Molecular lines studies at redshift greater than 1

TL;DR

This paper reviews high-redshift molecular line observations, highlighting their role in understanding star formation, galaxy evolution, and the biases introduced by selection effects, especially in CO and infrared-selected objects.

Contribution

It synthesizes current knowledge on molecular line studies at z>1, emphasizing the impact of selection biases and the diversity of galaxy types observed.

Findings

Gas fraction is higher at z>1.

Quasars are linked to starbursts and gas-rich environments.

Selection effects influence the detection of gaseous galaxies.

Abstract

Observations of CO molecules in the millimetrer domain at high redshift (larger than 1), have provided interesting informations about star formation efficiency, and its evolution with redshift. Due to the difficulty of the detections, selection effects are important. The detection if often due to gravitational amplification. Objects selected by their (far)infrared flux, are in general associated to ULIRGS, mergers with starburst in the nuclear regions. Quasars have been selected as powerful optical sources, and have been found to be associated to starbursts, rich in gas. The gas fraction appears to be much higher at redshift greater than 1. Quasars allow to probe the end of the reionisation period, and the relation between bulge and black hole mass. However these selection bias could have led us to miss some gaseous galaxies, with low-efficiency of star formation, such as the more quiescent objects selected by their BzK colors at z=1.5 or 2.