Complete Ionisation of the Neutral Gas in High Redshift Radio Galaxies and Quasars

TL;DR

This study finds a lack of neutral and molecular gas in high redshift radio galaxies and quasars with high UV luminosities, suggesting intense radiation ionizes all star-forming material, impacting galaxy evolution understanding.

Contribution

It demonstrates a critical UV luminosity threshold above which all gas in these galaxies is ionized, explaining the absence of detectable neutral and molecular gas.

Findings

No atomic or molecular absorption detected at z > 3.

High UV luminosity correlates with complete gas ionization.

Galaxies likely lack star-forming material, not just undetectable amounts.

Abstract

Cool neutral gas provides the raw material for all star formation in the Universe, and yet, from a survey of the hosts of high redshift radio galaxies and quasars, we find a complete dearth of atomic (HI 21-cm) and molecular (OH, CO, HCO+ & HCN) absorption at redshifts z > 3. Upon a thorough analysis of the optical photometry, we find that all of our targets have ionising ultra-violet continuum luminosities of logL > 23 W/Hz. We therefore attribute this deficit to the traditional optical selection of targets biasing surveys towards the most ultra-violet luminous objects, where the intense radiation excites the neutral gas to the point where it cannot engage in star formation. However, this hypothesis does not explain why there is a critical luminosity, rather than a continuum where the detections gradually become fewer and fewer as the harshness of the radiation increases. We show that by placing a quasar within a galaxy of gas there is always a finite ultra-violet luminosity above which all of the gas is ionised. This demonstrates that these galaxies are probably devoid of star-forming material rather than this being at abundances below the sensitivity limits of current radio telescopes.