Optically-Thin Spatially-Resolved Mg II Emission Maps the Escape of Ionizing Photons

TL;DR

This study uses spatially-resolved Mg II emission maps to infer the escape fraction of ionizing photons in a galaxy, providing a new indirect method to understand cosmic reionization.

Contribution

It demonstrates that Mg II emission lines can be used to estimate the escape fraction of ionizing photons, offering a novel indirect approach relevant to high-redshift galaxy studies.

Findings

Mg II emission is optically thin and spatially extends with stellar continuum.

The Mg II line ratio varies across the galaxy, indicating changes in neutral gas column density.

Regions with high Mg II ratio correspond to ionizing photon escape paths.

Abstract

Early star-forming galaxies produced copious ionizing photons. A fraction of these photons escaped gas within galaxies to reionize the entire Universe. This escape fraction is crucial for determining how the Universe became reionized, but the neutral intergalactic medium precludes direct measurement of the escape fraction at high-redshifts. Indirect estimates of the escape fraction must describe how the Universe was reionized. Here, we present new Keck Cosmic Web Imager spatially-resolved spectroscopy of the resonant Mg II 2800 doublet from a redshift 0.36 galaxy, J1503+3644, with a previously observed escape fraction of 6%. The Mg II emission has a similar spatial extent as the stellar continuum, and each of the Mg II doublet lines are well-fit by single Gaussians. The Mg II is optically thin. The intrinsic flux ratio of the red and blue Mg II emission line doublet, $R = F_{2796}/F_{2803}$, is set by atomic physics to be two, but Mg$^+$ gas along the line of sight decreases $R$ proportional to the Mg II optical depth. Combined with the metallicity, $R$ estimates the neutral gas column density. The observed $R$ ranges across the galaxy from 0.8-2.7, implying a factor of 2 spatial variation of the relative escape fraction. All of the ionizing photons that escape J1503+3644 pass through regions of high $R$. We combine the Mg II emission and dust attenuation to accurately estimate the absolute escape fractions for ten local Lyman Continuum emitting galaxies and suggest that Mg II can predict escape fraction within the Epoch of Reionization.