Sparks II: Panchromatic SED modeling and galaxy physical properties across the starburst to post-starburst sequence

TL;DR

The Sparks II survey analyzes galaxy properties across the starburst to post-starburst transition using multi-wavelength data, highlighting the importance of panchromatic SED fitting for accurate star formation rate estimates.

Contribution

This study compares different data types and modeling approaches, demonstrating the effectiveness of panchromatic SED fitting in deriving galaxy properties, especially SFRs and AGN activity.

Findings

Optical continuum fits underestimate SFRs compared to Hα-based measurements.

Panchromatic SED-based SFRs show higher star formation levels in many galaxies.

AGN torus models distinguish AGN from star-forming galaxies but suggest low covering factors.

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 (AGN). Interpreting these extreme systems requires reliable physical properties, yet these can vary substantially when derived from rest-frame optical spectroscopy versus multi-wavelength 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 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$\alpha$-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 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.