Sparks: The Magellan/FIRE survey from starburst to post-starburst

TL;DR

Sparks is an infrared survey that studies the transition from starburst to post-starburst in massive galaxies, revealing the timing of star formation and AGN activity through near-infrared spectra and multi-wavelength data.

Contribution

It provides new near-infrared spectral data and analysis of galaxy star formation histories across different evolutionary stages, including AGN hosts, in the local universe.

Findings

Galaxies are categorized into three groups based on star formation history.

AGN are mostly found in galaxies with recent or second major starbursts.

There is a short delay between starburst activity and black hole accretion.

Abstract

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 $z \gtrsim 3$. In the nearby Universe, their evolutionary stage remains uncertain: Balmer-strong galaxies hosting active galactic nuclei (AGN) 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 $\mu$m) for 93 local massive galaxies spanning three 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 $\sim 1$ 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. AGN appear predominantly in the second group, explaining why systems with strong Balmer absorption and AGN show elevated far-infrared emission, and implying a short delay between starburst and black hole accretion.