Using ultra-deep JWST NIRCam imaging from the GLIMPSE Survey, enhanced by gravitational lensing of the AbellS1063 cluster, we investigate the faintest galaxies ever observed in the redshift range z~9 to z~15. We identify 105 galaxy candidates within this range, spanning absolute ultraviolet (UV) magnitudes from MUV~-18 to MUV~-13 mag, about three magnitudes fainter, on average, than prior JWST studies. We place strong constraints on the ultra-faint end of the UV luminosity function (UVLF), finding minimal evolution in the faint-end slope, which varies from α=-2. 010. 20 at z=9 to α=-2. 100. 19 at z=13. This behaviour contrasts with the rapid evolution of the faint-end slope observed from z~0 to z~9. By integrating the UVLF down to MUV=-16, we derive the cosmic star formation rate density (SFRD) from z=9 to z=13, revealing a best-fit redshift evolution that follows (1+z) ^-2. 94^{+0. 06-₀. ₁₀}. This slope is significantly shallower than predictions from most theoretical models. Extending the integration limit to MUV=-13, we find that galaxies fainter than MUV=-16 contribute more than 50% of the total cosmic SFR density at z~12. The observed excess in the cosmic SFRD at these high redshifts may suggest an enhancement in the star formation efficiency during the earliest phases of galaxy formation. Alternatively, this could result from other physical mechanisms, such as bursty star formation histories; minimal dust attenuation; or an evolving initial mass function. However, existing models that incorporate these scenarios fail to fully reproduce the observed redshift evolution of SFRD. Finally, we acknowledge the potential impact of low-redshift contamination and cosmic variance, as the small survey volume may not represent the broader galaxy population. Similar observations in different fields and spectroscopic confirmation are required to validate these findings
Chemerynska et al. (Mon,) studied this question.