The birth of a galaxy – III. Propelling reionization with the faintest galaxies

Abstract

Starlight from galaxies plays a pivotal role throughout the process of cosmic reionisation. We present the statistics of dwarf galaxy properties at z > 7 in haloes with masses up to 10 9 M , using a cosmological radiation hydrodynamics simulation that follows their buildup starting with their Population III progenitors. We find that metal-enriched star formation is not restricted to atomic cooling (T vir 10 4 K) haloes, but can occur in haloes down to masses ∼ 10 6 M , especially in neutral regions. Even though these smallest galaxies only host up to 10 4 M of stars, they provide nearly 30 per cent of the ionising photon budget. We find that the galaxy luminosity function flattens above M UV ∼ −12 with a number density that is unchanged at z 10. The fraction of ionising radiation escaping into the intergalactic medium is inversely dependent on halo mass, decreasing from 50 to 5 per cent in the mass range log M/M = 7.0 − 8.5. Using our galaxy statistics in a semi-analytic reionisation model, we find a Thomson scattering optical depth consistent with the latest Planck results, while still being consistent with the UV emissivity constraints provided by Lyα forest observations at z = 4 − 6.

Paving the way for the JWST: witnessing globular cluster formation at z > 3

“…These new regimes of mass and luminosity are relevant for our understanding of the physics of dwarf and globular cluster formation at high redshift, as well as for the identification of sources possibly dominating the ionizing background (e.g., Yue et al 2014;Wise et al 2014).…”

“…A major uncertainty in the interpretation of these observations is represented by the observed fraction of ionizing radiation that escapes from galaxies and reach the intergalactic and circum-galactic media (see e.g., Grazian et al 2016;Siana et al 2015;Vanzella et al 2012b). Irrespective of the nature of the contributors, the general consensus is that the faintest objects represent the main producers of the ionizing background at high redshift (Wise et al 2014;Bouwens et al 2015;Ferrara & Loeb 2013;Kimm & Cen 2014). Noteworthy, proto-GCs may also represent good candidates for the sources of radiation which reionized the universe by z ∼ 6 (e.g., Ricotti 2002;Schaerer & Charbonnel 2011;Ricotti et al 2016).…”

The First Billion Years project: the escape fraction of ionizing photons in the epoch of reionization

“…However, high resolution is essential for the computation of the escape fraction, since the main constraint on the escape fraction is the local environment of the sources (Paardekooper et al 2011). Furthermore, this high resolution is necessary to resolve star formation in the abundant low-mass proto-galaxies that are likely the main drivers of reionization (Paper I, Wise et al 2014). From these simulations we have extracted all the dark-matter haloes that contain at least 1000 dark-matter particles, 100 gas particles and 1 star.…”

Large fluctuations in the hydrogen-ionizing background and mean free path following the epoch of reionization

“…The ratio of ionizing to non-ionizing UV luminosity is typically assumed to be fixed, but the stellar properties and environments of UV-faint and UV-luminous galaxies are likely to be different. Recent simulations by Wise et al (2014) found that more massive (and thus more luminous) galaxies exhibited smaller fesc than lower mass galaxies due to the relative robustness of their interstellar medium to supernova explosions. Reducing the ionizing photon output from high-luminosity galaxies would reduce the emissivity bias, leading to weaker fluctuations.…”