Emission line galaxies in the SHARDS Hubble frontier fields. II. Limits on Lyman-continuum escape fractions of lensed emission line galaxies at redshifts 2<z<3.5

Abstract

We present an investigation of escape fractions of UV photons from a unique sample of lensed low-mass emission line–selected galaxies at z < 3.5 found in the SHARDS Hubble Frontier Fields medium-band survey. We have used this deep imaging survey to locate 42 relatively low-mass galaxies down to log (M_(*)/M_(ꙩ))= 7 in the redshift range 2.4 <z<3.5 that are candidate line emitters. Using deep multiband Hubble UVIS imaging, we investigate the flux of escaping ionizing photons from these systems, obtaining 1σ upper limits of f^(rel)_(esc) ∼ 7% for individual galaxies and <2% for stacked data. We measure potential escaping Lyman-continuum flux for two low-mass line emitters with values at f^(rel)_(esc) = 0.032^(+0.081)_(-0.009) and 0.021^(+0.101)_(-0.006), both detected at the ∼3.2σ level. A detailed analysis of possible contamination reveals a <0.1% probability that these detections result from line-of-sight contamination. The relatively low Lyman-continuum escape fraction limit and the low fraction of systems detected are an indication that low-mass line-emitting galaxies may not be as important a source of reionization as hoped if these are analogs of reionization sources. We also investigate the structures of our galaxy sample, finding no evidence for a correlation of escape fraction with asymmetric structure.