Probing ionizing radiation of L<~0.1L* star-forming galaxies at z>~3 with strong lensing

TL;DR

This paper demonstrates that strong gravitational lensing can effectively probe the escape fraction of ionizing radiation from faint star-forming galaxies at high redshift, reaching lower luminosities than previous studies.

Contribution

It introduces a Monte Carlo simulation approach to estimate the potential of current observational capabilities in constraining the escape fraction at faint luminosities using strong lensing.

Findings

Lensing can constrain fesc down to 10-15% for galaxies at L<0.15L*

Current observational setups can probe fesc at these faint luminosities

Future observations with existing telescopes can improve constraints on ionizing radiation escape

Abstract

We show the effectiveness of strong lensing in the characterisation of Lyman continuum emission from faint L<~0.1L* star-forming galaxies at redshift >~ 3. Past observations of L>~L* galaxies at redshift >~3 have provided upper limits of the average escape fraction of ionising radiation of fesc~5%. Galaxies with relatively high fesc (>10%) seem to be particularly rare at these luminosities, there is therefore the need to explore fainter limits. Before the advent of giant ground based telescopes, one viable way to probe fesc down to 0.05-0.15L* is to exploit strong lensing magnification. This is investigated with Monte Carlo simulations that take into account the current observational capabilities. Adopting a lensing cross-section of 10 arcmin^2 within which the magnification is higher than 1 (achievable with about 4-5 galaxy clusters), with a U-band survey depth of 30(30.5) (AB, 1-sigma), it is possible to constrain fesc for z~3 star-forming galaxies down to 15(10)% at 3-sigma for L<0.15L* luminosities. This is particularly interesting if fesc increases at fainter luminosities, as predicted from various HI reionization scenarios and radiation transfer modelling. Ongoing observational programs on galaxy clusters are discussed and offer positive prospects for the future, even though from space the HST/WFC3 instrument represents the only option we have to investigate details of the spatial distribution of the Lyman continuum emission arising from z~2-4 galaxies.