Lyman Continuum Galaxy Candidates in COSMOS

Abstract

Star-forming galaxies are the sources likely to have reionized the universe. As we cannot observe them directly due to the opacity of the intergalactic medium at $z\gtrsim 5$, we study $z\sim 3\text{–}5$ galaxies as proxies to place observational constraints on cosmic reionization. Using new deep \textit{Hubble Space Telescope} rest-frame UV F336W and F435W imaging (30-orbit, $\sim 40$~arcmin${}^2$, $\sim 29\text{–}30$~mag depth at 5$\sigma$), we attempt to identify a sample of Lyman continuum galaxies (LCGs). These are individual sources that emit ionizing flux below the Lyman break ($<912\text{A˚}$). This population would allow us to constrain cosmic reionization parameters such as the number density and escape fraction ($f_{\mathrm{esc}}$) of ionizing sources. We compile a comprehensive parent sample that does not rely on the Lyman-break technique for redshifts. We present three new spectroscopic candidates at $z\sim 3.7\text{–}4.4$, and 32 new photometric candidates. The high-resolution multi-band HST imaging and new Keck/Low Resolution Imaging Spectrometer (LRIS) redshifts make these promising spectroscopic LCG candidates. Using both a traditional and probabilistic approach, we find the most likely $f_{\mathrm{esc}}$ values for the three spectroscopic LCG candidates are $>100\%$, and therefore not physical. We are unable to confirm the true nature of these sources with the best available imaging and direct blue Keck/LRIS spectroscopy. More spectra, especially from the new class of 30 m telescopes, will be required to build a statistical sample of LCGs to place firm observational constraints on cosmic reionization.

arXiv commentAccepted for publication in ApJ. 38 pages (28 in main body), 14 figures

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