Data from Local Analogs of Potential Ionizers of the Intergalactic Medium: Compact Star-Forming Galaxies with Intense C IV λ1550 Emission

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 CIVemittersatz>6:highHβequivalentwidths(EWs>200A ̊),extremeO32ratios,lowmetallicities (12+log(O/H) ≲ 7.8), low C/O abundances (log(C/O) < −0.6), and high ionization conditions (logU > −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 (C IV, He II, O III], C III]) in the HST spectra. Notably, all our targets show intense C IV emission lines withrest-frameEWs>10A ̊,indicativeofhardionizingradiation.Therest-frameUVemissionlinediagnostics disfavor an AGN 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, fesc,LyC > 10%) from these sources. This is consistent with their Lyα-inferred LyC escape fractions (fesc,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 ≳ 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.