Keck spectroscopy of z=1-3 ULIRGs from the Spitzer SWIRE survey

Abstract

(Abridged) High-redshift ultra luminous infrared galaxies contribute the bulk of the cosmic IR background and are the best candidates for very massive galaxies in formation at z>1.5. We present Keck/LRIS optical spectroscopy of 35 z>1.4 luminous IR galaxies in the Spitzer Wide-area Infra-Red Extragalactic survey (SWIRE) northern fields (Lockman Hole, ELAIS-N1, ELAIS-N2). The primary targets belong to the ``IR-peak'' class of galaxies, having the 1.6 micron (restframe) stellar feature detected in the IRAC Spitzer channels.The spectral energy distributions of the main targets are thoroughly analyzed, by means of spectro-photometric synthesis and multi-component fits (stars + starburst dust + AGN torus). The IR-peak selection technique is confirmed to successfully select objects above z=1.4, though some of the observed sources lie at lower redshift than expected. Among the 16 galaxies with spectroscopic redshift, 62% host an AGN component, two thirds being type-1 and one third type-2 objects. The selection, limited to r'<24.5, is likely biased to optically-bright AGNs. The SEDs of non-AGN IR-peakers resemble those of starbursts (SFR=20-500 Msun/yr) hosted in massive (M>1e11 Msun) galaxies. The presence of an AGN component provides a plausible explanation for the spectroscopic/photometric redshift discrepancies, as the torus produces an apparent shift of the peak to longer wavelengths. These sources are analyzed in IRAC and optical-IR color spaces. In addition to the IR-peak galaxies, we present redshifts and spectral properties for 150 objects, out of a total of 301 sources on slits.

Accepted for publications on Astronomy and Astrophysics (acceprance date March 8th, 2007). 33 pages. The quality of some figures have been degraded

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• The Cosmic Infrared Background Resolved by Spitzer. Contributions of Mid-Infrared Galaxies to the Far-Infrared Background
• Chandra and Spitzer unveil heavily obscured quasars in the SWIRE/Chandra Survey

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• The nature of star formation in distant ultraluminous infrared galaxies selected in a remarkably narrow redshift range
• HELP: The Herschel Extragalactic Legacy Project
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• A 610-MHz survey of the ELAIS-N1 field with the Giant Metrewave Radio Telescope - Observations, data analysis and source catalogue
• Unveiling the Important Role of Groups in the Evolution of Massive Galaxies: Insights from an Infrared Passive Sequence at Intermediate Redshift
• MAMBO 1.2mm observations of luminous starbursts at z~2 in the SWIRE fields
• An Evolutionary Paradigm for Dusty Active Galaxies at Low Redshift
• Multi-wavelength properties of Spitzer-selected starbursts at z~2
• The contribution of very massive high-redshift SWIRE galaxies to the stellar mass function
• The ESO-Spitzer Imaging extragalactic Survey (ESIS) II: VIMOS I,z wide field imaging of ELAIS-S1 and selection of distant massive galaxies
• The LOFAR Two-metre Sky Survey Deep Fields -- Data Release 1: IV. Photometric redshifts and stellar masses
• The Lockman-SpReSO project. Description, target selection, observations and preliminary results