LINKING Lyα AND LOW-IONIZATION TRANSITIONS AT LOW OPTICAL DEPTH

We suggest that low optical depth in the Lyman continuum (LyC) may relate the Ly alpha emission, C II and Si II absorption, and C II* and Si II* emission seen in high-redshift galaxies. We base this analysis on Hubble Space Telescope Cosmic Origins Spectrograph spectra of four Green Pea (GP) galaxies, which may be analogs of z > 2 Ly alpha emitters (LAEs). In the two GPs with the strongest Ly alpha emission, the Ly alpha line profiles show reduced signs of resonant scattering. Instead, the Ly alpha profiles resemble the H alpha line profiles of evolved star ejecta, suggesting that the Ly alpha emission originates from a low column density and similar outflow geometry. The weak C II absorption and presence of non-resonant C II* emission in these GPs support this interpretation and imply a low LyC optical depth along the line of sight. In two additional GPs, weak Ly alpha emission and strong C II absorption suggest a higher optical depth. These two GPs differ in their Ly alpha profile shapes and C II* emission strengths, however, indicating different inclinations of the outflows to our line of sight. With these four GPs as examples, we explain the observed trends linking Ly alpha, C II, and C II* in stacked LAE spectra, in the context of optical depth and geometric effects. Specifically, in some galaxies with strong Ly alpha emission, a low LyC optical depth may allow Ly alpha to escape with reduced scattering. Furthermore, C II absorption, C II* emission, and Ly alpha profile shape can reveal the optical depth, constrain the orientation of neutral outflows in LAEs, and identify candidate LyC emitters.