Detection of high Lyman continuum leakage from four low-redshift compact star-forming galaxies

Abstract

Following our first detection reported in Izotov et al. (2016), we present\nthe detection of Lyman continuum (LyC) radiation of four other compact\nstar-forming galaxies observed with the Cosmic Origins Spectrograph (COS)\nonboard the Hubble Space Telescope (HST). These galaxies, at redshifts of\nz~0.3, are characterized by high emission-line flux ratios [OIII]5007/[OII]3727\n> 5. The escape fractions of the LyC radiation fesc(LyC) in these galaxies are\nin the range of ~6%-13%, the highest values found so far in low-redshift\nstar-forming galaxies. Narrow double-peaked Lyalpha emission lines are detected\nin the spectra of all four galaxies, compatible with predictions for Lyman\ncontinuum leakers. We find escape fractions of Lyalpha, fesc(Lyalpha) ~20%-40%,\namong the highest known for Lyalpha emitters (LAEs). Surface brightness\nprofiles produced from the COS acquisition images reveal bright star-forming\nregions in the center and exponential discs in the outskirts with disc scale\nlengths alpha in the range ~0.6-1.4 kpc. Our galaxies are characterized by low\nmetallicity, ~1/8-1/5 solar, low stellar mass ~(0.2 - 4)e9 Msun, high star\nformation rates SFR~14-36 Msun/yr, and high SFR densities Sigma~2-35\nMsun/yr/kpc^2. These properties are comparable to those of high-redshift\nstar-forming galaxies. Finally, our observations, combined with our first\ndetection reported in Izotov et al. (2016), reveal that a selection for compact\nstar-forming galaxies showing high [OIII]5007/[OII]3727 ratios appears to pick\nup very efficiently sources with escaping Lyman continuum radiation: all five\nof our selected galaxies are LyC leakers.\n