Investigating the Lyman photon escape in local starburst galaxies with the Cosmic Origins Spectrograph★

We present a study of seven star-forming galaxies from the Cosmic Evolution Survey observed with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST). The galaxies are located at relatively low redshifts, z ∼ 0.3, with morphologies ranging from extended and disturbed to compact and smooth. To complement the HST observations, we also analyse observations taken with the Visible Multi-object Spectrograph (VIMOS) on the Very Large Telescope (VLT). In our galaxy sample, we identify three objects with double peak Lyman-α profiles similar to those seen in Green Pea compact galaxies and measure peak separations of 655, 374, and 275 km s−1. We measure Lyman-α escape fractions with values ranging between 5 per cent and 13 per cent. Given the low flux levels in the individual COS exposures, we apply a weighted stacking approach to obtain a single spectrum. From this COS combined spectrum, we infer upper limits for the absolute and relative Lyman continuum escape fractions of |fabs( LyC)| = 0.4|+10.1-0.4| per cent and |fᵣₑₛ( LyC)| = 1.7|+15.2-1.7|per cent, respectively. Finally, we find that most of these galaxies have moderate ultraviolet and optical star formation rates (SFRs) (SFRs ≲10 M⊙ yr−1).