On the contribution of fluorescence to Ly$\alpha$ halos (LAHs) around star forming galaxies

TL;DR

This paper investigates how Ly$ ext{alpha}$ fluorescence contributes to the extended halos observed around star-forming galaxies at redshift 3.1, using analytical models and simulations to quantify the effect.

Contribution

It introduces a novel approach to quantify the fluorescence contribution to Ly$ ext{alpha}$ halos by modeling ionizing source clustering and local background variations.

Findings

Fluorescence can account for up to 50-60% of the extended Ly$ ext{alpha}$ emission.

Maximum local ionizing background enhancement is between 50 and 200 at 10 kpc.

Clustering of ionizing sources significantly impacts the fluorescence contribution.

Abstract

We quantify the contribution of Ly$\alpha$ fluorescence to observed spatially extended Ly$\alpha$ halos around Ly$\alpha$ emitters (LAE) at redshift ${\rm z=3.1}$. The key physical quantities that describe the fluorescent signal include (${\it i}$) the distribution of cold gas in the circum-galactic medium (CGM); we explore simple analytical models and fitting functions to recent hydrodynamical simulations; (${\it ii}$) local variations in the ionizing background due to ionizing sources that cluster around the central galaxy. We account for clustering by boosting the observationally inferred volumetric production rate of ionizing photons, $\epsilon_{\rm LyC}$, by a factor of $1+\xi_{\rm LyC}(r)$, in which $\xi_{\rm LyC}(r)$ quantifies the clustering of ionizing sources around the central galaxy. We compute $\xi_{\rm LyC}(r)$ by assigning an 'effective' bias parameter to the ionizing sources. This novel approach allows us to quantify our ignorance of the population of ionizing sources in a simple parametrized form. We find a maximum enhancement in the local ionizing background in the range $50-200$ at $r \sim 10$ physical kpc. For spatially uncorrelated ionizing sources and fluorescing clouds we find that fluorescence can contribute up to $\sim 50-60\%$ of the observed spatially extended Ly$\alpha$ emission. We briefly discuss how future observations can shed light on the nature of Ly$\alpha$ halos around star forming galaxies.