The Far-Infrared, UV and Molecular Gas Relation in Galaxies up to z=2.5

TL;DR

This study investigates the relationship between UV attenuation, infrared excess, and molecular gas in galaxies up to redshift 2.5, revealing a tight correlation that enables estimating gas masses from UV and FIR data.

Contribution

It introduces a new empirical method to estimate molecular gas masses from UV and FIR photometry, based on the IRX-beta relation and its scatter.

Findings

Main sequence galaxies follow a tight IRX-beta relation with a flatter slope.

The IRX-beta scatter correlates with specific attenuation from molecular gas.

The new method estimates gas masses with 0.12-0.16 dex accuracy for normal galaxies.

Abstract

We use the infrared excess (IRX) FIR/UV luminosity ratio to study the relation between the effective UV attenuation (A_IRX) and the UV spectral slope (beta) in a sample of 450 1<z<2.5 galaxies. The FIR data is from very deep Herschel observations in the GOODS fields that allow us to detect galaxies with SFRs typical of galaxies with log(M)>9.3. Thus, we are able to study galaxies on and even below the main SFR-stellar mass relation (main sequence). We find that main sequence galaxies form a tight sequence in the IRX--beta plane, which has a flatter slope than commonly used relations. This slope favors a SMC-like UV extinction curve, though the interpretation is model dependent. The scatter in the IRX-beta plane, correlates with the position of the galaxies in the SFR-M plane. Using a smaller sample of galaxies with CO gas masses, we study the relation between the UV attenuation and the molecular gas content. We find a very tight relation between the scatter in the IRX-beta plane and the specific attenuation (S_A), a quantity that represents the attenuation contributed by the molecular gas mass per young star. S_A is sensitive to both the geometrical arrangement of stars and dust, and to the compactness of the star forming regions. We use this empirical relation to derive a method for estimating molecular gas masses using only widely available integrated rest-frame UV and FIR photometry. The method produces gas masses with an accuracy between 0.12-0.16 dex in samples of normal galaxies between z~0 and z~1.5. Major mergers and sub-millimeter galaxies follow a different S_A relation.