The effect of baryonic streaming motions on the formation of the first supermassive black holes.
Paper abstract: Observations of quasars at redshifts z ≳ 6 reveal that 109 M⊙ supermassive black holes (SMBHs) had already formed when the Universe was ≲ 0.9 Gyr old. One hypothesis for the origins of these SMBHs is that they grew from the remnants of the first generation of massive stars, which formed in low-mass ( ˜ 105-106 M⊙) dark matter minihaloes at z ≳ 20. This is the regime where baryonic streaming motions - the relative velocities of baryons with respect to dark matter in the early Universe - most strongly inhibit star formation by suppressing gas infall and cooling. We investigate the impact of this effect on the growth of the first SMBHs using a suite of high-fidelity, ellipsoidal-collapse Monte Carlo merger-tree simulations. We find that the suppression of seed BH formation by the streaming motions significantly reduces the number density of the most massive BHs at z > 15, but the residual effect at lower redshifts is essentially negligible. The streaming motions can reduce by a factor of few the number density of the most luminous quasars at z ≈ 10-11, where such objects could be detected by the James Webb Space Telescope. We conclude, with minor theoretical caveats, that baryonic streaming motions are unlikely to pose a significant additional obstacle to the formation of the observed high-redshift quasar SMBHs. Nor do they appreciably affect the heating and reionization histories of the Universe or the merger rates of nuclear BHs in the mass and redshift ranges of interest for proposed gravitational-wave detectors.
Simulating the assembly of galaxies at redshifts z = 6 - 12
Paper abstract: We use state-of-the-art simulations to explore the physical evolution of galaxies in the first billion years of cosmic time. First, we demonstrate that our model reproduces the basic statistical properties of the observed Lyman-break galaxy (LBG) population at z = 6 - 8, including the evolving ultra-violet (UV) luminosity function (LF), the stellar-mass density (SMD), and the average specific star-formation rates (sSFR) of LBGs with M_UV < -18 (AB mag). Encouraged by this success we present predictions for the behaviour of fainter LBGs extending down to M_UV <= -15 (as will be probed with the James Webb Space Telescope) and have interrogated our simulations to try to gain insight into the physical drivers of the observed population evolution. We find that mass growth due to star formation in the mass-dominant progenitor builds up about 90% of the total z~6 LBG stellar mass, dominating over the mass contributed by merging throughout this era. Our simulation suggests that the apparent ``luminosity evolution" depends on the luminosity range probed: the steady brightening of the bright end of the LF is driven primarily by genuine physical luminosity evolution and arises due to a fairly steady increase in the UV luminosity (and hence star-formation rates) in the most massive LBGs; e.g. the progenitors of the z~6 galaxies with M_UV < -18.5 comprised ~ 90% of the galaxies with M_UV < -18 at z~7, and ~75% at z~8. However, at fainter luminosities the situation is more complex, due in part to the more stochastic star-formation histories of lower-mass objects; the progenitors of a significant fraction of z~6 LBGs with M_UV > -18 were in fact brighter at z~7 (and even at z~8) despite obviously being less massive at earlier times. At this end, the evolution of the UV LF involves a mix of positive and negative luminosity evolution (as low-mass galaxies temporarily brighten then fade) coupled with both positive and negative density evolution (as new low-mass galaxies form, and other low-mass galaxies are consumed by merging). We also predict the average sSFR of LBGs should rise from sSFR~4.5/Gyr at z~6 to sSFR~11/Gyr by z~9.
Improving dark energy constraints with high-redshift Type Ia supernovae from CANDELS and CLASH
Paper abstract: We investigated the degree of improvement in dark energy constraints that can be achieved by extending Type Ia supernova (SN Ia) samples to redshifts z > 1.5 with the Hubble Space Telescope (HST), particularly in the ongoing Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) and the Cluster Lensing and Supernova survey with Hubble (CLASH) multi-cycle treasury programs. Using the popular Chevalier-Polarski-Linder (CPL) parametrization of the dark energy w = w0 + wa(1 - a) we generated mock SN Ia samples that can be projected out to higher redshifts. The synthetic datasets thus generated were fitted to the CPL model, and we evaluated the improvements that a high-z sample can add to improve the statistical and systematic uncertainties on cosmological parameters. In an optimistic but still very achievable scenario, we find that extending the HST sample beyond CANDELS+CLASH to reach a total of 28 SN Ia at z > 1.0 could improve the uncertainty in the wa parameter ?waby up to 21%. The corresponding improvement in the figure of merit (FoM) would be as high as 28%. Finally, we consider the use of high-redshift SN Ia samples to detect non-cosmological evolution in SN Ia luminosities with redshift, finding that these tests could be undertaken by future space-based infrared surveys using the James Webb Space Telescope (JWST).