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 tackles how HI gas mass scales with existing stellar mass in galaxies at $0.25<z<0.5$ by applying a Bayesian stacking framework to MIGHTEE-HI DR1 data, enabling recovery of the full conditional distribution $P(M_{\rm HI}|M_\star)$ beyond traditional detections. It compares Normal, Schechter, and Skew-normal HI distributions, finding strong evidence for asymmetry and an upper HI-mass envelope, and derives the HI mass function for $\log_{10}(M_\star/M_\odot)>9.5$. The results show color and morphology dependencies, with blue/spiral systems more HI-rich and described by asymmetric models, while red/elliptical systems favor symmetric or smaller HI contents; comparisons with SIMBA and local HIMFs suggest mild evolution in the high-HI population by $z\sim0.5$. The work highlights the importance of modeling the intrinsic HI distribution to accurately infer the HIMF and informs models of gas accretion and star formation fueling in massive galaxies across cosmic time.

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 $z\sim 0.5$.

Figures (19)

• Figure 1: DEVILS spectroscopic samples overlaid on the MIGHTEE field. The colour scheme indicates the noise level of MIGHTEE DR1 L1 spectral data on a projected 2-dimensional sky.
• Figure 2: Stellar mass as a function of redshift. The dashed line is the limit of the sample completeness of $\log_{10}(M_\star/M_\odot$) = 9.5.
• Figure 3: Colour (u-r) against the stellar mass. The blue stars are spirals while the orange dots are ellipticals. The dashed line is from Schawinski2014 for seperating the full sample into blue and red sub-samples.
• Figure 4: Histogram of the stellar mass for all galaxy samples. The orange, blue, red, green and purple are the full sample, blue, red, spiral and elliptical sub-samples.
• Figure 5: Distribution of SNR of the measured flux for all galaxy samples. The orange, blue, red, green and purple are the full sample, blue, red, spiral and elliptical sub-samples. The dashed grey line indicates a nominal detection threshold of 5$\sigma$. The dashed line is for the measured noise at positions offset from the optical galaxy locations.