Sparks: The Magellan/FIRE Survey from Starburst to Post-starburst

Rapid transitions from starburst to quiescence constitute a key evolutionary pathway in galaxy formation. Post-starburst galaxies trace this brief phase, exhibiting optical spectra dominated by intermediate-age stellar populations with strong Balmer absorption features. Although rare locally, such systems are commonly revealed by JWST observations among massive galaxies at z ≳ 3. In the nearby Universe, their evolutionary stage remains uncertain: Balmer-strong galaxies hosting active galactic nuclei (AGNs) show conflicting star formation rates (SFRs), with optical diagnostics implying quenching while far-infrared emission suggests ongoing obscured star formation. We present Sparks, an infrared survey designed to study the transition from starburst to post-starburst. Using the FIRE spectrograph on the Magellan Telescope, Sparks provides near-infrared spectra (0.82–2.51 μm) for 93 local massive galaxies spanning 3 orders of magnitude in SFR, from starbursts to quenched post-starbursts, including AGN hosts. Here, we describe the survey goals, sample selection, observations, and data reduction, and examine galaxy properties derived from stellar population synthesis fitting of photometric data covering far-ultraviolet to far-infrared. Our new panchromatic-based SFR and star formation history measurements divide the sample into three groups: galaxies undergoing their first major starburst in the past ∼1 Gyr; galaxies undergoing their second major starburst, with optical continua dominated by intermediate-age stellar populations formed during the previous recent burst; and post-burst quenching systems. AGNs appear predominantly in the second group, explaining why systems with strong Balmer absorption and AGNs show elevated far-infrared emission, and implying a short delay between starburst and black hole accretion.