Small-scale Intensity Mapping: Extended Halos as a Probe of the Ionizing Escape Fraction and Faint Galaxy Populations during Reionization

TL;DR

This paper introduces a method to measure the ionizing escape fraction and detect ultra-faint galaxies during reionization using diffuse emission profiles, with predictions for JWST observations to distinguish emission sources.

Contribution

It develops a new modeling approach for diffuse halos around high-redshift galaxies and predicts JWST can detect fluorescent Hα emission to constrain escape fractions.

Findings

Favored low escape fraction of ~5% for certain galaxy magnitudes.

Predicted JWST can detect extended fluorescent Hα emission.

Stacking observations can constrain faint galaxy populations.

Abstract

We present a new method to quantify the value of the escape fraction of ionizing photons, and the existence of ultra-faint galaxies clustered around brighter objects during the epoch of cosmic reionization, using the diffuse Ly$\alpha$, continuum and H$\alpha$ emission observed around galaxies at $z\sim6$. We model the surface brightness profiles of the diffuse halos considering the fluorescent emission powered by ionizing photons escaping from the central galaxies, and the nebular emission from satellite star-forming sources, by extending the formalisms developed in Mas-Ribas & Dijkstra (2016) and Mas-Ribas et al. (2017). The comparison between our predicted profiles and Ly$\alpha$ observations at $z=5.7$ and $z=6.6$ favors a low ionizing escape fraction, $f_{\rm esc}^{\rm ion}\sim5\%$, for galaxies in the range $-19\gtrsim M_{\rm UV} \gtrsim -21.5$. However, uncertainties and possible systematics in the observations do not allow for firm conclusions. We predict H$\alpha$ and rest-frame visible continuum observations with JWST, and show that JWST will be able to detect extended (a few tens of kpc) fluorescent H$\alpha$ emission powered by ionizing photons escaping from a bright, $L\gtrsim 5L^*$, galaxy. Such observations can differentiate fluorescent emission from nebular emission by satellite sources. We discuss how observations and stacking of several objects may provide unique constraints on the escape fraction for faint galaxies and/or the abundance of ultra-faint radiation sources.