A DEEPHUBBLE SPACE TELESCOPESEARCH FOR ESCAPING LYMAN CONTINUUM FLUX ATz∼ 1.3: EVIDENCE FOR AN EVOLVING IONIZING EMISSIVITY

We have obtained deep Hubble Space Telescope far-UV images of 15 starburst galaxies at z ~ 1.3 in the GOODS fields to search for escaping Lyman continuum (LyC) photons. These are the deepest far-UV images (m_(AB) = 28.7, 3σ, 1" diameter) over this large an area (4.83 arcmin^2) and provide some of the best escape fraction constraints any galaxies at any redshift. We do not detect any individual galaxies, with 3σ limits to the LyC (~700 Å) 50–149 times fainter (in f_ν) than the rest-frame UV (1500 Å) continuum fluxes. Correcting for the mean medium (IGM) attenuation (factor ~2), as well as an intrinsic stellar Lyman break (factor ~3), these translate to relative escape fraction limits of f_(esc,rel) < [0.03, 0.21]. The stacked limit is f_(esc,rel)(3σ) < 0.02. We use a Monte Carlo simulation to properly account for the expected distribution of line-of-sight IGM opacities. When including constraints from previous surveys at z ~ 1.3 we find that, at the 95% confidence level, no more than 8% of star-forming galaxies at z ~ 1.3 can have relative escape fractions greater than 0.50. Alternatively, if the majority of galaxies have low, but non-zero, escaping LyC, the escape fraction cannot be more than 0.04. In light of some evidence for strong LyC emission from UV-faint regions of Lyman break galaxies (LBGs) at z ~ 3, we also stack sub-regions of our galaxies with different surface brightnesses and detect no significant LyC flux at the f_(esc,rel) < 0.03 level. Both the stacked limits and the limits from the Monte Carlo simulation suggest that the average ionizing emissivity (relative to non-ionizing UV emissivity) at z ~ 1.3 is significantly lower than has been observed in LBGs at z ~ 3. If the ionizing emissivity of star-forming galaxies is in fact increasing with redshift, it would help to explain the high photoionization rates seen in the IGM at z > 4 and reionization of the IGM at z > 6.