K. B. Mantha | Searcharxiv

arXiv2026

Galaxy mergers drive enhancements in ionization states

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.

astro-ph.GA↗

arXiv2025

Galaxy Zoo JWST: Up to 75% of discs are featureless at $3<z<7$

We have not yet observed the epoch at which disc galaxies emerge in the Universe. While high-$z$ measurements of large-scale features such as bars and spiral arms trace the evolution of disc galaxies, such methods cannot directly quantify featureless discs in the early Universe. Here we identify a substantial population of apparently featureless disc galaxies in the Cosmic Evolution Early Release Science (CEERS) survey by combining quantitative visual morphologies of $\sim 7,000$ galaxies from the Galaxy Zoo JWST CEERS project with a public catalogue of expert visual and parametric morphologies. While the highest-redshift featured disc we identify is at $z_{\rm{phot}}=5.5$, the highest-redshift featureless disc we identify is at $z_{\rm{phot}}=7.4$. The distribution of Sérsic indices for these featureless systems suggests that they truly are dynamically cold: disc-dominated systems have existed since at least $z\sim 7.4$. We place upper limits on the featureless disc fraction as a function of redshift, and show that up to $75\%$ of discs are featureless at $3.0<z<7.4$. This is a conservative limit assuming all galaxies in the sample truly lack features. With further consideration of redshift effects and observational constraints, we find the featureless disc fraction in CEERS imaging at these redshifts is more likely $\sim29-38\%$. We hypothesise that the apparent lack of features in a third of high-redshift discs is due to a higher gas fraction in the early Universe, which allows the discs to be resistant to buckling and instabilities.

astro-ph.GA↗