The infrared-dark dust content of high redshift galaxies

We present a theoretical model aimed at explaining the IRX-β relation for high redshift (z ≳ 5) galaxies. Recent observations have shown that early Lyman-Break Galaxies, although characterized by a large ultraviolet (UV) attenuation (e.g. flat UV β slopes), show a striking far-infrared (FIR) deficit, i.e. they are ‘infrared-dark’. This marked deviation from the local IRX-β relation can be explained by the larger molecular gas content of these systems. While dust in the diffuse interstellar medium attains relatively high temperatures (Td ≃ 45 K for typical size a = 0.1 μm; smaller grains can reach Td ≳ 60 K), a sizable fraction of the dust mass is embedded in dense gas, and therefore remains cold. If confirmed, the FIR deficit might represent a novel, powerful indicator of the molecular content of high-z galaxies which can be used to pre-select candidates for follow-up deep CO observations. Thus, high-z CO line searches with ALMA might be much more promising than currently thought.