The origin of the H$\alpha$ line profiles in simulated disc galaxies

Observations of ionised H$\alpha$ gas in high-redshift disc galaxies have ubiquitously found significant line broadening, ${\sigma }_{\mathrm{H}\alpha }\sim 10-100$ 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 (RHD) simulations of isolated Milky Way-mass disc galaxies in a gas-poor (low-redshift) and gas rich (high-redshift) condition and create mock H$\alpha$ emission line profiles. We find that the majority of the total (integrated) H$\alpha$ emission is confined within the ISM, with extraplanar gas contributing $\sim 45$% of the extended profile wings ($v_z\ge 200$ km s${}^{-1}$) in the gas-rich galaxy. This substantiates using the H$\alpha$ emission line as a tracer of mid-plane disc dynamics. We investigate the relative contribution of diffuse and dense H$\alpha$ emitting gas, corresponding to diffuse ionised gas (DIG; $\rho \lesssim 0.1$ cm${}^{-3}$, $T\sim 8000$ K) and HII regions ($\rho \gtrsim 10$ cm${}^{-3}$, $T\sim 10000$ K), respectively, and find that DIG contributes $f_{\mathrm{D}\mathrm{I}\mathrm{G}}\lesssim 10$% of the total L${}_{\mathrm{H}\alpha }$. However, the DIG can reach upwards of ${\sigma }_{\mathrm{H}\alpha }\sim 60-80$ km s${}^{-1}$ while the HII regions are much less turbulent ${\sigma }_{\mathrm{H}\alpha }\sim 10-40$ km $s^{-1}$. This implies that the ${\sigma }_{\mathrm{H}\alpha }$ observed using the full H$\alpha$ emission line is dependent on the relative H$\alpha$ contribution from DIG/HII regions and a larger $f_{\mathrm{D}\mathrm{I}\mathrm{G}}$ would shift ${\sigma }_{\mathrm{H}\alpha }$ to higher values. Finally, we show that ${\sigma }_{\mathrm{H}\alpha }$ 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.