Narrowband Lyman-continuum Imaging of Galaxies at z ~ 2.85

Abstract

We present results from a survey for z ~ 2.85 Lyman-continuum (LyC) emission in the HS1549+1933 field and place constraints on the amount of ionizing radiation escaping from star-forming galaxies. Using a custom narrowband filter (NB3420) tuned to wavelengths just below the Lyman limit at z >= 2.82, we probe the LyC spectral region of 49 Lyman break galaxies (LBGs) and 91 Lyα emitters (LAEs) spectroscopically confirmed at z >= 2.82. Four LBGs and seven LAEs are detected in NB3420. Using V-band data probing the rest-frame nonionizing UV, we observe that many NB3420-detected galaxies exhibit spatial offsets between their LyC and nonionizing UV emission and are characterized by extremely blue NB3420-V colors, corresponding to low ratios of nonionizing to ionizing radiation (F _UV/F _LyC) that are in tension with current stellar population synthesis models. We measure average values of (F _UV/F _LyC) for our LBG and LAE samples, correcting for foreground galaxy contamination and H I absorption in the intergalactic medium. We find (F_{UV}/F_{LyC})_{corr}^{LBG}=82+/- 45 and (F_{UV}/F_{LyC})_{corr}^{LAE}=7.4+/- 3.6. These flux density ratios correspond, respectively, to relative LyC escape fractions of f_{esc,\,rel}^{LBG}=5%{--}8% and f_{esc,\,rel}^{LAE}=18%{--}49%, absolute LyC escape fractions of f_{esc}^{LBG}=1%{--}2% and f_{esc}^{LAE}=5%{--}15%, and a comoving LyC emissivity from star-forming galaxies of 8.8-15.0 × 10²⁴ erg s^-1 Hz^-1 Mpc^-3. In order to study the differential properties of galaxies with and without LyC detections, we analyze narrowband Lyα imaging and rest-frame near-infrared imaging, finding that while LAEs with LyC detections have lower Lyα equivalent widths on average, there is no substantial difference in the rest-frame near-infrared colors of LBGs or LAEs with and without LyC detections. These preliminary results are consistent with an orientation-dependent model where LyC emission escapes through cleared paths in a patchy interstellar medium.