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

By R. Naidu, S. Tacchella, C. Mason, S. Bose, P. Oesch, C. Conroy. Astrophysical Journal, 2019

187 citations recorded by the source corpus.

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 ( ). Here we update this picture via an empirical model that successfully predicts latest observations (e.g., the rapid drop in star-formation density ( at ). We generate an ionizing spectrum for each galaxy in our model and constrain by leveraging latest measurements of the reionization timeline (e.g., Lyα damping of quasars and galaxies at z > 7). Assuming a constant across all sources at z > 6, we find < −13.5 galaxies need = to complete reionization. The inferred Intergalactic Medium neutral fraction is [0.9, 0.5, 0.1] at —that is, the bulk of reionization transpires rapidly in 300 Myr, driven by the z > 8 ρSFR 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 ( ), we propose a physically motivated model relating to and find . Since falls by ∼2.5 dex between z = 8 and z = 0, our model explains the humble upper limits on at lower redshifts and its required evolution to ∼ 0.2 at z > 6. Within this model, strikingly, <5% of galaxies with < −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 ( > −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 ( > −2) and/or distributions skewed toward massive galaxies produce the required late and rapid reionization. We predict that LyC leakers like COLA1 (z = 6.6, ∼ 30%, = −21.5) will become increasingly common toward 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.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/e766c28c201f4f3128367920f77dd5fdf0d0ca50

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In Astronomy and Astrophysics

  1. P. V. van Dokkum
  2. Richard S. Ellis
  3. T. Treu
  4. M. Franx
  5. C. Conroy
  6. G. Brammer
  7. L. Hernquist
  8. A. Fontana

In Instrumentation

  1. P. V. van Dokkum
  2. T. Treu
  3. G. Brammer
  4. M. Franx
  5. Richard S. Ellis
  6. A. Fontana
  7. C. Conroy
  8. D. Marchesini

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