Absorption-Line Spectroscopy of Gravitationally Lensed Galaxies: Further Constraints on the Escape Fraction of Ionizing Photons at High Redshift

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-tonoise ratio to demonstrate the presence of such spatial variations and scatter in its dependence on the Lyα 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.

The HDUV Survey: Six Lyman Continuum Emitter Candidates at z ∼ 2 Revealed by HST UV Imaging*

“…( Leethochawalit et al 2016;Matthee et al 2017). The plot does not show population-average escape fractions, which still have to be measured reliably based on large samples of galaxies with HST imaging in the future (e.g., Siana et al 2015).…”

“…So in order to study f esc in the EoR, we need to link it to quantities that may be measured at very high redshifts. Nakajima & Ouchi 2014;Faisst 2016); (2) the strengths of nebular emission lines such as Hβ compared with the total star formation rate (Zackrisson et al 2013(Zackrisson et al , 2017; (3) the shape of the Lyα line profile (Verhamme et al 2015(Verhamme et al , 2017; or (4) the absorption strength of low-ionization lines and Lyman series lines, which are related to the covering fraction of absorbing gas (e.g., Heckman et al 2011;Leethochawalit et al 2016;Reddy et al 2016b). With the limited data we already have on our candidates, we can discuss the first two indicators, which we do in the following sections.…”

Small-scale Intensity Mapping: Extended Halos as a Probe of the Ionizing Escape Fraction and Faint Galaxy Populations during Reionization

“…Various indirect alternative approaches, including relations between the gas covering fraction and reddening (e.g., Jones et al 2012;Leethochawalit et al 2016;Reddy et al 2016), Hβ line equivalent width and UV spectral slope (Zackrisson et al 2013(Zackrisson et al , 2017, or analyses of the Lyα spectral line profile Verhamme et al 2017), have been proposed, but the values for the escape fraction remain within a fairly broad range: f 0.3 0.01 esc ion   . Additionally, measurements of the Thomson scattering optical depth to the CMB have placed the "galaxypopulation-averaged" ionizing escape fraction value within the range f 0.1 0.2 esc ion~-(e.g., Robertson et al 2015;Mitra et al 2016;Sun & Furlanetto 2016).…”

Small-scale Intensity Mapping: Extended Lyα, Hα, and Continuum Emission as a Probe of Halo Star Formation in High-redshift Galaxies

“…These more detailed calculations are beyond the scope of our current work. Observations of extended halos complement other recently proposed ways to constraint escape fractions, such as using Hβ EWs (Zackrisson et al 2013(Zackrisson et al , 2017, Lyα line profiles (Verhamme et al 2015(Verhamme et al , 2017Dijkstra et al 2016), and covering factor values (Jones et al 2012(Jones et al , 2013Leethochawalit et al 2016;Reddy et al 2016). We will apply our method to investigate the average ionizing escape fraction of galaxies during the epoch of cosmic reionization in an upcoming work.…”