NARROWBAND IMAGING OF ESCAPING LYMAN-CONTINUUM EMISSION IN THE SSA22 FIELD

We present the results of an ultradeep, narrowband imaging survey for Lyman-continuum (LyC) emission at z similar to 3 in the SSA22a field. We employ a custom narrowband filter centered at lambda = 3640 angstrom (NB3640), which probes the LyC region for galaxies at z >= 3.06. We also analyze new and archival NB4980 imaging tuned to the wavelength of the Ly alpha emission line at z = 3.09, and archival broadband B, V, and R images of the non-ionizing UV continuum. Our NB3640 images contain 26 z >= 3.06 Lyman break galaxies (LBGs) as well as a set of 130 Ly alpha emitters (LAEs), identified by their excess NB4980 flux relative to the BV continuum. Six LBGs and 28 LAEs are detected in the NB3640 image. LBGs appear to span a range of NB3640-R colors, while LAEs appear bimodal in their NB3640-R properties. We estimate average UV-to-LyC flux density ratios, corrected for foreground contamination and intergalactic medium absorption, finding < F-UV/F-LyC >(LBG)(corr) = 11.3(-5.4)(+10.3), which implies an LBG LyC escape fraction f(esc)(LyC) similar to 0.1, and < F-UV/F-LyC >(LAE)(corr) = 2.2(-0.6)(+0.9). The strikingly blue LAE flux density ratios defy interpretation in terms of standard stellar population models. Assuming < F-UV/F-LyC >(LBG)(corr) applies down to L = 0.1L*, we estimate a galaxy contribution to the intergalactic hydrogen ionization rate that is consistent with independent estimates based on the Ly alpha forest opacity at z similar or equal to 3. If we assume that < F-UV/F-LyC >(LAE)(corr) holds at the faintest luminosities, the galaxy contribution significantly exceeds that inferred from the Ly alpha forest. We interpret our results in terms of a model where LyC photons escape over only similar to 10%-20% of solid angle. When advantageously oriented, a galaxy will exhibit a low UV-to-LyC ratio, an effect enhanced for more compact galaxies. This model, however, does not adequately explain the extremely blue NB3640-R colors measured for some LAEs in our sample. Further follow-up study of these faint LAEs is crucial, given the potentially important contribution similar objects make to the process of reionization.