A new model framework for circumgalactic Ly$\alpha$ radiative transfer constrained by galaxy-Ly$\alpha$ forest clustering

TL;DR

This paper introduces a new perturbative model for Ly$ ext{alpha}$ radiative transfer in the circum- and inter-galactic medium, constrained by galaxy-Ly$ ext{alpha}$ forest clustering data, revealing the CGM/IGM's significant impact on Ly$ ext{alpha}$ observations at redshifts 2-3.

Contribution

The paper develops a physically motivated, observationally constrained model for Ly$ ext{alpha}$ RT in the CGM and IGM, linking clustering data to radiative transfer effects.

Findings

Approximately 80% of Ly$ ext{alpha}$ photons escape the CGM/IGM at z~2-3.

The CGM influences Ly$ ext{alpha}$ line properties and escape fractions.

Ly$ ext{alpha}$ haloes follow a power-law surface brightness profile ∝ r^{-2.4}.

Abstract

We present a new perturbative approach to "constrained Ly$\alpha$ radiative transfer'" (RT) through the circum- and inter-galactic medium (CGM and IGM). We constrain the HI content and kinematics of the CGM and IGM in a physically motivated model, using the galaxy-Ly$\alpha$ forest clustering data from spectroscopic galaxy surveys in QSO fields at $z\sim2-3$. This enables us to quantify the impact of the CGM/IGM on Ly$\alpha$ emission in an observationally constrained, realistic cosmological environment. Our model predicts that the CGM and IGM at these redshifts transmit $\approx80~\%$ of Ly$\alpha$ photons after having escaped from galaxies. This implies that while the inter-stellar medium primarily regulates Ly$\alpha$ escape, the CGM has a non-negligible impact on the observed Ly$\alpha$ line properties and the inferred Ly$\alpha$ escape fraction, even at $z\sim 2-3$. Ly$\alpha$ scattering in the CGM and IGM further introduces an environmental dependence in the (apparent) Ly$\alpha$ escape fraction, and the observed population of Ly$\alpha$ emitting galaxies: the CGM/IGM more strongly suppresses direct Ly$\alpha$ emission from galaxies in overdense regions in the Universe, and redistributes this emission into brighter Ly$\alpha$ haloes. The resulting mean surface brightness profile of the Ly$\alpha$ haloes is generally found to be a power-law $\propto r^{-2.4}$. Although our model still contains arbitrariness, our results demonstrate how (integral field) spectroscopic surveys of galaxies in QSO fields constrain circumgalactic Ly$\alpha$ RT, and we discuss the potential of these models for studying CGM physics and cosmology.