Rapid Reionization by the Oligarchs: The Case for Massive, UV-bright, Star-forming Galaxies with High Escape Fractions

Abstract

The protagonists of the last great phase transition of the universe -cosmic reionization -remain elusive. Faint star-forming galaxies are leading candidates because they are found to be numerous and may have significant ionizing photon escape fractions (f esc ). Here we update this picture via an empirical model that successfully predicts latest observations (e.g., the rapid drop in star-formation density (ρ SFR ) at z > 8). We generate an ionizing spectrum for each galaxy in our model and constrain f esc by leveraging latest measurements of the reionization timeline (e.g., Lyα damping of quasars and galaxies at z > 7). Assuming a constant f esc across all sources at z > 6, we find M UV < −13.5 galaxies need f esc =0.21 +0.06 −0.04 to complete reionization. The inferred IGM neutral fraction is [0.9, 0.5, 0.1] at z = [8.2, 6.8, 6.2] ± 0.2, i.e., the bulk of reionization transpires rapidly in 300 Myrs, driven by the z > 8 ρ SFR and favored by high neutral fractions (∼60−90%) measured at z ∼ 7 − 8. Inspired by the emergent sample of Lyman Continuum (LyC) leakers spanning z ∼ 0−6.6 that overwhelmingly displays higher-than-average star-formation surface density (Σ SFR ), we propose a physically motivated model relating f esc to Σ SFR and find f esc ∝ Σ 0.4±0.1 SFR . Since Σ SFR falls by ∼ 2.5 dex between z = 8 and z = 0, our model explains the humble upper limits on f esc at lower redshifts and its required evolution to f esc ∼ 0.2 at z > 6. Within this model, strikingly, <5% of galaxies with M UV < −18 and log(M /M ) > 8 (the 'oligarchs') account for 80% of the reionization budget -a stark departure from the canonical 'democratic' reionization led by copious faint sources. In fact, faint sources (M UV >−16) must be relegated to a limited role in order to ensure high neutral fractions at z = 7 − 8. Shallow faint-end slopes of the UV luminosity function (α UV > −2) and/or f esc distributions skewed toward massive galaxies produce the required late and rapid reionization. We predict LyC leakers like COLA1 (z = 6.6, f esc ∼ 30%, M UV = −21.5) become increasingly common towards z ∼ 6 and that the drivers of reionization do not lie hidden across the faint-end of the luminosity function, but are already known to us.

No missing photons for reionization: moderate ionizing photon escape fractions from the FIRE-2 simulations

“…As dṅion/d log Mvir = Qesc dn/d log Mvir, for the canonical α = 2 slope, dṅion/d log Mvir increases with Mvir at the low-mass end and decreases dramatically above M * vir . We find at z ∼ 6, dṅion/d log Mvir peaks at approximate Mvir ∼ 10 10.5 M (M * ∼ 10 8 M ), which means that intermediatemass galaxies dominate the cosmic ionizing photon budget at z ∼ 6 (see also Naidu et al 2019). However, the HMF starts to decline at a much smaller mass at z ∼ 10, so we find dṅion/d log Mvir peaks at Mvir ∼ 10 9 M (M * 10 6 M ).…”

“…Models that describe the reionization history from the galaxy populations at z 5 need to make assumptions about fesc, either a constant fesc for all galaxies or some mass-and redshift-dependent form of fesc (e.g. Finkelstein et al 2012Finkelstein et al , 2019Kuhlen & Faucher-Giguère 2012;Robertson et al 2013Robertson et al , 2015Bouwens et al 2015b;Naidu et al 2019;Yung et al 2020). Currently available observational constraints on the reionization history, such as the integrated Thomson scattering optical depths (e.g.…”

Three Lyα Emitting Galaxies within a Quasar Proximity Zone at z ∼ 5.8

“…Faint galaxies are expected to provide the majority of reionization photons, but scenarios driven by bright galaxies are still permitted (Meyer et al 2019c). In the model of Naidu et al (2020), galaxies brighter than = -M 20 UV can power reionization alone if their escapes fractions are  f 0.20 esc . Proximate LAEs provide the most direct way of measuring f esc in individual bright and faint galaxies, and they will eventually rule in favor or against existing predictions.…”

The Ionizing Photon Production Efficiency (ξion) of Lensed Dwarf Galaxies at z ∼ 2*

“…What is not well understood is what are the sources that ionized the universe and provided the IGM thereafter (Fan et al 2001;Somerville et al 2003;Madau et al 2004;Bouwens et al 2015b). In fact, it is not clear whether the galaxies that we have detected at high redshift are capable of ionizing the IGM (Robertson et al 2015;Finkelstein et al 2019;Naidu et al 2020;Mason et al 2019). In order to determine this, we need to know the rate of ionizing photons emitted into the IGM as a function of redshift (often referred to as Γ(z)).…”

“…It is not clear what type of galaxies contribute the most to the total ionizing photon budget necessary for reionization. Some studies suggest that perhaps rare Lyman continuum leakers with substantial star formation surface densities have led to a rapid, recent reionization at z∼6 (Naidu et al 2020). Other studies predict that low-mass galaxies should have a greater contribution to reionization because of the steep faint-end slope of the UV 1500 luminosity function of high-redshift galaxies (Reddy & Steidel 2009;Bouwens et al 2012;Alavi et al 2014;Finkelstein et al 2015;Ishigaki et al 2015;Atek et al 2015;Livermore et al 2017;Mehta et al 2017).…”