Steep Faint-end Slopes of Galaxy Mass and Luminosity Functions at z>=6 and the Implications for Reionisation

TL;DR

This study uses cosmological simulations to analyze galaxy properties at high redshifts, revealing steep faint-end slopes in galaxy luminosity and mass functions, which imply low-mass galaxies significantly contributed to cosmic reionization.

Contribution

It provides the first detailed prediction of evolving steep faint-end slopes of galaxy luminosity and mass functions at z≥6, highlighting the importance of low-mass galaxies in reionization.

Findings

Steep faint-end slopes of galaxy functions increase with redshift.

Simulations predict many low-mass galaxies beyond current observational limits.

Low-mass galaxies likely played a key role in reionizing the universe.

Abstract

We present the results of a numerical study comparing photometric and physical properties of simulated z=6-9 galaxies to the observations taken by the WFC3 instrument aboard the Hubble Space Telescope. Using cosmological hydrodynamical simulations we find good agreement with observations in color-color space at all studied redshifts. We also find good agreement between observations and our Schechter luminosity function fit in the observable range, Muv<= -18, provided that a moderate dust extinction effect exists for massive galaxies. However beyond what currently can be observed, simulations predict a very large number of low-mass galaxies and evolving steep faint-end slopes from alpha_L = -2.15 at z=6 to alpha_L = -2.64 at z=9, with a dependence of |alpha_L| \propto (1+z)^0.59. During the same epoch, the normalization phi* increases and the characteristic magnitude Muv* becomes moderately brighter with decreasing redshift. We find similar trends for galaxy stellar mass function with evolving low-mass end slope from alpha_M = - 2.26 at z=6 to alpha_M = -2.87 at z=9, with a dependence of |alpha_M| \propto (1+z)^0.65. Together with our recent result on the high escape fraction of ionizing photons for low-mass galaxies, our results suggest that the low-mass galaxies are important contributor of ionizing photons for the reionisation of the Universe at z>=6.