A search for the first galaxies across $>0.6$ deg${}^2$ of JWST imaging: new evidence for a rapid decline in star-formation activity at $z>12$

Abstract

We present a new determination of the evolving galaxy UV luminosity function (LF) over the extreme redshift range $12.5<z<18.5$, based on a wide-area search of $>$0.6 deg${}^2$ of JWST NIRCam imaging containing $>150$ independent sight-lines. We find evidence for an accelerated decline in the UV LF, and hence inferred star-formation rate density ($\ddot{I}{}_{\mathrm{SFR}}$), over the $\simeq 100\mathrm{Myr}$ cosmic time interval between $z=11$ and $z=13.5$. Moreover, based on a notable lack of galaxy candidates at $z>14.5$, we find evidence for an even more rapid descent in star-formation activity towards earlier times, with our new measurement of $\ddot{I}{}_{\mathrm{SFR}}$ at $z\simeq 15.5$ lying significantly below an extrapolation of the log-linear $\ddot{I}{}_{\mathrm{SFR}}(\mathrm{z})$ relation inferred from early JWST LF studies. Instead, we find that the evolution in $\ddot{I}{}_{\mathrm{SFR}}(\mathrm{z})$ at these very early times is better described by a piece-wise log-linear relation, in which the decline in $\ddot{I}{}_{\mathrm{SFR}}(\mathrm{z})$ at $z>12$ is $\simeq 4$ times steeper than at redshifts $z<12$. Our observational results are consistent with a number of theoretical models of galaxy evolution which have incorporated a range of treatments in an attempt to explain the prevalence of UV-bright galaxies at least out to $z\simeq 12$ (e.g., increased star-formation efficiency, stochastic star-formation histories, an evolving stellar initial mass function and/or a shift towards attenuation-free stellar populations). However, our results are also entirely consistent with a relatively simple galaxy evolution model with no such adjustments, in which the rapid evolution of the dark-matter halo mass function at early times is for a while partially masked by progressively younger stellar ages, with the inferred epoch of first galaxy formation lying at $z\simeq 15$.

21 pages, 10 figures, to be submitted to MNRAS