Silicate-break galaxies: an efficient selection method of distant ultraluminous infrared galaxies

We present a photometric selection method for ULIRGs in the redshift range of z = 1 – 2. We utilize the most prominent spectral feature of ULIRGs, i.e. the silicate absorption feature at 9.7 μm and an optimized filter system at mid-infrared wavelengths. These ‘silicate-break’ galaxies could be selected by using colour anomalies owing to the silicate absorption feature around 9.7(1+z) μm. Such filter systems are available on the Spitzer Space Telescope but we suggest that the most promising selection criteria would be given with mid-infrared bands of ASTRO-F satellite due to a more comprehensive set of filter bands than that of Spitzer. We study the selection method of silicate-break galaxies with the SED model of both starbursts and quiescent star-forming galaxies, and then verify the method by using the observed spectra of nearby galaxies. We would expect that about 1000 candidates of silicate-break galaxies could be detected per square degree in current and future mid-infrared surveys. The silicate-break selection criteria will break the degeneracy between various galaxy evolution models for extragalactic source counts and place strong limits on the star formation activity at z = 1 – 2. Applying our silicate-break technique to initial Spitzer results we have tentatively identified the first candidate silicate-break galaxy at z = 1.6.