JWST PRIMER: A new multi-field determination of the evolving galaxy UV luminosity function at redshifts $\mathbf{z}\simeq 9-15$

Abstract

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 $\simeq 370$ sq. arcmin of JWST/NIRCam imaging, reaching (5-$\ddot{I}ƒ$) depths of $\simeq 30$ 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 $\simeq 4$ magnitudes in UV luminosity at $z=9-12.5$, placing new constraints on both the shape and evolution of the LF at early times. Our measurements yield a new estimate of the early evolution of cosmic star-formation rate density ($\ddot{I}{}_{\mathrm{SFR}}$) confirming the gradual decline deduced from early JWST studies, at least out to $z\simeq 12$. Finally we show that the observed early evolution of the galaxy UV LF (and $\ddot{I}{}_{\mathrm{SFR}}$) can be reproduced in a $\hat{\mathrm{I}}›$CDM Universe, with no change in dust properties or star-formation efficiency required out to $z\simeq 12$. Instead, a progressive trend towards younger stellar population ages can reproduce the observations, and the typical ages required at $z\simeq$ 8, 9, 10, and 11 all converge on $\simeq 380-330$ Myr after the Big Bang, indicative of a rapid emergence of early galaxies at $z\simeq 12-13$. This is consistent with the first indications of a steeper drop-off in $\ddot{I}{}_{\mathrm{SFR}}$ we find beyond $z\simeq 13$, possibly reflecting the rapid evolution of the halo mass function at earlier times.

15 pages, 9 figures, accepted for publication in MNRAS