Uncertainties in high-$z$ galaxy properties inferred from SED fitting using JWST NIRCam photometry

Abstract

Numerous high-$z$ galaxies have recently been observed with 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 SFR$_{10}>0.3\,M_\odot/yr$ at $z=6$ from the SPHINX cosmological simulation, we examine the uncertainties related to the recovery of stellar masses, star formation rates (SFR$_{10}$), 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 SFR$_{10}$ 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-$\chi^2$ estimates are used), the slope of the main sequence of star formation acutely depends on the adopted fitting model. [Abridged]