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Spectroscopic confirmation of escaping ionizing flux from individual high-redshift galaxies

Published onJun 01, 2020
Spectroscopic confirmation of escaping ionizing flux from individual high-redshift galaxies

During cosmic reionization (z ~ 14-6), the universe went from being entirely neutral to almost completely ionized. The main contributors to the ionizing ultraviolet (UV) flux during reionization are likely to be young star-forming galaxies, as active galactic nuclei alone appear to be insufficient. Neutral gas along the line of sight to distant galaxies acts as an opaque veil to this ionizing radiation, preventing us from measuring it directly at z≳5. Therefore, identifying galaxies as close to this redshift as possible that exhibit escaping ionizing flux, observed as continuum below the Lyman break at λ < 912 A, is imperative to understanding cosmic reionization. We present a sample of high-redshift (z=3-5) Lyman continuum (LyC) galaxies (LCGs). This elusive population has previously been missed in searches for high-redshift star-forming galaxies, typically selected with the classic Lyman break technique, due to the non-zero flux where a break is expected. We are developing a novel and less biased selection method for finding individual high-redshift LCGs that is proving to be an effective way to identify this rare population. We present the results from our successful pilot study to search for LCGs in the COSMOS field. COSMOS was selected for its extensive multi-band photometric coverage, including high-resolution optical Hubble Space Telescope (HST) imaging, and accurate (Δz ~ 2%) 30-band photometric redshifts from the ZFOURGE survey. Our pilot study combined four untargeted deep rest-frame far-UV HST/Wide Field Camera 3 pointings to directly image leaking LyC flux and Keck/LRIS ground-based spectroscopic follow-up to confirm their redshifts. We spectroscopically confirmed four individual high-redshift LCGs in our HST footprints, demonstrating our method as a highly-effective strategy for identifying a statistically significant sample of individual LCGs. We present this sample, their properties and morphologies, escape fractions and distribution to reveal vital clues for understanding the role of LCGs in reionizing the universe.

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