Strong Lyman continuum emitting galaxies show intense C IV λ1550 emission

Abstract

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 C IV λ1550, He II λ1640, O III] λ1666, and C III] λ1909, whose origin is presumably essentially nebular. C IV λ1550 emission is detected above 4σ in six out of eight galaxies, with equivalent widths of EW(C IV) = 12 − 15 Å for two galaxies, which exceeds the previously reported maximum emission in low-z star-forming galaxies. We detect C IV λ1550 emission in all LyC emitters with escape fractions f_esc > 0.1 and find a tentative increase in the flux ratio C IV λ1550/C III] 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): C IV λ1550/C III] λ1909 ≳ 0.75. Finally, we also find He II 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 He II 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.