Identifying Silicate-absorbed ULIRGs at z ~1-2 in the Bootes Field Using the Spitzer IRS

Using the 16 (mu)m peak-up imager on the Infrared Spectrograph (IRS) on Spitzer, we present a serendipitous survey of 0.0392 deg2 within the area of the NOAO Deep Wide-Field Survey in Bootes. Combining our results with the available Multiband Imaging Photometer for Spitzer (MIPS) 24 (mu)m survey of this area, we produce a catalog of 150 16 (mu)m sources brighter than 0.18 mJy (3 (sigma)) for which we derive measures or limits on the 16 (mu)m/24 (mu)m colors. Such colors are especially useful in determining redshifts for sources whose mid-infrared spectra contain strong emission or absorption features that characterize these colors as a function of redshift. We find that the 9.7 (mu)m silicate absorption feature in ultraluminous infrared galaxies (ULIRGs) results in sources brighter at 16 (mu)m than at 24 (mu)m at z ~1-1.8 by at least 20%. With a threshold flux ratio of 1.2, restricting our analysis to >5 (sigma)detections at 16 (mu)m, and using a 3 (sigma)limit on 24 (mu)m nondetections, the number of silicate-absorbed ULIRG candidates is 36. This defines a strong upper limit of ~920 sources deg-2 on the population of silicate-absorbed ULIRGs at z ~1-1.8. This source count is about half of the total number of sources predicted at z ~1-2 by various phenomenological models. We note that the high 16 (mu)m/24 (mu)m colors measured cannot be reproduced by any of the mid-IR spectral energy distributions assumed by these models, which points to the strong limitations currently affecting our phenomenological and theoretical understanding of infrared galaxy evolution.