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 $8.5<z<15.5$ using a combination of several major Cycle-1 JWST imaging programmes - PRIMER, JADES and NGDEEP. This multi-field approach yields a total of 370 sq. arcmin of JWST/NIRCam imaging, reaching (5-σ) depths of 30 AB mag in the deepest regions. We select a sample of 2548 galaxies with a significant probability of lying at high redshift ($p(z>8.5)>0.05$) 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 (ρSFR) 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 ρSFR) can be reproduced in a ΛCDM 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.