Measuring PAH Emission in Ultradeep Spitzer IRS Spectroscopy of High Redshift IR Luminous Galaxies

The study of the dominant population of high redshift IR-luminous galaxies (10 10 L⊙ at 1 < z < 3), requires observation of sources at the ∼ 0.1 mJy level in the mid-IR. We present the deepest spectra taken to date with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. We targeted two faint (f24 ∼ 0.15 mJy) sources in the Southern GOODS field at z = 1.09 and z = 2.69. Spectra of the lower redshift target were taken in the observed-frame 8–21 micron range, while the spectrum of the higher redshift target covered 21– 37 microns. We also present the spectra of two secondary sources within the slit. We detect strong PAH emission in all four targets, and 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 IR-luminous galaxy, while the z = 2.69 source is a composite source with strong star formation and a prominent AGN. The IRAC colors of this source show no evidence of rest-frame near-infrared stellar photospheric emission. We demonstrate that an AGN which contributes only a small (∼ 10%) fraction of the bolometric luminosity can produce enough hot Based on observations obtained with the Spitzer Space Telescope, which is operated by JPL, California Institute of Technology for the National Aeronautics and Space Administration The IRS is a collaborative venture between Cornell University and Ball Aerospace Corporation that was funded by NASA through JPL. Spitzer Science Center, MS 220-6, Caltech, Pasadena, CA 91125. [email protected] Astronomy Department, Caltech, Pasadena, CA 91125 Astronomy Department, Cornell University, Ithaca, NY 14853 Dept. of Physics & Astronomy, University of British Columbia, Vancouver, BC V6T1Z1, Canada Dept. of Physics, University of Crete, GR-71003 Heraklion, Greece IESL / Foundataion for Research and Technology-Hellas, GR-71110, Heracklion, Greece, and Chercheur Associé, Observatoire de Paris, F-75014, Paris, France