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 on 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 $log(M_{*}/M_{\odot}) = 7$, between redshifts 2.4 < z < 3.5 which are candidate line emitters. Using deep multi-band Hubble UVIS imaging we investigate the flux of escaping ionizing photons from these systems, obtaining 1$\sigma$ 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 $f^{\rm rel}{\rm esc} = 0.032^{+0.081}{-0.009}$ and $f^{\rm rel}{\rm esc} = 0.021^{+0.101}{-0.006}$, both detected at the ~3.2$\sigma$ 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, is 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.