Data from Cluster-based Star Formation History of The Late-Stage Merger NGC 34

The luminous infrared galaxy NGC 34 is a late-stage merger in the process of transitioning into an elliptical galaxy. Observations of NGC 34 taken with the Hubble Space Telescope in six filters – covering the near-ultraviolet, optical, and narrow-band Hα – reveal a few hundred star clusters throughout the system. We fit this multiband photometry with cluster evolution models to estimate the
age, reddening, and mass of each star cluster. We use their age and mass distributions to reconstruct the recent (<3 Gyr) star formation history (SFH) of NGC 34, which varies significantly across the galaxy. A global starburst began ≈400 Myr ago and halted abruptly ≈80 Myr ago, with an average star formation rate (SFR) of 23.8 ± 12.6 M⊙ yr−1. Following this global burst, star formation halted in all but the central 1 kpc. For the past 10 Myr, this nuclear region has experienced a more intense burst of star formation, with literature estimates on the order of ∼100 M⊙ yr−1. We suggest that the burst at 80 Myr coincides with the last close pericenter passage of the progenitors, while the current nuclear burst is the result of final coalescence. Comparing the details of NGC 34’s SFH to other transitioning galaxies indicates that there are a variety of pathways leading to the global cessation of star formation in major mergers. The global moderate burst, dip, intense central burst evolution of NGC 34’s star formation is strikingly similar to both more evolved systems, including the classical post-starburst galaxy S12 (∼100 Myr past final coalescence), and predictions from galaxy mergers in hydrodynamic26
simulations.