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

TL;DR

This study uses radiation hydrodynamic simulations of disc galaxies to investigate the origins of Hα line broadening, revealing that turbulence mainly arises within the galactic disc and is influenced by the diffuse ionised gas component.

Contribution

It provides new insights into the sources of Hα line broadening, emphasizing the role of diffuse ionised gas and galaxy gas fraction in shaping observed turbulence signatures.

Findings

Most Hα emission originates within the galactic disc.

Diffuse ionised gas significantly contributes to line broadening.

Hα turbulence correlates with galaxy gas fraction.

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.