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 ] λλ 1907, 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 ] λ 5007 emission. The composite UV spectrum shows evidence for the He ii λ 1640 emission line and interstellar absorption features (e.g., C iv λλ 1548, 1550, Al iii λλ 1854, 1862). We do not detect the O iii ] λλ 1661, 1666 emission with >3 σ 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 ] λ 1909 and [O iii ] λ 5007 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 Ly α and C iii ] EWs, and the observed scatter is likely due to the variations in the optical depth of Ly α 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.