New constraints on the Lyman continuum escape fraction at z ∼ 1.3

We examine deep far-UV (1600 angstrom) imaging of the HDF-N and the HUDF to search for leaking Lyman continuum radiation from starburst galaxies at z similar to 1.3. There are 21 (primarily sub-L*) galaxies with spectroscopic redshifts between 1.1 < z < 1.5, and none are detected in the far-UV. We fit stellar population templates to the galaxies' optical/near-infrared SEDs to determine the starburst age and level of dust attenuation for each individual galaxy, giving a more accurate estimate of the intrinsic Lyman continuum ratio, f(1500)/f(700), and allowing a conversion from f(700) to relative escape fraction (f(esc,rel)). We show that previous high-redshift studies may have underestimated the amplitude of the Lyman break, and thus the relative escape fraction, by a factor similar to 2. Once the starburst age and intergalactic H-i absorption are accounted for, 18 galaxies in our sample have limits to the relative escape fraction, f(esc,rel) < 1.0 with some limits as low as f(esc,rel) < 0.10 and a stacked limit of f(esc,rel) < 0.08. This demonstrates, for the first time, that most sub-L* galaxies at this redshift do not have large escape fractions. When combined with a similarstudy of more luminous galaxies at the same redshift, we show that, if all star-forming galaxies at z similar to 1 have similar relative escape fractions, the value must be less than 0.14 (3 sigma). We also show that less than 20% (3 sigma) of star-forming galaxies at z similar to 1 have relative escape fractions near unity. These limits contrast with the large escape fractions found at z similar to 3 and suggest that the average escape fraction has decreased between z similar to 3 and z similar to 1.