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Exploring the Nature of Ionizing Stars in Galaxies with Extreme [OIII]/[OII] Ratios

Presentation #102.02 in the session “Emission Line Galaxies in the Low Redshift Universe (Meeting-in-a-Meeting)”.

Published onJun 18, 2021
Exploring the Nature of Ionizing Stars in Galaxies with Extreme [OIII]/[OII] Ratios

The source of Lyman continuum photons and their escape from galaxies is key to understanding the reionization of the Universe. Star-forming dwarf galaxies at z>6 are thought to be the primary sources of ionizing photons responsible for reionization. Since the increasing IGM opacity makes the direct detection of these photons impossible, various rest-UV spectral diagnostics are being identified that can be used to infer the escape fraction of Lyman continuum. Among the local analogs of reionization sources, the galaxies with high [OIII]/[OII] ratios have yielded the most promising Lyman continuum emitters. I will present an overview of the galaxies with high [OIII]/[OII] ratios and the nature of ionizing stars and ISM conditions revealed by their rest-UV and optical emission lines. Our photoionization models show that low metallicity, high ionization parameters and young stellar ages can account for their observed extreme emission lines. We have explored the optical depth diagnostics and the effects of metallicity and stellar ages on the leakage of ionizing radiation. The galaxies with high [OIII]/[OII] are most likely density-bounded with low optical depth which explains the prevelance of Lyman continuum emitters among them. However, the optical depth diagnostics offer only weak constraints on the escape fraction since the role of low metallicity and stellar ages on the ionizing radiation can be dominant. Although, most of the low-metallicity dwarf galaxies at low redshifts exhibit the UV emission lines (eg; CIV, HeII, OIII] and CIII]) seen in high redshift galaxies, the extreme equivalent widths (> 15 Angstrom) are rarely observed among the local analogs. Our ongoing HST/COS program to obtain UV spectroscopy of compact star-forming galaxies with extreme [OIII]/[OII]ratios (>22) have revealed strong CIV and HeII emission lines requiring high ionization comparable to that observed in z>6 galaxies. These new observations will offer the best constraints on the shape of the ionizing spectrum and the stellar populations that are responsible for the high ionization.

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