Spitzer- and Herschel-based Spectral Energy Distributions of 24 μm Bright z ~ 0.3-3.0 Starbursts and Obscured Quasars

Abstract

In this paper, we characterize the infrared spectral energy distributions (SEDs) of mid-IR-selected z ∼ 0.3–3.0 and LIR ∼ 10^(11)–10^(13) L_☉ galaxies, and study how their SEDs differ from those of local and high-z analogs. Infrared SEDs depend both on the power source (AGN or star formation) and the dust distribution. Therefore, differences in the SEDs of high-z and local galaxies provide clues as to differences in their physical conditions. Our mid-IR flux-limited sample of 191 sources is unique in size, and spectral coverage, including Spitzer mid-IR spectroscopy. Here, we add Herschel photometry at 250μm, 350μm, and 500μm, which allows us, through fitting an empirical SED model, to obtain accurate total IR luminosities, as well as constrain the relative contributions of AGNs and starbursts to those luminosities. Our sample includes three broad categories of SEDs: ∼23% of the sources are AGNs (i.e., where the AGN contributes >50% of L_(IR)), ∼30% are starbursts where an AGN contributes <20% of LIR, and the mid-IR spectra are starburst-like (i.e., strong polycyclic aromatic hydrocarbon features); and the largest group (∼47%) are composites which show both significant AGN and starburst activity. The AGN-dominated sources divide into ones that show a strong silicate 9.7μm absorption feature, implying highly obscured systems, and ones that do not. The high-τ_(9.7) sources are half of our z > 1.2 AGNs, but show SEDs that are extremely rare among local AGNs. The 30% of the sample that are starbursts, even the z ∼ 2, L_(IR) ∼ 10^(13) L_☉ ones, have lower far-IR to mid-IR continuum ratios than local Ultra Luminous Infrared Galaxies (ULIRGs) or the z ∼ 2 submm galaxies—effectively the SEDs of our z ∼ 2 starburst-dominated ULIRGs are much closer to those of local Luminous Infrared Galaxies than ULIRGs. This is consistent with our earlier finding that, unlike local ULIRGs, our high-z starbursts are typically only in the early stages of a merger. The SEDs of the composite sources are most similar to the local archetypal warm ULIRG, Mrk231, which supports the interpretation of their consisting of both AGN and starburst components. In summary, our results show that there is strong evolution in the SEDs between local and z ∼ 2 IR-luminous galaxies, as well as that there is a wide range of SEDs among high redshift IR-luminous sources. The publicly available SED templates we derive from our sample will be particularly useful for infrared population synthesis models, as well as in the interpretation of other mid-IR high-z galaxies, in particular those detected by the recent all sky Wide-field Infrared Survey Explorer.

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