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Systematics in the SED Fitting Parameter Estimation of Composite Galaxies

Derivation of physical properties of galaxies using spectral energy distribution (SED) fitting is a powerful method, but can suffer from various systematics arising from model assumptions. Previously, such biases were mostly studied in the context of individual galaxies. In this study, we investigate potential biases arising from performing the SED fitting on the combined light of two galaxies, as would be the case in post-merger systems. We use GALEX-SDSS-WISE Legacy Catalog (GSWLC) of z<0.3 galaxies to identify 9,000 galaxy pairs that could eventually merge. For these we investigate if the UV/optical SED fitting accurately determines the stellar mass and (specific) star formation rate if the pair was unresolved (merged). The sum of the stellar masses (and SFRs) of individual galaxies in the pair establishes the ground truth for these quantities. For star forming galaxies no biases (<0.1 dex) are found in the stellar mass, SFR, or sSFRs. Moderate systematics in SFR (~0.1 dex) are found for systems with an extreme contrast in dust content between the two galaxies. We conclude that biases that would arise in the determination of masses and SFRs of post-merger systems on account of the two original galaxies having potentially very different star formation histories and different dust properties are small and that the approach with simple two-component star formation histories is adequate. The approach presented in this study, using flux compositing with empirically determined ground truth, offers new opportunities for testing the results of SED fitting in general.