Constraining Baryonic Feedback at $z\sim 1$ with the H$α$ Luminosity Function

Abstract

Baryonic feedback remains one of the primary uncertainties in galaxy formation models because of its impact on the interstellar medium (ISM) and star formation. We investigate its effect on the H$α$ luminosity function (LF) using the Illustris and IllustrisTNG simulations, incorporating direction-dependent dust attenuation and a physically motivated treatment of the propagation of Lyman-series photons. We model three sources of H$α$ emission: star-forming H II regions, collisional excitation and recombination in the diffuse ISM, and AGN-induced photo-excitation of diffuse gas. The models are calibrated against observed H$α$ LFs at $z\sim 1-1.5$ using a single free parameter relating the star formation rate to the H II-region H$α$ luminosity. We find that emission from diffuse gas contributes $\sim 1\%-10\%$ of the total H$α$ luminosity in both simulation sets. AGN photo-excitation produces only a modest enhancement of the bright end of the LF, although it dominates the H$α$ emission of Illustris galaxies with $L_{\text{H}α}\gtrsim 3\times10^{42}\ \rm{erg \ s^{-1}}$. The low-feedback IllustrisTNG simulations provide good agreement with the observed LFs for all emission models considered, whereas the high-feedback Illustris simulation fails to simultaneously reproduce the faint and bright ends of the LF at $z\sim 1$. This tension between H$α$ LFs and stacked kSZ measurements highlights the combined power of these probes for constraining the interplay between baryonic feedback, gas properties, and the physics governing emission-line galaxies.