On the Absence of High Metallicity-High Column Density Damped Lyman Alpha Systems: Molecule Formation in a Two-Phase Interstellar Medium

TL;DR

This paper explains the scarcity of high metallicity, high column density damped Lyman alpha systems by proposing that molecule formation in a cold phase of a two-phase interstellar medium prevents their observation, regardless of radiation environment.

Contribution

It introduces a radiation-insensitive model linking molecule formation to the absence of certain DLA systems, accounting for observations across different environments.

Findings

Molecule formation in the cold phase explains the lack of high metallicity, high column DLAs.

The maximum atomic gas column constrains the warm phase mass fraction.

The model accounts for the absence of in situ star formation in DLAs.

Abstract

We argue that the lack of observed damped Lyman alpha (DLA) systems that simultaneously have high HI columns densities and high metallicities results naturally from the formation of molecules in the cold phase of a two-phase atomic medium in pressure balance. Our result applies equally well in diffuse systems where the ultraviolet radiation field is dominated by the extragalactic background and in dense star-forming ones, where the local radiation field is likely to be orders of magnitude higher. We point out that such a radiation-insensitive model is required to explain the absence of high column - high metallicity systems among DLAs observed using gamma-ray burst afterglows, since these are likely subjected to strong radiation fields created by active star formation in the GRB host galaxy. Moreover, we show that the observed relationship between the maximum atomic gas column in DLAs sets a firm upper limit on the fraction of the mass in these systems that can be in the warm, diffuse phase. Finally, we argue that our result explains the observed lack of in situ star formation in DLA systems.