Flux sensitivity requirements for the detection of Lyman continuum radiation drop-ins from star-forming galaxies below redshifts of 3

Abstract

Flux estimates for ionizing radiation escaping from star-forming galaxies with characteristic UV luminosities ($L^{*}_{1500(1+z)}$) derived from GALEX and the VIMOS-VLT Deep Survey, are presented as a function of redshift and assumed escape fraction. These estimates offer guidance to the design of instrumentation and observing strategies, be they spectroscopic or photometric, attempting to detect LyC escaping star-forming galaxies for redshifts $ z < 3$. Examples are given that relate the absolution escape fraction ($f^{e}_{LyC}$) of ionizing photons, integrated over the entire extreme UV (EUV) bandpass, to the relative escape fraction ($f^{e}_{900}$) observed just shortward of the ionization edge at 911.8 \AA\ as a function of HI, HeI, and HeII column densities. We find that for $\log{N_{HI}(cm^{-2})} \gtrsim 17.0$ $f^{e}_{LyC}$ is significantly greater than $f^{e}_{900}$. Detection of LyC "drop-ins" in the rest-frame EUV will provide enhanced fidelity to determinations of the integrated fraction of ionizing photons $f^{e}_{LyC}$ that escape star-forming galaxies and contribute to the meta-galactic ionizing background.