MUSE integral-field spectroscopy towards the Frontier Fields cluster Abell S1063

In spite of their conjectured importance for the Epoch of Reionization, the properties of low-mass galaxies are currently still very much under debate. In this article, we study the stellar and gaseous properties of faint, low-mass galaxies at z > 3. We observed the Frontier Fields cluster Abell S1063 with MUSE over a 2 arcmin(2) field, and combined integral-field spectroscopy with gravitational lensing to perform a blind search for intrinsically faint Ly alpha emitters (LAEs). We determined in total the redshift of 172 galaxies of which 14 are lensed LAEs at z = 3-6.1. We increased the number of spectroscopically-confirmed multiple-image families from 6 to 17 and updated our gravitational-lensing model accordingly. The lensing-corrected Ly alpha luminosities are with L-Ly alpha less than or similar to 10(41.5) erg/s among the lowest for spectroscopically confirmed LAEs at any redshift. We used expanding gaseous shell models to fit the Ly alpha line profile, and find low column densities and expansion velocities. This is, to our knowledge, the first time that gaseous properties of such faint galaxies at z greater than or similar to 3 are reported. We performed SED modelling to broadband photometry from the U band through the infrared to determine the stellar properties of these LAEs. The stellar masses are very low (10(6-8) M-circle dot), and are accompanied by very young ages of 1-100 Myr. The very high specific star-formation rates (similar to 100 Gyr (-1)) are characteristic of starburst galaxies, and we find that most galaxies will double their stellar mass in less than or similar to 20 Myr. The UV-continuum slopes beta are low in our sample, with beta < -2 for all galaxies with M-star < 10(8) M-circle dot. We conclude that our low-mass galaxies at 3 < z < 6 are forming stars at higher rates when correcting for stellar mass effects than seen locally or in more massive galaxies. The young stellar populations with high star-formation rates and low HI column densities lead to continuum slopes and LyC-escape fractions expected for a scenario where low mass galaxies reionise the Universe