A SPITZER UNBIASED ULTRADEEP SPECTROSCOPIC SURVEY

We carried out an unbiased, spectroscopic survey using the low-resolution module of the infrared spectrograph (IRS) onboard Spitzer targeting two 2.6 square arcminute regions in the GOODS-North field. The IRS was used in a spectral mapping mode with 5 hr of effective integration time per pixel. One region was covered between 14 and 21 μm and the other between 20 and 35 μm. We extracted spectra for 45 sources. About 84% of the sources have reported detections by GOODS at 24 μm, with a median f ν(24 μm) ∼ 100 μJy. All but one source are detected in all four IRAC bands, 3.6 to 8 μm. We use a new cross-correlation technique to measure redshifts and estimate IRS spectral types; this was successful for ∼60% of the spectra. Fourteen sources show significant polycyclic aromatic hydrocarbon emission, four mostly SiO absorption, eight present mixed spectral signatures (low PAH and/or SiO) and two show a single line in emission. For the remaining 17, no spectral features were detected. Redshifts range from z ∼ 0.2 to z ∼ 2.2, with a median of 1. IR luminosities are roughly estimated from 24 μm flux densities, and have median values of 2.2 x 1011 Lsun and 7.5 x 1011 Lsun at z ∼ 1 and z ∼ 2, respectively. This sample has fewer active galactic nuclei than previous faint samples observed with the IRS, which we attribute to the fainter luminosities reached here.