Gas fraction and star formation efficiency at z < 1.0

TL;DR

This study investigates how gas fraction and star formation efficiency evolve at z<1, revealing both increase significantly and jointly influence the decline in cosmic star formation during this epoch.

Contribution

It provides the first comprehensive CO survey in the 0.2<z<1 range, demonstrating the joint evolution of gas fraction and star formation efficiency.

Findings

Gas fraction increases by a factor of 3 from z=0 to 1.

Star formation efficiency also increases by a factor of 3 from z=0 to 1.

Both factors significantly influence the decline in star formation rate.

Abstract

After new observations of 39 galaxies at z = 0.6-1.0 obtained at the IRAM 30m telescope, we present our full CO line survey covering the redshift range 0.2 < z < 1. Our aim is to determine the driving factors accounting for the steep decline in the star formation rate during this epoch. We study both the gas fraction, defined as Mgas/(Mgas+Mstar), and the star formation efficiency (SFE) defined by the ratio between far-infrared luminosity and molecular gas mass (LFIR/M(H2), i.e. a measure for the inverse of the gas depletion time. The sources are selected to be ultra-luminous infrared galaxies (ULIRGs), with LFIR greater than 10^12 Lo and experiencing starbursts. When we adopt a standard ULIRG CO-to-H2 conversion factor, their molecular gas depletion time is less than 100 Myr. Our full survey has now filled the gap of CO observations in the 0.2<z<1 range covering almost half of cosmic history. The detection rate in the 0.6 < z < 1 interval is 38% (15 galaxies out of 39), compared to 60% for the 0.2<z<0.6 interval. The average CO luminosity is L'CO = 1.8 10^10 K km/s pc^2, corresponding to an average H2 mass of 1.45 10^10 Mo. From observation of 7 galaxies in both CO(2-1) and CO(4-3), a high gas excitation has been derived; together with the dust mass estimation, this supports the choice of our low ULIRG conversion factor between CO luminosity and H2, for our sample sources. We find that both the gas fraction and the SFE significantly increase with redshift, by factors of 3 +-1 from z=0 to 1, and therefore both quantities play an important role and complement each other in cosmic star formation evolution.