What is the maximum mass of a Population III galaxy?

Abstract

We utilize cosmological hydrodynamic simulations to study the formation of Population III (Pop III) stars in dark matter halos exposed to strong ionizing radiation. We simulate the formation of three halos subjected to a wide range of ionizing fluxes, and find that for high flux, ionization and photoheating can delay gas collapse and star formation up to halo masses significantly larger than the atomic cooling threshold. The threshold halo mass at which gas first collapses and cools increases with ionizing flux for intermediate values, and saturates at a value approximately an order of magnitude above the atomic cooling threshold for extremely high flux (e.g. ≈ 5 × 10 8 M ⊙ at z ≈ 6). This behavior can be understood in terms of photoheating, ionization/recombination, and Lyα cooling in the pressure-supported, self-shielded gas core at the center of the growing dark matter halo. We examine the spherically-averaged radial velocity profiles of collapsing gas and find that a gas mass of up to ≈ 10 6 M ⊙ can reach the central regions within 3 Myr, providing an upper limit on the amount of massive Pop III stars that can form. The ionizing radiation increases this limit by a factor of a few compared to strong Lyman-Werner (LW) radiation alone. We conclude that the bright HeII 1640 Å emission recently observed from the high-redshift galaxy CR7 cannot be explained by Pop III stars alone. However, in some halos, a sufficient number of Pop III stars may form to be detectable with future telescopes such as the James Webb Space Telescope (JWST).

Evidence for a direct collapse black hole in the Lymanαsource CR7

“…Haiman et al 1997). Furthermore, Visbal et al (2016) demonstrate that if local reionization occurs early enough and with sufficient ionizing flux the photoheated gas around a M vir ∼ 10 9 M halo with starfree progenitors has a cosmological Jeans mass that is comparably large. In such cases the metal enrichment is considerably delayed and potentially results in a rapid Pop III starburst with Lyα luminosity comparable to that of CR7.…”

“…Because a normal stellar population fails to reproduce the excess Lyα and He ii luminosities, Sobral et al (2015) suggest a composite spectrum in which clump A is fit by a pure Pop III SED and clumps B+C are comprised of enriched populations. If CR7 does indeed host a Pop III star cluster with a top-heavy initial mass function (IMF) then we are likely viewing a rare starburst with an age of less than a few Myr (Pallottini et al 2015;Hartwig et al 2015;Visbal et al 2016).…”

Enhanced direct collapse due to Lyman α feedback

“…Sobral et al 2015;Visbal et al 2016; see also Johnson 2010), is dubious since it is unknown how a sufficiently high mass of Population III stars could be assembled rapidly enough to explain the observed extremely bright emission (e.g. Hartwig et al 2016;Yajima & Khochfar 2016;Xu et al 2016;Visbal et al 2017). …”

Blossoms from black hole seeds: properties and early growth regulated by supernova feedback

“…The only remaining coolant is atomic hydrogen, which cannot cool gas below ∼ 8000 K, thus leading to a very large Jeans mass of 10 5 M . In the very centre of a metal-free halo where molecular hydrogen formation is suppressed for at least 10 Myr (Visbal, Haiman & Bryan 2014;Latif & Volonteri 2015), and in the presence of large inflow rates (> 0.1 M /yr), a supermassive star can form, and from it a BH, retaining up to 90% of the stellar mass. This models predicts massive, but rare seeds, which can possibly explain the quasar population at z > 6 (Fan et al 2006;Jiang & et al, 2009;Mortlock et al 2011), but has more difficulties with accounting for the presence of BHs in almost all galaxies today (Habouzit et al 2016a).…”