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
by
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 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 (ρSFR), 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 ρSFR at z≃15.5 lying significantly below an extrapolation of the log-linear ρSFR(z) relation inferred from early JWST LF studies. Instead, we find that the evolution in ρSFR(z) at these very early times is better described by a piece-wise log-linear relation, in which the decline in ρSFR(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 z≃15.
Link to the paper: https://arxiv.org/abs/2604.16666