Astronomy and Astrophysics – Astronomy
Scientific paper
Mar 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008aspc..381..343y&link_type=abstract
The Second Annual Spitzer Science Center Conference: Infrared Diagnostics of Galaxy Evolution. ASP Conference Series, Vol. 381,
Astronomy and Astrophysics
Astronomy
Scientific paper
We present the mid-IR spectra taken with the Spitzer InfraRed Spectrograph (IRS) for a sample of 52 infrared luminous, high redshift galaxies selected in the Extragalactic First Look Survey (XFLS). The target sample is selected to have 24μm flux densities brighter than 0.9mJy, and very red 24μm-to-R and 24-to-8μm colors. The color criteria were designed to pick out high redshift sources with either PAH emission and/or steep mid-IR continua. Based only on the mid-IR PAH emission and silicate absorption features, we are able to derive redshifts for 48 (48/52=92%) of the sources. The majority of these redshifts (36/48=75%) are beyond z=1, in the range of 1.2 ≲ z ≲ 3.2. The Keck optical and near-IR spectroscopy of a subset has confirmed the mid-IR redshift identifications. The 52 spectra generally fall into three broad groups: (1) strong PAH emission, (2) heavily absorbed systems, and (3) mid-IR continuum dominated with weak PAH and/or silicate absorption features. Of the full sample, 35% have spectra with strong PAH emission (18/52, with 11 at z ≳ 1.6), 31% (16/52, at 1.77 ≲ z ≲ 3.2) are heavily absorbed systems with strong silicate absorption at 9.8μm and also broad emission peak around 7.7μm (likely distorted PAH emission). The remainder 34% of the sample are mid-IR continuum dominated systems, where 14 spectra have weak PAH emission and/or silicate absorption (thus yielding redshifts), and 4 sources are without any identifiable features. The detections of deep silicate absorption galaxies imply that our sample has a significant population (at least 31%) of extremely luminous and heavily absorbed systems at z˜2. Combining the spectra and broad band photometries at 8μm and 70μm, we fit the spectral continua and derived monochromatic, rest-frame 12μm, continuum luminosities (ν L_ν), ranging from 10^{11} - 10^{12.5} L_&sun;. The monochromatic ν L_ν(12μm) is crudely 1/10th of L_{ir}. Although our sample has a limited dynamic range in luminosity, we detect a weak trend that at the same redshift, sources with strong PAH emission tend to have lower luminosities than those of deeply embedded as well as continuum dominated systems.
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