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

Abstract

Using the 16$\mu$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$\mu$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 16/24$\mu$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 $\sim$ 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 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$\mu$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.

Accepted for Publication in Astrophysical Journal Letters 4 pages, 2 figures, 1 table

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