Modeling spatially-resolved galactic H$α$ emission for galaxy clustering

TL;DR

This work develops a physically grounded, radiative-transfer–based model to predict spatially resolved $H\alpha$ emission from galaxies by combining collisional excitation, photo-excitation (from stars and AGN), recombination, and HII-region contributions within IllustrisTNG simulations. The method yields $H\alpha$ LFs and halo occupation distributions that align with Euclid-era expectations only after applying modest dust extinction, revealing substantial diffuse ISM contributions to the total $H\alpha$ output. It demonstrates that, while a direct $L_{H\alpha}$–SFR mapping captures a portion of the emission, the diffuse ISM and AGN-related processes are essential for realistic counts, and it provides a framework extendable to other lines and next-generation surveys. The results underscore the importance of baryonic feedback and ISM physics in shaping the observable emission-line galaxy population and offer a versatile tool for forecasting and interpreting high-redshift galaxy clustering in Euclid-like surveys.

Abstract

Near-infrared spectroscopic surveys target high-redshift emission-line galaxies (ELGs) to probe cosmological scenarios. Understanding the clustering properties of ELGs is essential to derive optimal constraints. We present a simple radiative transfer model for spatially resolved galactic H$α$ emission, which includes emission from the warm-hot diffuse interstellar medium. The atomic level populations are in steady-state and computed in the coronal approximation. The model is applied to multiple IllustrisTNG simulations in the redshift range $1\leq z \leq 2$ to produce the luminosity function (LF) and the halo occupation distribution (HOD). Collisional processes account for a significant fraction of $\approx 40\%$ of the total ${\rm H}α$ luminosity ($L_{{\rm H}α}$). Our LFs are in reasonable agreement with measurements from H$α$ surveys if a uniform extinction of $0.3<A_{{\rm H}α}<0.85$ mag is assumed. Our HOD is consistent with that of the ${\it Euclid}$ Flagship galaxy mock up to differences that can be attributed to baryonic feedback, which is absent from the latter. When H$α$ luminosities are computed from an empirical relation between $L_{{\rm H}α}$ and the total star formation rate (SFR) the resulting LFs are in tension with previous observations. Our approach can be extended to other atomic lines, which should be helpful for the mining of high-redshift galaxy spectra in forthcoming surveys.

Figures (14)

• Figure 1: The effective rate coefficient $\alpha_{\mathrm{H}\alpha}^{\text{CE}}(T)$ for $\mathrm{H}\alpha$ emission from collisional excitations (CE) (solid blue curve), and the conversion probability to produce an $\mathrm{H}\alpha$ photon in a CE event (dashed red curve). Both are shown as a function of the gas cell temperature $T$.
• Figure 2: Photo-excitation rate (normalized by $f_{ij}$) as a function of the photon energy $E_{ij}$ for a gas cell with a number density $n_*=10^3{\ {\rm kpc}^{-3}}$ of stars with mass $M_*\geq 15\ {\rm M_\odot}$. The vertical lines mark a few selected transitions $1\to j$ from the Lyman series.
• Figure 3: The effective rate coefficient $\alpha_{\mathrm{H}\alpha}^{\text{RR}}(T)$ for $\mathrm{H}\alpha$ emission from radiative recombination as a function of the gas cell temperature (solid blue curve), and the conversion probability (per recombination event) to produce an $\mathrm{H}\alpha$ photon (dashed red curve).
• Figure 4: Total galactic SFR ($\rm{SFR_{gal}}$) - $L_{\mathrm{H}\alpha}$ relation of all the star-forming ELGs in TNG300-1 ($z=1$). Each black dot of the scatter plot marks an ELG. The yellow line is Eq. \ref{['eq:L_HII']}, the red line represents the empirical relation (cf. main text), and the cyan line displays the best fit linear regression $L_{\mathrm{H}\alpha}=10^{41.47}\rm{SFR_{gal}}$ of the scatter plot.
• Figure 5: Face-on maps of the $\mathrm{H}\alpha$ surface brightness $\Sigma_{\mathrm{H}\alpha}$ of two ELGs. The panels from left to right are the separate contributions of collisional excitations, recombinations, photo-excitations and $\rm{HII}$ regions. They display $\log_{10}\Sigma_{\mathrm{H}\alpha}$ (in unit of $\ {\rm erg\, s^{-1} pc^{-2}}$) according to the color scale shown above the panels. The top row shows a present-day galaxy extracted from the TNG MW-like galaxy catalogue of pillepich2023milkywayandromedaanalogs. The bottom panel shows an ELG from the $z=1$ snapshot of TNG300-1.