Molecular gas in high redshift galaxies

TL;DR

This paper reviews recent observational advances in studying molecular gas in high-redshift galaxies, highlighting how star formation efficiency and gas content evolved over cosmic time, aided by new instruments like IRAM, Herschel, and ALMA.

Contribution

It summarizes recent observational findings on molecular gas in high-redshift galaxies and discusses the impact of new instruments in extending these studies to earlier cosmic epochs.

Findings

Star formation was more active in the past due to higher gas fractions.

The Kennicutt-Schmidt relation remains similar up to z ~ 2.5.

Gravitational lensing and advanced telescopes enable exploration of galaxies up to z = 6.

Abstract

Recent observations with the IRAM instruments have allowed to explore the star formation efficiency in galaxies as a function of redshift, in detecting and mapping their molecular gas. Some galaxies stand on what is called the "main sequence", forming stars with a rate that can be sustained over time-scales of 1 Gyr, some are starbursts, with a much shorter depletion time. Star formation was more active in the past, partly because galaxies contained a larger gas fraction, and also because the star formation efficiency was higher. The global Kennicutt-Schmidt relation was however similar until z \sim 2.5. Magnification by gravitational lenses have been used to explore in details galaxies at higher redshift up to 6. Herschel has discovered many of these candidates, and their redshift has been determined through the CO lines. ALMA is beginning to extend considerably these redshift searches, with its broad-band receivers, for a large range of objects too obscured to be seen in the optical.