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

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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>85</mml:mn> <mml:mi>%</mml:mi> </mml:mrow> </mml:math> of the entire KCWI sample exhibit large physical sizes () and low surface brightnesses ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mn>24</mml:mn> <mml:mo>≲</mml:mo> <mml:msub> <mml:mi>μ</mml:mi> <mml:mrow> <mml:mn>0</mml:mn> <mml:mo>,</mml:mo> <mml:mi>g</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≲</mml:mo> <mml:mn>25</mml:mn> </mml:mrow> </mml:math> mag arcsec -2 ) which categorize them as UDGs. We find <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mn>20</mml:mn> <mml:mi>%</mml:mi> </mml:mrow> </mml:math> of the confirmed UDG population contain post-starburst (or K+A) features, characterized by minimal to no emission in H <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>β</mml:mi> </mml:math> 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 away from potential nearby massive hosts ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⋆</mml:mo> </mml:msub> <mml:mo>&gt;</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>10</mml:mn> </mml:msup> <mml:mspace width="0.222em"/> <mml:msub> <mml:mi mathvariant="normal">M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> </mml:mrow> </mml:math> ), 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.