The Internal Kinematics, Stellar Population, and Gas-phase Properties of The Pseudobulge in An Ultra-diffuse Galaxy: AGC721966

Abstract

Leveraging spectroscopic data from the Sloan Digital Sky Survey, we conduct a comprehensive analysis of the central stellar velocity dispersion, stellar population properties, star formation history, and gas-phase chemical abundances in AGC721966, a unique ultra-diffuse galaxy (UDG) harboring a pseudobulge. Our findings reveal that the pseudobulge formed in the early universe but underwent a recent episode of rejuvenated star formation. The system exhibits a mass-weighted (light-weighted) stellar population age of $\ddot{I}{„}_{\star }\sim 7.4\pm 2.5$ ($2.9\pm 1.5$)~Gyr, a stellar metallicity of [M/H]$\sim -0.62\pm 0.26$ ($-0.55\pm 0.20$), an $\hat{I}\pm$-element enhancement of [$\hat{I}\pm$/Fe]$\sim 0.36\pm 0.09$ ($0.37\pm 0.07$), and a gas-phase oxygen abundance of \Oabund$\sim 8.15\pm 0.03$. The central stellar velocity dispersion is measured as $\ddot{I}{ƒ}_{\mathrm{c}}\sim 57.9\pm 15.7$~km/s. These results provide robust evidence supporting the early halo-halo merging formation scenario proposed by \cite{Rong25}, while unequivocally ruling out the ``failed'' $L^{\star }$ formation model, at least for AGC721966. Furthermore, through systematic application of the baryonic Tully-Fisher relation, we establish that these pseudobulge-hosting UDGs are neither misidentified nuclear star cluster-bearing dwarf galaxies nor bulge-dominated massive galaxies, thereby affirming their distinct evolutionary pathway.

Submitted to ApJ Letters; comments welcome