Ultradeep Spectroscopy with the Spitzer IRS

Abstract

Mid-IR spectroscopy has detected the signatures of star-formation (PAH emission) in high redshift (z > 1) ultra- and hyper-luminous infrared galaxies. However, the study of the dominant population of IR-luminous galaxies (10^(11) - 10^(12) Lsun at 1 < z < 3), requires observation of sources a at the 0.1 mJy level. We present the deepest spectra taken to date in the Long-Low module of the the Infrared Spectrometer (IRS) on the Spitzer Space Telescope. We targeted two faint (~0.15 mJy) sources in the Southern GOODS field at z = 1.09 and z = 2.69 as likely star-forming galaxies. Spectra of the lower redshift target were taken in 8-21 micron range (short-low first order and long-low second order), while the higher redshift target was observed from 21-37 microns (longlow first order). Observing times were 3 and 9 hours on-source for SL-1 and LL-2, respectively, and 12 hours for LL-1. We also present the spectra of two serendipitous sources. We detect strong PAH emission in four targets. We compare the spectra to those of local galaxies observed by the IRS. The z = 1.09 source appears to be a typical, star-formation dominated LIRG, while the z = 2.69 source is a composite source with strong star formation and a prominent AGN. The AGN component dominates the IRAC colors of this source, obscuring the 1.6 μm “bump.” Such sources would be excluded from IRAC surveys for starbursts which might then underestimate the star formation density.