The impact of broad mid-infrared emission bands on extragalactic surveys - a case study: the unidentified 22-μm feature

Abstract

The effect of unidentified dust emission features in the mid-infrared (MIR) spectral energy distributions of dusty star-forming galaxies is investigated. As a case study, we discuss the broad emission feature at 2 μm of Chan & Onaka. By modelling a distribution of feature strengths using a starburst galaxy template, we speculatively investigate the influence of the feature on galaxy fluxes, colours and detected numbers. We find that the fluxes and colours of galaxies are systematically affected by the feature from redshifts 0-3, at wavelengths ranging from 20 to 70 μm by up to factors of ∼2, resulting in bluer MIR/far-infrared (FIR) and FIR/MIR colours at local and high redshifts, respectively, as the feature is redshifted in and out of the various observation passbands. Furthermore, we find that if the feature is prolific, it will manifest itself in the same manner as evolution in the infrared (IR) galaxy population in a similar way to the unidentified IR bands at shorter wavelengths. We also consider the effect of the feature on the future IR SIRTF (Spitzer) and ASTRO-F space missions. We conclude that although the bulk effects of the feature may be small, careful consideration will have to be made when discussing the colours of sources in the relevant bands and redshift ranges affected by the feature emission.