ABSORPTION-LINE SPECTROSCOPY OF GRAVITATIONALLY LENSED GALAXIES: FURTHER CONSTRAINTS ON THE ESCAPE FRACTION OF IONIZING PHOTONS AT HIGH REDSHIFT

• Published in: The Astrophysical journal Vol. 831; no. 2; pp. 152 - 163
• Main Authors: Leethochawalit, Nicha, Jones, Tucker A., Ellis, Richard S., Stark, Daniel P., Zitrin, Adi
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 10.11.2016; IOP Publishing
• Subjects: Absorption; ABSORPTION SPECTRA; Astrophysics; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; CLOUDS; Computer simulation; COSMIC DUST; COSMOLOGY; cosmology: dark ages, reionization, first stars; GALAXIES; galaxies: evolution; galaxies: formation; galaxies: ISM; GRAVITATIONAL LENSES; IONIZATION; LYMAN LINES; PHOTONS; RED SHIFT; SIGNAL-TO-NOISE RATIO; SIMULATION; SPECTROSCOPY; Spectrum analysis; Star & galaxy formation; STAR EVOLUTION; Star formation; Star formation rate; STARS; Stars & galaxies; TIME DELAY; Time lag; VARIATIONS
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/0004-637X/831/2/152

ABSTRACT The fraction of ionizing photons escaping from high-redshift star-forming galaxies is a key obstacle in evaluating whether galaxies were the primary agents of cosmic reionization. We previously proposed using the covering fraction of low-ionization gas, measured via deep absorption-line spectroscopy, as a proxy. We now present a significant update, sampling seven gravitationally lensed sources at 4 < z < 5. We show that the absorbing gas in our sources is spatially inhomogeneous, with a median covering fraction of 66%. Correcting for reddening according to a dust-in-cloud model, this implies an estimated absolute escape fraction of 19% 6%. With possible biases and uncertainties, collectively we find that the average escape fraction could be reduced to no less than 11%, excluding the effect of spatial variations. For one of our lensed sources, we have sufficient signal-to-noise ratio to demonstrate the presence of such spatial variations and scatter in its dependence on the Ly equivalent width, consistent with recent simulations. If this source is typical, our lower limit to the escape fraction could be reduced by a further factor 2. Across our sample, we find a modest anticorrelation between the inferred escape fraction and the local star formation rate, consistent with a time delay between a burst and leaking Lyman continuum photons. Our analysis demonstrates considerable variations in the escape fraction, consistent with being governed by the small-scale behavior of star-forming regions, whose activities fluctuate over short timescales. This supports the suggestion that the escape fraction may increase toward the reionization era when star formation becomes more energetic and burst-like.