Redshift Evolution of the H2/HI Mass Ratio In Galaxies

TL;DR

This study investigates how the ratio of molecular to neutral gas in galaxies evolves with redshift, revealing a slight decrease contrary to expectations, based on high-resolution spatial data of nearby galaxies.

Contribution

It introduces a novel approach to estimate the redshift evolution of the H2/HI ratio using spatially resolved data and two different relations, challenging common assumptions.

Findings

H2/HI ratio slightly decreases with redshift within galaxies.

Contradicts the expectation that high-redshift galaxies are more H2 dominated.

Provides insights into the internal gas dynamics of high-redshift galaxies.

Abstract

In this paper we present an attempt to estimate the redshift evolution of the molecular to neutral gas mass ratio within galaxies (at fixed stellar mass). For a sample of five nearby grand design spirals located on the Main Sequence (MS) of star forming galaxies, we exploit maps at 500 pc resolution of stellar mass and star formation rate ($M_{\star}$ and SFR). For the same cells, we also have estimates of the neutral ($M_{\rm HI}$) and molecular ($M_{\rm H_2}$) gas masses. To compute the redshift evolution we exploit two relations: {\it i)} one between the molecular-to-neutral mass ratio and the total gas mass ($M_{\rm gas}$), whose scatter shows a strong dependence with the distance from the spatially resolved MS, and {\it ii)} the one between $\log(M_{\rm{H_2}}/M_{\star})$ and $\log(M_{\rm{HI}}/M_{\star})$. For both methods, we find that $M_{\rm H_2}$/$M_{\rm HI}$ within the optical radius slightly decreases with redshift, contrary to common expectations of galaxies becoming progressively more dominated by molecular hydrogen at high redshifts. We discuss possible implications of this trend on our understanding of the internal working of high redshift galaxies.