MIGHTEE-HI: The HI mass-stellar mass relation of massive galaxies and the HI mass function at 0.25<z<0.5

TL;DR

This study measures the HI mass-stellar mass relation and the HI mass function of galaxies at redshifts 0.25 to 0.5 using Bayesian stacking, revealing an asymmetric HI distribution and a potential decline in massive HI galaxies.

Contribution

First measurement of the bivariate HI mass and stellar mass distribution at intermediate redshifts using Bayesian stacking techniques.

Findings

Asymmetric HI mass distribution is strongly preferred.

Shallow slopes suggest an upper HI mass limit for galaxies.

Tentative evidence for a decrease in massive HI galaxies at z~0.5.

Abstract

The relationship between the already formed stellar mass in a galaxy and the gas reservoir of neutral atomic hydrogen, is a key element in our understanding of how gas is turned into stars in galaxy haloes. In this paper, we measure the $M_{\rm HI}-M_{\star}$ relation based on a stellar-mass selected sample at $0.25 < z < 0.5$ and the MIGHTEE-HI DR1 spectral data. Using a powerful Bayesian stacking technique, for the first time we are also able to measure the underlying bivariate distribution of HI mass and stellar mass of galaxies with $M_\star > 10^{9.5}$ M$_{\odot}$, finding that an asymmetric underlying HI distribution is strongly preferred by our complete samples. We define the concepts of the average of the logarithmic HI mass, $\langle\log_{10}(M_{\rm HI})\rangle$, and the logarithmic average of the HI mass, $\log_{10}(\langle M_{\rm HI}\rangle)$, and find that the difference between $\langle\log_{10}(M_{\rm HI})\rangle$ and $\log_{10}(\langle M_{\rm HI}\rangle)$ can be as large as $\sim$0.5 dex for the preferred asymmetric HI distribution. We observe shallow slopes in the underlying $M_{\rm HI}-M_{\star}$ scaling relations, suggesting the presence of an upper HI mass limit beyond which a galaxy can no longer retain further HI gas. From our bivariate distribution we also infer the HI mass function at this redshift and find tentative evidence for a decrease of 2-10 times in the co-moving space density of the most HI massive galaxies up to $z\sim 0.5$.