The infrared-dark dust content of high redshift galaxies

TL;DR

This paper proposes a theoretical model explaining why high-redshift galaxies are infrared-dark despite high UV attenuation, linking their FIR deficit to large molecular gas content and suggesting new observational strategies.

Contribution

It introduces a model connecting FIR deficit in early galaxies to their molecular gas content, offering a new method to identify candidates for CO observations.

Findings

High-z galaxies show a FIR deficit linked to large molecular gas reservoirs.

Dust temperatures in diffuse ISM are around 45-60 K, depending on grain size.

FIR deficit can serve as an indicator for molecular content in early galaxies.

Abstract

We present a theoretical model aimed at explaining the IRX-$\beta$ relation for high redshift (z >5) galaxies. Recent observations (Capak+2015; Bouwens+2016) have shown that early Lyman Break Galaxies, although characterized by a large UV attenuation (e.g. flat UV beta slopes), show a striking FIR deficit, i.e. they are "infrared-dark". This marked deviation from the local IRX-beta relation can be explained by the larger molecular gas content of these systems. While dust in the diffuse ISM attains relatively high temperatures (Td = 45 K for typical size a=0.1 um; 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.