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

TL;DR

This study investigates how primordial non-Gaussianities influence the formation of early low-mass galaxies and their role in cosmic reionization, using simulations to connect initial conditions with observable reionization signatures.

Contribution

It demonstrates that primordial non-Gaussianities significantly affect the UV luminosity function of high-redshift galaxies and the reionization history, providing a potential observational probe.

Findings

Non-Gaussianities enhance low-mass galaxy formation at high redshift.

Reionization history and optical depth are sensitive to initial density fluctuation statistics.

Current uncertainties limit constraints, but future measurements of tau_E could reveal primordial non-Gaussianities.

Abstract

[Abridged] 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 statistical properties of initial density fluctuations affect the formation of early galaxies. Following Habouzit et al. (2014), we run 5 N-body simulations with Gaussian and (scale-dependent) non-Gaussian initial conditions, all consistent with Planck constraints. By appealing to a galaxy formation model and to a population synthesis code, we compute the far-UV galaxy luminosity function down to M_UV = -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 in low-mass galaxies. We adopt a reionization model calibrated from state-of-the-art hydrodynamical simulations and show that such non-Gaussianities leave a clear imprint on the Universe reionization history and electron Thomson scattering optical depth tau_E. Although current uncertainties in the physics of reionization and on the determination of tau_E still dominate the signatures of non-Gaussianities, our results suggest that tau_E could ultimately be used to constrain the statistical properties of initial density fluctuations.