2022A&A...658L..11S

Using the Space Telescope Imaging Spectrograph, we have obtained ultraviolet spectra from ∼1200 to 2000 Å of known Lyman continuum (LyC) emitting galaxies at low redshift (z ∼ 0.3 - 0.4) with varying absolute LyC escape fractions (f_esc ∼ 0.01 - 0.72). Our observations include in particular the galaxy J1243+4646, which has the highest known LyC escape fraction at low redshift. While all galaxies are known Lyman alpha emitters, we consistently detect an inventory of additional emission lines, including CIV λ1550, HeII λ1640, OIII] λ1666, and CIII] λ1909, whose origin is presumably essentially nebular. CIV λ1550 emission is detected above 4σ in six out of eight galaxies, with equivalent widths of EW(CIV) = 12 - 15 Å for two galaxies, which exceeds the previously reported maximum emission in low-z star-forming galaxies. We detect CIV λ1550 emission in all LyC emitters with escape fractions f_esc > 0.1 and find a tentative increase in the flux ratio CIV λ1550/CIII] λ1909 with f_esc. Based on the data, we propose a new criterion to select and classify strong leakers (galaxies with f_esc > 0.1): CIV λ1550/CIII] λ1909 ≥ 0.75. Finally, we also find HeII λ1640 emission in all the strong leakers with equivalent widths from 3 to 8 Å rest frame. These are among the highest values observed in star-forming galaxies and are primarily due to a high rate of ionizing photon production. The nebular HeII λ1640 emission of the strong LyC emitters does not require harder ionizing spectra at > 54 eV compared to those of typical star-forming galaxies at similarly low metallicity.