Behind the spotlight: a systematic assessment of outshining using NIRCam medium bands in the JADES Origins Field

ABSTRACT The spatial resolution and sensitivity of James Webb Space Telescope’s NIRCam instrument have revolutionized our ability to probe the internal structure of early galaxies. By leveraging deep medium-band imaging in the Jades Origins Field, we assemble comprehensive spectral energy distributions (SEDs) using 19 photometric bands for over 200 high-redshift galaxies ($z \ge 4.5$). We use this sample to investigate the ‘outshining’ phenomenon, which can bias the inferred stellar populations by masking the presence of evolved stars ($\ge$ 100 Myr) with the light of bright, young O- and B-type stars. We address this problem by performing spatially resolved SED fitting of both binned and full pixel-by-pixel photometry, which we compare to the traditional integrated approach. We find evidence for systematic underestimation of stellar mass in low-mass galaxies ($\le 10^9 \rm M_\odot$) with bursty star formation histories (SFHs), which can exceed a factor of 10 in individual cases, but on average is typically a factor of 1.25–2.5, depending on the binning methodology and SFH model used. The observed mass offset correlates with burstiness (SFR$_{10 \ \rm Myr}$/SFR$_{100 \ \rm Myr}$) and sSFR (specific star formation rate), such that galaxies with recently rising SFHs have larger mass offsets. While no integrated model reproduces all resolved SFHs, the models which produce the closest stellar mass estimates are the double power-law and non-parametric ‘continuity’ models. We apply an outshining correction factor to the $z=7$ stellar mass function, finding little impact within the uncertainties. We conclude that outshining can be important in individual low-mass galaxies, but the overall impact is limited and should be considered alongside other systematic SED-fitting effects.