Galaxy mergers drive enhancements in ionization states

Abstract

Recent studies have suggested that galaxy mergers may help drive the escape of ionizing photons from galaxies. However, the ionization properties of merging galaxies have not yet been systematically studied across the full merger sequence in a large, well-defined sample. Here, we investigate the impact of mergers on the line ratio $\rm{O}_{32}$ tracing the ionization state and study its evolution as a function of merger progression. We identify 7641 galaxy mergers with robust ($\rm{SNR}>3$) emission line fluxes from the Sloan Digital Sky Survey Data Release 17, using spectroscopic pair analysis and visual classifications from the participatory science experiment "Cosmic Disco: Characterizing Galaxy Collisions". We find a continuous increase in $\rm{O}_{32}$ as a function of merger progression traced by pair separation and visual merger stages, with median and 90th percentile peaking post-coalescence. $\rm{O}_{32}$ is significantly enhanced in mergers past the first pericenter passage as compared to a control sample of isolated galaxies. We investigate the properties of galaxy mergers with $\rm{O}_{32}>3$ and find that these are mainly low-mass ($\widetilde{\rm{M}_{*}}=6\times 10^{8}\,\rm{M}{_\odot}$) mergers close to coalescence. In this population, large $\rm{O}_{32}$ values are due to mergers driving a simultaneous increase in specific star formation rate and decrease in metallicity. Thus, galaxy mergers enhance the ionization state of galaxies and likely facilitate the escape of ionizing radiation from galaxies, even at low stellar masses. Finally, we infer that Green Pea galaxies, a population characterized by high $\rm{O}_{32}$ , are low-mass galaxy mergers at coalescence, a scenario that reconciles findings from environmental and interferometric 21cm studies.