A search for the first galaxies across >0.6 deg² of JWST imaging: new evidence for a rapid decline in star-formation activity at z>12

We present a new determination of the evolving galaxy UV luminosity function (LF) over the extreme redshift range 12. 50. 6 deg² 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 (? ₒ₅ₑ), over the 100Myr 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? ₒ₅ₑ at z15. 5 lying significantly below an extrapolation of the log-linear? ₒ₅ₑ (z) relation inferred from early JWST LF studies. Instead, we find that the evolution in? ₒ₅ₑ (z) at these very early times is better described by a piece-wise log-linear relation, in which the decline in? ₒ₅ₑ (z) at z>12 is 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 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 z15.