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 deg(2) 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 mm 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 similar to 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 similar to 920 sources deg(-2) on the population of silicate-absorbed ULIRGs at. This source count is about half of the total number of sources predicted z similar to 1 - 2 at 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.