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

¹ INAF – OAR, Via Frascati 33, 00078 Monte Porzio Catone, (Roma), Italy
² Dipartimento di Fisica, Sapienza, Università di Roma, Piazzale Aldo Moro 5, 00185 Roma, Italy
³ Centro de Astrobiología (CAB), CSIC–INTA, Cra. de Ajalvir km. 4, E28850 Torrejón de Ardoz, Madrid, Spain
⁴ University of Massachusetts, 710 N. Pleasant St, LGRB-520, Amherst, MA 01003, USA
⁵ NSF’s NOIRLab, Tucson, AZ 85719, USA
⁶ Department of Astronomy, The University of Texas at Austin, Austin, TX, USA
⁷ Department of Physics and Astronomy, University of California, Los Angeles, 430 Portola Plaza, Los Angeles, CA 90095, USA
⁸ Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain
⁹ Astrophysics Science Division, NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA
¹⁰ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, I-20133 Milano, Italy
¹¹ INAF – OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129 Bologna, Italy
¹² INAF-Osservatorio Astronomico di Padova, Via dell’Osservatorio 5, 35122 Padova, Italy
¹³ CEA, IRFU, DAp, AIM, Université Paris-Saclay, Université Paris Cité, Sorbonne Paris Cité, CNRS, 91191 Gif-sur-Yvette, France
¹⁴ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
¹⁵ Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
¹⁶ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
¹⁷ Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova, Via Marzolo 8, 35131 Padova, Italy
¹⁸ INAF – IASF Milano, Via A. Corti 12, I-20133 Milano, Italy
¹⁹ Università di Napoli “Federico II”, C.U. Monte Sant’Angelo, Via Cinthia, 80126 Napoli, Italy
²⁰ Space Telescope Science Institute, Baltimore, MD, USA
²¹ University of Louisville, Department of Physics and Astronomy, 102 Natural Science Building, 40292 KY, Louisville, USA
²² School of Physics, The University of Melbourne, Parkville, VIC 3010, Australia
²³ IPAC, California Institute of Technology, MC 314-6, 1200 East California Boulevard, Pasadena, CA 91125, USA
²⁴ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, PO Box 218 Hawthorn, VIC 3122, Australia
²⁵ Center for Astrophysics, Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
²⁶ Department of Physics & Astronomy, University College London, Gower St., London WC1E 6BT, UK
²⁷ Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara, Via Saragat 1, I-44122 Ferrara, Italy
²⁸ Center for Computational Astrophysics, Flatiron Institute, 162 5th Avenue, New York, NY 10010, USA
²⁹ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
³⁰ Como Lake Center for Astrophysics, DiSAT, Università degli Studi dell’Insubria, Via Valleggio 11, I-22100 Como, Italy
³¹ School of Astronomy and Space Science, University of Chinese Academy of Sciences (UCAS), Beijing 100049, China
³² National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
³³ Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, China
³⁴ Laboratory for Multiwavelength Astrophysics, School of Physics and Astronomy, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623, USA

^⋆ Corresponding author: marco.castellano@inaf.it

Received: 8 April 2025
Accepted: 20 October 2025

Abstract

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.