Molecular Gas in Intermediate Redshift ULIRGs

TL;DR

This study investigates molecular gas properties in intermediate redshift ULIRGs through CO(1-0) observations, revealing evolution in dust-to-gas ratios and a significant increase in molecular gas mass over cosmic time.

Contribution

First comprehensive CO(1-0) survey of intermediate redshift ULIRGs, showing evolution in dust-to-gas ratios and molecular gas mass with redshift.

Findings

Detected CO(1-0) in stacked spectrum at z=0.38 with 10σ significance.

Dust-to-gas ratio declines to levels similar to the Small Magellanic Cloud at intermediate redshift.

Molecular gas mass in ULIRGs increases by nearly an order of magnitude from present to 5 Gyr ago.

Abstract

We report on the results of observations in the CO(1-0) transition of a complete sample of Southern, intermediate redshift (z = 0.2 - 0.5) Ultra-Luminous Infra-Red Galaxies using the Mopra 22m telescope. The eleven ULIRGs with L_FIR > 10^12.5 L_Sun south of Dec = -12 deg were observed with integration times that varied between 5 and 24 hours. Four marginal detections were obtained for individual targets in the sample. The "stacked" spectrum of the entire sample yields a high significance, 10{\sigma} detection of the CO(1-0) transition at an average redshift of z = 0.38. The tightest correlation of L_FIR and L_CO for published low redshift ULIRG samples (z < 0.2) is obtained after normalisation of both these measures to a fixed dust temperature. With this normalisation the relationship is linear. The distribution of dust-to-molecular hydrogen gas mass displays a systematic increase in dust-to-gas mass with galaxy luminosity for low redshift samples but this ratio declines dramatically for intermediate redshift ULIRGs down to values comparable to that of the Small Magellanic Cloud. The upper envelope to the distribution of ULIRG molecular mass as function of look-back time demonstrates a dramatic rise by almost an order of magnitude from the current epoch out to 5 Gyr. This increase in maximum ULIRG gas mass with look-back time is even more rapid than that of the star formation rate density.