Sparks. II. Panchromatic Spectral Energy Distribution Modeling and Galaxy Physical Properties across the Starburst to Post-starburst Sequence

Abstract The Sparks survey provides rest-frame near-infrared spectroscopy for 93 local massive galaxies spanning the rapid transition from starburst to post-starburst, including Balmer-strong galaxies as well as systems with active galactic nuclei (AGNs). Interpreting these extreme systems requires reliable physical properties, yet these can vary substantially when derived from rest-frame optical spectroscopy versus multiwavelength photometry, and across different fitting codes and assumptions. We assemble far-ultraviolet to far-infrared photometry for the Sparks sample and compare the resulting galaxy properties across data types and modeling approaches, identifying the final measurements adopted for the survey. With stellar masses recovered relatively robustly, we focus on the more model-dependent quantities of star formation rates (SFRs) and star formation histories (SFHs), and AGN activity. Fits to optical stellar continuum alone, dominated by strong Balmer absorption, systematically favor rapidly declining SFHs and suppress ongoing star formation. Benchmarking against H α -based SFRs in the star-forming Sparks galaxies shows that Prospector fits to the optical continuum spectroscopy underestimate the SFR by 0.76 dex (scatter 0.42 dex), whereas panchromatic spectral energy distribution (SED)-based SFRs perform better, with a −0.15 dex offset and 0.14 dex scatter. We therefore adopt the panchromatic SED-based SFRs for composite and AGN hosts, finding that many exhibit higher levels of star formation than previously inferred. Finally, we test the AGN torus model in Prospector , finding that it successfully distinguishes optically classified AGN from star-forming galaxies, but yields torus luminosities an order of magnitude below expectations from AGN bolometric luminosities, possibly indicating intrinsically low covering factors in Sparks AGN shaped by black hole feedback during coalescence.