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Paper: Spitzer Mid-Infrared Spectroscopy of Infrared Luminous Galaxies at z ~ 2
Volume: 381, The Second Annual Spitzer Science Center Conference: Infrared Diagnostics of Galaxy Evolution
Page: 343
Authors: Yan, L.
Abstract: 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 24 μm flux densities brighter than 0.9mJy, and very red 24 μm– to–R and 24–to–8 μm 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 9.8 μm and also broad emission peak around 7.7 μm (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 8 μm and 70 μm, we fit the spectral continua and derived monochromatic, rest-frame 12 μm, continuum luminosities (νLν), ranging from 1011 − 1012.5L. The monochromatic νLν(12 μm) is crudely 1/10th of Lir. 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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