Pushing JWST to the extremes: Search and scrutiny of bright galaxy candidates at z ≃ 15–30

Aims. We investigate the galaxy ultraviolet luminosity function (UV LF) at z ≃ 15 − 30 to constrain early galaxy formation scenarios aimed at explaining the mild evolution of the UV LF bright end reported by JWST at z ≈ 10 − 15. Methods. We designed customised Lyman-break colour selection techniques to identify galaxy candidates in the redshift ranges 15 ≤ z ≤ 20 and 20 ≤ z ≤ 28. The selection was performed on the ASTRODEEP-JWST multi-band catalogues of the CEERS, Abell-2744, JADES, NGDEEP, and PRIMER survey fields, covering a total area of ∼0.2 sq. deg. Results. We identified five candidates at 15 ≤ z ≤ 20, whereas no objects were found based on the z ≳ 20 colour selection criteria. Despite exhibiting a > 1.5 mag break, all the objects display multi-modal redshift probability distributions across different SED-fitting codes and methodologies. The alternative solutions correspond to poorly understood populations of low-mass quiescent or dusty galaxies at z ∼ 3 − 7. This conclusion is supported by the analysis of five F200W dropout objects that we found to be interlopers based on NIRSpec PRISM spectra: four dusty star-forming galaxies at z ∼ 2.2 − 6.6 and a passive galaxy at z = 4.91, with log(M _star/M_⊙)≲9. We measured the UV LF under different assumptions on the contamination level within our sample. We find that if even a fraction of the candidates were indeed found at z ≳ 15, the resulting UV LF would point to a very mild evolution compared to estimates at z < 15, implying a significant tension with existing theoretical models. In particular, confirming our bright (M _UV < −21) candidates would require substantial revisions to the theoretical framework. In turn, if all these candidates ended up being confirmed as interlopers, we would have to conclude that future surveys might require ten times wider areas to be able to select M _UV ≲ −20 galaxies at z > 15. Observations in the F150W and F200W filters at depths comparable to those in the NIRCam LW bands would also be required to mitigate contamination from rare red objects at z ≲ 8.