Caught in the Act of Quenching? -- A Population of Post-Starburst Ultra-Diffuse Galaxies

TL;DR

This study reports the discovery of post-starburst ultra-diffuse galaxies (UDGs) through spectroscopic analysis, revealing their properties, environments, and potential quenching mechanisms, thus shedding light on the diverse evolutionary pathways of UDGs.

Contribution

First spectroscopic confirmation of post-starburst UDGs, analyzing their properties and environments to understand their quenching processes and diversity.

Findings

20% of confirmed UDGs show post-starburst features.

Nearly half of post-starburst UDGs are isolated systems.

Some post-starburst UDGs are located far from massive hosts, indicating environmental independence.

Abstract

We report the discovery of post-starburst ultra-diffuse galaxies (UDGs), identified through spectroscopic analysis with KCWI at the Keck II Telescope. Our analysis is based on a sample of 44 candidate UDGs selected from the Systematically Measuring Ultra-Diffuse Galaxies (SMUDGes) program. Our measured spectroscopic redshifts reveal $\sim 85\%$ of the entire KCWI sample exhibit large physical sizes ($R_{e} \gtrsim 1~{\rm kpc}$) and low surface brightnesses ($24 \lesssim \mu_{0,g} \lesssim 25$ mag arcsec$^{-2}$) which categorize them as UDGs. We find $20\%$ of the confirmed UDG population contain post-starburst (or K+A) features, characterized by minimal to no emission in H$\beta$ indicative of quenched star formation and a predominant presence of spectral A-type stars. In surveying the local environments of the post-starburst UDGs, we find that nearly half are isolated systems, which is unusual given that isolated UDGs are most commonly found to be star-forming. Two of these systems reside $2-3~R_{\rm vir}$ away from potential nearby massive hosts ($M_{\star} >10^{10}~\mathrm{M}_{\odot}$), indicating the absence of environmental influence. These post-starburst UDGs may represent systems experiencing star formation feedback such that a recent burst may lead to (at least temporary) quenching. Overall, our results highlight the potentially diverse quenching pathways of UDGs in the local Universe.