Effect of primordial non-Gaussianities on the far-UV luminosity function of high-redshift galaxies: implications for cosmic reionization

Jacopo Chevallard (1) , Joseph Silk (1) , Takahiro Nishimichi (1) , Melanie Habouzit (1) , Gary A. Mamon (1) , Sébastien Peirani (1)

1 IAP - Institut d'Astrophysique de Paris

Understanding how the intergalactic medium (IGM) was reionized at z ≳ 6 is one of the big challenges of current high-redshift astronomy. It requires modelling the collapse of the first astrophysical objects (Pop III stars, first galaxies) and their interaction with the IGM, while at the same time pushing current observational facilities to their limits. The observational and theoretical progress of the last few years have led to the emergence of a coherent picture in which the budget of hydrogen-ionizing photons is dominated by low-mass star-forming galaxies, with little contribution from Pop III stars and quasars. The reionization history of the Universe therefore critically depends on the number density of low-mass galaxies at high redshift. In this work, we explore how changes in the cosmological model, and in particular in the statistical properties of initial density fluctuations, affect the formation of early galaxies. Following Habouzit et al. (2014), we run five different N-body simulations with Gaussian and (scale-dependent) non-Gaussian initial conditions, all consistent with Planck constraints. By appealing to a phenomenological galaxy formation model and to a population synthesis code, we compute the far-UV galaxy luminosity function down to M_FUV = -14 at redshift 7 ≤ z ≤ 15. We find that models with strong primordial non-Gaussianities on ≲ Mpc scales show a far-UV luminosity function significantly enhanced (up to a factor of 3 at z = 14) in low-mass galaxies. We adopt a reionization model calibrated from state-of-the-art hydrodynamical simulations and show that such scale-dependent non-Gaussianities leave a clear imprint on the Universe reionization history and electron Thomson scattering optical depth τ_e. Although current uncertainties in the physics of reionization and on the determination of τ_e still dominate the signatures of non-Gaussianities, our results suggest that τ_e could ultimately be used to constrain the statistical properties of initial density fluctuations.

Mots clés: galaxies: high-redshift; intergalactic medium; galaxies: luminosity function; mass function; dark ages; reionization; first stars; Astrophysics - Cosmology and Nongalactic Astrophysics

Domaines: Planète et Univers [physics]; Astrophysique [astro-ph]

Jacopo Chevallard, Joseph Silk, Takahiro Nishimichi, Melanie Habouzit, Gary A. Mamon, et al.. Effect of primordial non-Gaussianities on the far-UV luminosity function of high-redshift galaxies: implications for cosmic reionization. Monthly Notices of the Royal Astronomical Society, 2015, 446, pp.3235-3252. ⟨10.1093/mnras/stu2280⟩. ⟨insu-03644780⟩