Uncertainties in high-z galaxy properties inferred from spectral energy distribution fittings using JWST NIRCam photometry

¹ Department of Astronomy, Yonsei University, 50 Yonsei-ro Seodaemun-gu Seoul 03722, Republic of Korea
² Department of Astronomy & Astrophysics, University of Chicago 5640 S Ellis Avenue Chicago IL 60637, USA
³ Kavli Institute for Cosmological Physics, University of Chicago Chicago IL 60637, USA
⁴ Centre de Recherche Astrophysique de Lyon UMR5574, Univ Lyon, Univ Lyon1, Ens de Lyon F-69230 Saint-Genis-Laval, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 1 September 2025
Accepted: 15 January 2026

Abstract

Numerous high-z galaxies have recently been observed with the James Webb Space Telescope (JWST), providing new insights into early galaxy evolution. Their physical properties are typically derived through spectral energy distribution (SED) fitting, but the reliability of this approach remains uncertain owing to limited constraints on star formation histories (SFHs) and on the contribution from emission for such early systems. Applying BAGPIPES on simulated SEDs with SFR₁₀ > 0.3 M_⊙ yr⁻¹ at z = 6 from the SPHINX²⁰ cosmological simulation, we examine the uncertainties related to the recovery of stellar masses, star formation rates (SFR₁₀), and stellar metallicities from mock JWST/Near-Infrared Camera photometry, spanning F115W–F444W. Even without dust or emission lines, fitting the intrinsic stellar continuum overestimates the stellar mass by about 60%, on average (and by up to a factor of five for low-mass galaxies with recent starbursts). It also underestimates the SFR₁₀ by a factor of 2, due to inaccurate SFHs and age–metallicity degeneracies. In full SED-fitting models that include dust attenuation and nebular emission, stellar mass estimates are primarily affected by age–metallicity degeneracy and emission lines. Short-term SFRs are most sensitive to dust attenuation and nebular emission, while long-term SFRs additionally depend on the assumed SFHs. Incorporating bands that are free of strong emission lines, such as F410M, helps mitigate stellar mass overestimation by disentangling line emission from older stellar populations. We also find that best fit or likelihood-weighted estimates are generally more accurate than median posterior values. Although stellar mass functions are reproduced reasonably well (particularly when the minimum-χ² estimates are used), the slope of the main sequence of star formation acutely depends on the adopted fitting model. Overall, these results underscore the importance of careful modelling when interpreting high-z photometry, particularly for galaxies with recent star formation burst and/or strong emission lines, to minimise systematic biases in derived physical properties.