The Semiforbidden C iii] λ1909 Emission in the Rest-ultraviolet Spectra of Green Pea Galaxies
Swara Ravindranath, TalaWanda Monroe, Anne Jaskot, Henry C. Ferguson, Jason Tumlinson
Abstract
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.