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

Abstract

Observations of ionised H$\alpha$ gas in high-redshift disc galaxies have ubiquitously found significant line broadening, $\sigma_{\rm H\alpha}\sim10-100$ 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 $\sim45$% of the extended profile wings ($v_z\geq200$ 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; $\rho\lesssim0.1$ cm$^{-3}$, $T\sim8\,000$ K) and HII regions ($\rho\gtrsim10$ cm$^{-3}$, $T\sim10\,000$ K), respectively, and find that DIG contributes $f_{\rm DIG}\lesssim10$% of the total L$_{\rm H\alpha}$. However, the DIG can reach upwards of $\sigma_{\rm H\alpha}\sim60-80$ km s$^{-1}$ while the HII regions are much less turbulent $\sigma_{\rm H\alpha}\sim10-40$ km $s^{-1}$. This implies that the $\sigma_{\rm H\alpha}$ observed using the full H$\alpha$ emission line is dependent on the relative H$\alpha$ contribution from DIG/HII regions and a larger $f_{\rm DIG}$ would shift $\sigma_{\rm H\alpha}$ to higher values. Finally, we show that $\sigma_{\rm H\alpha}$ 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.