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

S. Berta; C. J. Lonsdale; B. Siana; D. Farrah; H. E. Smith; M. Polletta; A. Franceschini; J. Fritz; I. Perez-Fournon; M. Rowan-Robinson; D. Shupe; J. Surace

doi:10.1051/0004-6361:20066795

arXiv · astro-ph/0703214

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

S. Berta·C. J. Lonsdale·B. Siana·D. Farrah·H. E. Smith·M. Polletta·A. Franceschini·J. Fritz·I. Perez-Fournon·M. Rowan-Robinson·D. Shupe·J. Surace

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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.

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S. Berta, C. J. Lonsdale, B. Siana, D. Farrah, H. E. Smith, M. Polletta, A. Franceschini, J. Fritz, I. Perez-Fournon, M. Rowan-Robinson, D. Shupe, J. Surace. 2007-03-09. Keck spectroscopy of z=1-3 ULIRGs from the Spitzer SWIRE survey. https://doi.org/10.1051/0004-6361%3A20066795

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