Beyond UV: Rest-frame B-band and Apparent Luminosity Functions of z=5-9 Galaxies

Nicha Leethochawalit, Takahiro Morishita, Tirawut Worrakitpoonpon, Michele Trenti

First listed 2026-01-15 | Last updated 2026-01-15

Abstract

We present new measurements of galaxy luminosity functions (LFs) from JWST/NIRCam imaging over the redshift range z=4.5-9.7, using photometric catalogs from JADES and public extragalactic fields. Our analysis includes rest-frame UV and B-band LFs, as well as apparent LFs in F090W, F115W, F200W, F356W, and F444W. We present the first constraints on the rest-frame B-band LF at z~7-8 and extend existing measurements at z~5 to M(B) = -18 mag. The B-band LFs evolve more strongly with redshift than UV LFs, though both decline more gradually than predicted by simulations at z>5. No single existing simulation reproduces all observed trends, with discrepancies likely driven by assumptions about binary evolution and stellar population synthesis models. The apparent LFs in F356W and F444W show hints of a bright-end excess at all redshifts, extending to fainter magnitudes at higher redshift. While extreme emission line galaxies may partially account for it, the excess may also indicate a population of moderately red, optically bright sources - potentially dusty star-forming galaxies or obscured AGNs. Finally, we find that rest-frame B-band luminosity correlates more tightly with stellar mass than UV, making it a powerful tracer of mass assembly and reinforcing the diagnostic value of rest-frame optical LFs in uncovering the physical processes that drive early galaxy formation.

Short digest

JWST/NIRCam imaging from JADES plus compiled public fields yields rest-UV, rest-frame B, and apparent-band luminosity functions at z=4.5–9.7, including the first B-band constraints at z~7–8 and an extension to M(B)=−18 at z~5. The B-band LFs evolve more strongly than UV while both decline more slowly than simulations at z>5, and the apparent F356W/F444W LFs show a persistent bright-end excess that reaches fainter magnitudes toward higher redshift—pointing to moderately red, optically bright sources such as dusty star-formers or obscured AGN. Rest-frame B luminosity correlates more tightly with stellar mass than UV, underscoring optical LFs as incisive tracers of early mass assembly. A caveat is that no single simulation matches all trends, likely reflecting assumptions about binary evolution and stellar population synthesis.

Key figures to inspect

• Figure 3: Compare the green LF points/fit to simulation curves across the three dropout bins to see where the bright end in B (vs UV) departs from predictions and how Schechter M* and α differ between bands.
• Figure 4: Track the redshift evolution—note the steeper change in the B-band LF relative to UV and isolate where Ma et al. and TNG predictions under- or over-shoot the observed normalization and bright end.
• Figure 2: Inspect completeness for UV, B, and F356W in the F090W-dropout field to gauge selection depth and potential Eddington bias, ensuring the bright-end excess is not a byproduct of incompleteness/contamination modeling.
• Figure 1: Review field tiling and sub-areas used for injection–recovery; this frames the effective survey volume and the strategy for mitigating low-z interlopers that propagate into the LF bins.