JWST PRIMER: A new multi-field determination of the evolving galaxy UV luminosity function at redshifts z 9-15

We present a new determination of the evolving galaxy UV luminosity function (LF) over the redshift range 8. 58. 5) >0. 05) to undertake a statistical calculation of the evolving UV LF. Our new measurements span 4 magnitudes in UV luminosity at z=9-12. 5, placing new constraints on both the shape and evolution of the LF at early times. We fit our observational data-points with a double-power law (DPL) function and explore the evolution of the DPL parameters. Our UV LF measurements yield a new estimate of the early evolution of cosmic star-formation rate density (ₒ₅ₑ) which confirms the gradual, log-linear decline deduced from early JWST studies, at least out to z 12. Finally we show that the observed early evolution of the galaxy UV LF (and ₒ₅ₑ) can be reproduced in a CDM Universe, with no change in dust properties or star-formation efficiency required out to z 12. Instead, we show that a progressive trend towards younger stellar population ages can reproduce the observations, and we show that the typical ages required at z 8, 9, 10, and 11 all converge on a time 380-330 Myr after the Big Bang, indicative of a rapid emergence of early galaxies at z 12 - 13. This is consistent with the first indications of a steeper drop-off in the galaxy population we find beyond z 13, possibly reflecting the rapid evolution of the halo mass function at earlier times.