The Semiforbidden C III] λ1909 Emission in the Rest-ultraviolet Spectra of Green Pea Galaxies

Abstract

We used the Space Telescope Imaging Spectrograph on the Hubble Space Telescope (HST) to observe the semiforbidden C III] λλ1907, 1909 doublet emission in green pea galaxies at 0.13 ≤ z ≤ 0.3. We detect C III] emission in 7/10 galaxies with C III] equivalent widths (EWs) that range from 2 to 10 Å, confirming that C III] emission is almost ubiquitous in low-mass, low-metallicity (12+log(O/H) < 8.4) galaxies that are characterized by strong optical [O III] λ5007 emission. The composite UV spectrum shows evidence for the He II λ1640 emission line and interstellar absorption features (e.g., C IV λλ1548, 1550, Al III λλ1854, 1862). We do not detect the O III] λλ1661, 1666 emission with >3σ significance. The observed C III] emission line strengths are consistent with the predictions from photoionization models that incorporate the effects of binary stellar evolution with young stellar ages ≤3-5 Myr and high ionization parameters (log U > -2). The hard ionizing radiation from young massive stars and high nebular temperatures at low metallicities can account for the observed high EWs of C III] λ1909 and [O III] λ5007 emission lines. Some of the star-forming galaxies at high redshift and local blue compact dwarf galaxies show offsets from the EW(C III]) versus EW([O III]) model grids, indicating an additional contribution to the continuum emission from composite stellar populations or different C/O abundances, nebular temperatures, and electron densities than assumed in the photoionization models. The green pea galaxies do not show a significant correlation between the Lyα and C III] EWs, and the observed scatter is likely due to the variations in the optical depth of Lyα to the neutral gas. Green pea galaxies are likely to be density-bounded, and we examined the dependence of C III] emission on the Lyman continuum optical depth. The potential LyC leaker galaxies in our sample have high C III] EWs that can only be reproduced by starburst ages as young as <3 Myr and harder ionizing spectra than the nonleakers. Among the galaxies with similar metallicities and ionization parameters, the C III] EW appears to be stronger for those with higher optical depth to LyC, as expected from the photoionization models. There are various factors that affect the C III] emission line strengths, and further investigation of a larger sample of C III] emitters is necessary to calibrate the dependence of C III] emission on the escape of LyC radiation and enable application of the C III] diagnostics to galaxies in the reionization epoch.