The origin of the Hα line profiles in simulated disc galaxies

Observations of ionized H α gas in high-redshift disc galaxies have ubiquitously found significant line broadening, σ H\,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 α emission line profiles. We find that the majority of the total (integrated) H\,α emission is confined within the ISM, with extraplanar gas contributing ~ 45~\ per\ cent of the extended profile wings (vz≥ 200{\, {km\, s⁻¹} }$) in the gas-rich galaxy. This substantiates using the ${ H\,α }$ emission line as a tracer of mid-plane disc dynamics. We investigate the relative contribution of diffuse and dense ${ H\,α }$ emitting 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 $fDIG 10~{{\ per\ cent}}$ of the total L$Hα$. However, the DIG can reach upwards of $σ H\,α ~ 60{\!-\!}80\, { km\, s⁻¹}$ while the H ii regions are much less turbulent $σ H\,α~ 10{\!-\!}40\, { km\, s⁻¹}$. This implies that the $σ H\,α$ observed using the full ${ H\,α }$ emission line is dependent on the relative ${ H\,α }$ contribution from DIG/H ii regions and a larger $fDIG$ would shift $σ H\,α$ to higher values. Finally, we show that $σ H\,α$ evolves, in both the DIG and H ii 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.