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Implications for the Epoch of Reionization in the Local Universe

Presentation #228.03D in the session “Elliptical and Starburst Galaxies”.

Published onJan 11, 2021
Implications for the Epoch of Reionization in the Local Universe

The epoch of reionization is a critical phase transition in the cosmic history, during which radiations from very early astronomical objects ionized the neutral hydrogen that once filled the entire universe. Direct observations of the ionizing process, however, are precluded by the neutral intergalactic medium at high redshifts. In recent years, small samples of ionizing-photon (i.e., Lyman-continuum; LyC) emitters (LCEs) at low redshifts have been discovered, and clues have begun to emerge as to both the indirect signposts of LyC leakage and of the processes that enable its escape. We propose a new technique for finding LCEs — use the weakness of the [S II] nebular emission lines relative to typical star-forming galaxies as evidence that the interstellar medium is optically thin to LyC. Significant LyC fluxes are detected in two out of three [S II]-deficient-selected star-forming galaxies at z ~ 0.3 using the Cosmic Origins Spectrograph onboard the Hubble Space Telescope (HST). We show that these galaxies differ markedly in their properties from the benchmark sample of leaky “Green-Pea” galaxies at similar redshifts. Statistical analyses are then performed with data recently available from the HST Low-Redshift LyC Survey. We reaffirm the robustness of the [SII] diagnostic. Moreover, preliminary results show a complex relation between optical emission-line properties and escape fractions of LyC. We plan to investigate this issue further and focus on the role played by galactic outflows. Previously we find that most of the outflowing material is likely created/injected at radii much larger than what is commonly assumed in a sample of local starbursts. In the study we use a combination of ultraviolet spectroscopy and imaging of resonance absorption and fluorescence emission lines to infer the radial structure of outflows, which is not accessible by conventional absorption-line probes in “down-the-barrel” spectra. We aim to adapt this novel approach to examine extreme conditions required for the escape of LyC, as well as implications for the regulating mechanism of the galactic baryonic cycle.


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