2025ApJ...988....4J

We performed spectroscopic analyses of five local compact star-forming galaxies (CSFGs) with extremely high [O III]/[O II] (O₃₂) ratios (>20). These targets remarkably share similar properties with high-redshift C IV emitters at z > 6: high Hβ equivalent widths (EWs > 200Å), extreme O₃₂ ratios, low metallicities (12+log(O/H) ≲ 7.8), low C/O abundances (log(C/O) < -0.7), and high ionization conditions (logU > -2). The ultraviolet (UV) spectra were acquired using the Hubble Space Telescope's (HST) Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph. We have identified a wealth of rest-frame UV emission lines (C IV, He II, O III], C III]) in the HST spectra. Notably, all our targets show intense C IV emission lines with rest-frame EWs > 10 Å, indicative of hard ionizing radiation. The rest-frame UV emission line diagnostics disfavor an active galactic nucelus and could be consistent with significant shock contributions to the source of ionizing radiation. Four of our targets show high C IV/C III] ratios (≥1.4), suggestive of strong Lyman-continuum leakage (LyC escape fraction, f_esc,LyC > 10%) from these sources. This is consistent with their Lyα-inferred LyC escape fractions (f_esc,LyC = 9%-31%). We derive relative C/O abundances from our sources, showing log(C/O) values from -1.28 to -0.77, comparable to those of reionization-era galaxies at z ≳ 6. The properties of the CSFGs, particularly their intense C IV emission and high O₃₂ ratios, which suggest significant LyC escape fractions, are similar to those of the reionization-era C IV emitters. These similarities reinforce the hypothesis that these CSFGs are the closest analogs of significant contributors to the reionization of the intergalactic medium.