?p=find+eprint+astro-ph/0612297