New Constraints on the Lyman Continuum Escape Fraction atz ∼ 1.3

We examine deep far-UV (1600 Å) imaging of the HDF-N and the HUDF to search for leaking Lyman continuum radiation from starburst galaxies at z ~ 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 ~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 similar study of more luminous galaxies at the same redshift, we show that, if all star-forming galaxies at z ~ 1 have similar relative escape fractions, the value must be less than 0.14 (3 σ). We also show that less than 20% (3 σ) of star-forming galaxies at z ~ 1 have relative escape fractions near unity. These limits contrast with the large escape fractions found at z ~ 3 and suggest that the average escape fraction has decreased between z ~ 3 and z ~ 1.