Local Analogs of Potential Ionizers of the Intergalactic Medium: Compact Star-Forming Galaxies with Intense CIV $\hat{I}»$1550 Emission

Abstract

We performed spectroscopic analyses of five local compact star-forming galaxies (CSFGs) with extremely high [OIII]/[OII] (O${}_{32}$) ratios ($>20$). These targets remarkably share similar properties with high-redshift CIV emitters at $z>6$: high H${\hat{I}}^2$ equivalent widths (EWs $>200$Ã ), extreme O${}_{32}$ ratios, low metallicities (12+log(O/H) $\lesssim 7.8$), low C/O abundances (log(C/O) $<-0.6$), and high ionization conditions (log$U>-2$). The UV spectra were acquired using the Hubble Space Telescope's (HST) Cosmic Origins Spectrograph (COS) and Space Telescope Imaging Spectrograph (STIS). We have identified a wealth of rest-frame UV emission lines (CIV, HeII, OIII], CIII]) in the HST spectra. Notably, all our targets show intense CIV emission lines with rest-frame EWs $>10$Ã , indicative of hard ionizing radiation. The rest-frame UV emission line diagnostics disfavor an AGN and could be consistent with significant shock contributions to the source of ionizing radiation. Four of our targets show high CIV/CIII] ratios ($\ge 1.4$), suggestive of strong Lyman-continuum leakage (LyC escape fraction, $f_{\mathrm{esc},\mathrm{LyC}}>10$%) from these sources. This is consistent with their Ly$\hat{I}\pm$-inferred LyC escape fractions ($f_{\mathrm{esc},\mathrm{LyC}}=$ 9 - 31%). We derive relative C/O abundances from our sources, showing log(C/O) values from $-1.12$ to $-0.61$, comparable to those of reionization-era galaxies at $z\gtrsim 6$. The properties of the CSFGs, particularly their intense CIV emission and high O${}_{32}$ ratios, which suggest significant LyC escape fractions, are similar to those of the reionization-era CIV emitters. These similarities reinforce the hypothesis that these CSFGs are the closest analogs of significant contributors to the reionization of the intergalactic medium.

Submitted to ApJ (21 pages, 7 figures, 6 tables)