JWST PRIMER: a new multifield determination of the evolving galaxy UV luminosity function at redshifts z ≃ 9 – 15

We present a new determination of the evolving galaxy ultraviolet (UV) luminosity function (LF) over the redshift range $8.5< z< 15.5$ using a combination of several major Cycle-1 JWST imaging programmes – Public Release IMaging for Extragalactic Research, JWST Advanced Deep Extragalactic Survey, and Next Generation Deep Extragalactic Exploratory Public Survey. This multifield approach yields a total of 370 arcmin2 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 UV LF. Our new measurements span 4 mag in UV luminosity at $z=9-12.5$, placing new constraints on both the shape and evolution of the LF at early times. Our measurements yield a new estimate of the early evolution of cosmic star-formation rate density (ρ SFR) confirming the gradual 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 ρ SFR) can be reproduced in a Λcold dark matter Universe, with no change in dust properties or star-formation efficiency required out to z 12. Instead, a progressive trend towards younger stellar population ages can reproduce the observations, and the typical ages required at z 8, 9, 10, and 11 all converge on 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 ρ SFR we find beyond z 13, possibly reflecting the rapid evolution of the halo mass function at earlier times.