The production and escape of Lyman-Continuum radiation from star-forming galaxies atz∼ 2 and their redshift evolution

Abstract

We study the production rate of ionizing photons of a sample of 588 Hα emitters (HAEs) and 160 Lyman-α emitters (LAEs) at z = 2.2 in the COSMOS field in order to assess the implied emissivity from galaxies, based on their UV luminosity. By exploring the rest-frame Lyman Continuum (LyC) with GALEX/N U V data, we find f esc < 2.8 (6.4)% through median (mean) stacking. By combining the Hα luminosity density with IGM emissivity measurements from absorption studies, we find a globally averaged f esc of 5.9 +14.5 −4.2 % at z = 2.2 if we assume HAEs are the only source of ionizing photons. We find similarly low values of the global f esc at z ≈ 3 − 5, also ruling out a high f esc at z < 5. These low escape fractions allow us to measure ξ ion , the number of produced ionizing photons per unit UV luminosity, and investigate how this depends on galaxy properties. We find a typical ξ ion ≈ 10 24.77±0.04 Hz erg −1for HAEs and ξ ion ≈ 10 25.14±0.09 Hz erg −1 for LAEs. LAEs and low mass HAEs at z = 2.2 show similar values of ξ ion as typically assumed in the reionization era, while the typical HAE is three times less ionizing. Due to an increasing ξ ion with increasing EW(Hα), ξ ion likely increases with redshift. This evolution alone is fully in line with the observed evolution of ξ ion between z ≈ 2 − 5, indicating a typical value of ξ ion ≈ 10 25.4 Hz erg −1 in the reionization era.

Metallicity-constrained merger rates of binary black holes and the stochastic gravitational wave background

“…This new value can be accomodated if the escape fraction is lowered to fesc = 0.1, consistent with some recent measurements e.g. Matthee et al (2016). We intend to explore the implications of these results in future work (see also the discussion in Vangioni et al 2015, on the effect of the escape fraction).…”

The efficiency of ionizing photon production and the radiation energy balance in compact star-forming galaxies

“…Average values of β shown in Fig. 6c by horizontal lines and various symbols are comparable to those for z ∼ 4 galaxies (Finkelstein et al 2012) and for z > 1 galaxies (Reddy et al 2010(Reddy et al , 2015Kurczynski et al 2014;Forrest et al 2016;Matthee et al 2016). …”

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).…”

“…Compilation of absolute f esc measurements for star-forming sources reported in the literature. The red symbols include direct detections from galaxies at low redshift (Leitet et al 2011;Borthakur et al 2014;Izotov et al 2016aIzotov et al , 2016bLeitherer et al 2016), as well as detections or limits from z 2 > sources using different methods (Mostardi et al 2015;Leethochawalit et al 2016;Matthee et al 2017;Shapley et al 2016;Vanzella et al 2016;Vasei et al 2016). The plot does not show population-average escape fractions and depicts f esc measured from individual galaxies and small subsamples.…”