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) o v er 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 Surv e y, and Ne xt Generation Deep Extragalactic Exploratory Public Surv e y. This multifield approach yields a total of ≃ 370 arcmin 2 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 ( ρSFR ) 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 ρSFR ) can be reproduced in a Λ cold 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 ρSFR we find beyond z ≃ 13, possibly reflecting the rapid evolution of the halo mass function at earlier times.