Origin of the galaxy H I size–mass relation

Adam R.H. Stevens, Benedikt Diemer, Claudia P. Del Lagos, Dylan Nelson, Danail Obreschkow, Jing Wang, Federico Marinacci

Research output: Contribution to journalArticlepeer-review

32 Citations (Scopus)
64 Downloads (Pure)


We analytically derive the observed size–mass relation of galaxies’ atomic hydrogen (H I), including limits on its scatter, based on simple assumptions about the structure of H I discs. We trial three generic profiles for H I surface density as a function of radius. First, we assert that H I surface densities saturate at a variable threshold, and otherwise fall off exponentially with radius or, secondly, radius squared. Our third model assumes the total gas surface density is exponential, with the H I fraction at each radius depending on local pressure. These are tested against a compilation of 110 galaxies from the THINGS, LITTLE THINGS, LVHIS, and Bluedisk surveys, whose H I surface density profiles are well resolved. All models fit the observations well and predict consistent size–mass relations. Using an analytical argument, we explain why processes that cause gas disc truncation – such as ram-pressure stripping – scarcely affect the H I size–mass relation. This is tested with the IllustrisTNG(100) cosmological, hydrodynamic simulation and the DARK SAGE semi-analytic model of galaxy formation, both of which capture radially resolved disc structure. For galaxies with m ≥ 109 M and mH I ≥ 108 M, both simulations predict H I size–mass relations that align with observations, show no difference between central and satellite galaxies, and show only a minor, second-order dependence on host halo mass for satellites. Ultimately, the universally tight H I size–mass relation is mathematically inevitable and robust. Only by completely disrupting the structure of H I discs, e.g. through overly powerful feedback, could a simulation predict the relation poorly.

Original languageEnglish
Pages (from-to)96-113
Number of pages18
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Publication statusPublished - 21 Nov 2019


Dive into the research topics of 'Origin of the galaxy H I size–mass relation'. Together they form a unique fingerprint.

Cite this