The origin of the Hα line profiles in simulated disc galaxies

Observations of ionised Hα gas in disc galaxies with high star formation rates have ubiquitous and significant line broadening with widths σ_ Hα≳ 50-100 km s^-1. To understand whether this broadening reflects gas turbulence within the interstellar medium (ISM) of galactic discs, or arises from off-the-plane emission in mass-loaded galactic winds, we perform radiation hydrodynamic (RHD) simulations of isolated Milky Way-mass disc galaxies in a gas-poor (low-redshift) and gas rich (high-redshift) condition and create mock Hα emission line profiles. We find that the vast majority of the Hα emission is confined within the ISM, with extraplanar gas contributing mainly to the extended profile wings. This substantiates the emission line as a tracer of mid-plane disc dynamics. We investigate the relative contribution of diffuse and dense Hα emitting gas, corresponding to DIG (ρ≲ 0.1 cm^-3, T∼ 8 000 K) and HII regions (ρ≳ 10 cm^-3, T∼ 10 000 K), respectively, and find that DIG contributes ≲ 10 % of the total L_ Hα. However, the DIG can reach upwards of σ_ Hα∼ 60-80 km s^-1 while the HII regions are much less turbulent σ_ Hα∼10-40 km s^-1. This implies that the σ_ Hα observed using the full Hα emission line is dependent on the relative Hα contribution from DIG/HII regions and a larger f_ DIG would shift σ_ Hα to higher values. Finally, we show that σ_ Hα evolves, in both the DIG and HII regions, with the galaxy gas fraction. Our high-redshift equivalent galaxy is roughly twice as turbulent, except for in the DIG which has a more shallow evolution.