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Identifying silicate-absorbed ULIRGs at z~1-2 in the Bootes Field using Spitzer/IRS

Using the 16$μ$m peakup 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$μ$m survey of this area, we produce a catalog of 150 16$μ$m sources brighter than 0.18 mJy (3$σ$) for which we derive measures or limits on the 16/24$μ$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$μ$m silicate absorption feature in Ultraluminous Infrared Galaxies (ULIRGs) results in sources brighter at 16$μ$m than at 24$μ$m at z $\sim$ 1--1.8 by at least 20%. With a threshold flux ratio of 1.2, restricting our analysis to $>5σ$ detections at 16$μ$m, and using a $3σ$ limit on 24$μ$m non-detections, the number of silicate-absorbed ULIRG candidates is 36. This defines a strong upper limit of $\sim$920 sources deg$^{-2}$, on the population of silicate-absorbed ULIRGs at z $\sim$ 1--1.8. This source count is about half of the total number of sources predicted at z $\sim$ 1--2 by various phenomenological models. We note that the high 16/24$μ$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.