The origin of the H α line profiles in simulated disc galaxies

Observations of ionized H αgas in high-redshift disc galaxies have ubiquitously found significant line broadening, σH α∼10 -100 km s-1 . To understand whether this broadening reflects gas turbulence within the interstellar medium (ISM) of galactic discs, or arises from out-of-plane emission in mass-loaded outflows, we perform radiation hydrodynamic simulations of isolated Milky Way-mass disc galaxies in a gas-poor (low-redshift) and gas rich (high-redshift) condition and create mock H αemission line profiles. We find that the majority of the total (integrated) H αemission is confined within the ISM, with extraplanar gas contributing ∼45 per cent of the extended profile wings ( v z ≥200 km s-1 ) in the gas-rich galaxy. This substantiates using the H αemission line as a tracer of mid-plane disc dynamics. We investigate the relative contribution of diffuse and dense H αemitting gas, corresponding to diffuse ionized gas (DIG; ρ ≲ 0 . 1 cm -3 , T ∼8 000 K) and H II regions ( ρ ≳ 10 cm -3 , T ∼10 000 K), respectively, and find that DIG contributes f DIG ≲ 10 per cent of the total L H α. Ho we ver, the DIG can reach upwards of σH α∼60 -80 km s-1 while the H II regions are much less turbulent σH α∼10 -40 km s-1 . This implies that the σH αobserved using the full H αemission line is dependent on the relative H αcontribution from DIG/H II regions and a larger f DIG would shift σH αto higher v alues. Finally, we sho w that σH αevolves, in both the DIG and H II regions, with the galaxy gas fraction. Our high-redshift equi v alent galaxy is roughly twice as turbulent, except for in the DIG which has a more shallow evolution.