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Behind the Spotlight: A systematic assessment of outshining using NIRCam medium-bands in the JADES Origins Field

7 Apr 2025arXiv:2504.05244links table onlyarchive 2025-07-28

Thomas Harvey, Christopher J. Conselice, Nathan J. Adams, Duncan Austin, Qiong Li, Vadim Rusakov, Lewi Westcott, Caio M. Goolsby, Christopher C. Lovell, Rachel K. Cochrane, Aswin P. Vijayan, James Trussler

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The spatial resolution and sensitivity of JWST's NIRCam instrument has revolutionised 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 ≥4.5). We present an analysis of this sample with particular emphasis on investigating the "outshining" phenomenon, which can bias the inferred stellar populations by masking the presence of evolved stellar populations (≥ 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 (≤10⁹ M_⊙) with bursty star formation, 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 Myr)/SFR_(100 Myr)) and sSFR, such that galaxies with recently rising SFHs have larger mass offsets. The integrated SFH models which produce the most consistent stellar masses are the double power-law and non-parametric `continuity' models, although no integrated model fully reproduces all resolved SFHs. We apply an outshining correction factor to the Stellar Mass Function at z=7, 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.

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