Realistic HI scale heights of Milky Way-mass galaxies in the FIREbox cosmological volume

TL;DR

This study measures the HI scale heights in 22 Milky Way-mass galaxies from the FIREbox simulation, finding results consistent with observations and highlighting the importance of realistic ISM and feedback processes.

Contribution

First to measure radially resolved HI scale heights in a large sample of simulated Milky Way-mass galaxies using multiple methods.

Findings

HI scale heights range from ~100 pc in centers to ~800 pc in outskirts.

Results agree well with observational data.

Realistic ISM and stellar feedback are key to reproducing observed HI scale heights.

Abstract

Accurately reproducing the thin cold gas discs observed in nearby spiral galaxies has been a long standing issue in cosmological simulations. Here, we present measurements of the radially resolved HI scale height in 22 non-interacting Milky Way-mass galaxies from the FIREbox cosmological volume. We measure the HI scale heights using five different approaches commonly used in the literature: fitting the vertical volume density distribution with a Gaussian, the distance between maximum and half-maximum of the vertical volume density distribution, a semi-empirical description using the velocity dispersion and the galactic gravitational potential, the analytic assumption of hydrostatic equilibrium, and the distance from the midplane which encloses $\gtrsim$60 per cent of the HI mass. We find median HI scale heights, measured using the vertical volume distribution, that range from ~100 pc in the galactic centres to ~800 pc in the outskirts and are in excellent agreement with recent observational results. We speculate that the presence of a realistic multiphase interstellar medium, including cold gas, and realistic stellar feedback are the drivers behind the realistic HI scale heights.