Letter to the Editor

An isolated early-type dwarf galaxy that ran away from the group environment

¹ Department of Astronomy, Yonsei University, Seoul 03722, Republic of Korea
² Center for Galaxy Evolution Research, Yonsei University, Seoul 03722, Republic of Korea
³ Natural Science Research Institute (NSRI), University of Seoul, Seoul 02504, Republic of Korea
⁴ Department of Physics, Astronomy, Geology and Environmental Sciences, Youngstown State University, Youngstown, OH, USA
⁵ Department of Physics and Astronomy, The University of Tennessee, Knoxville, TN 37996, USA

^⋆ Corresponding author: sjyoon0691@yonsei.ac.kr

Received: 17 July 2025
Accepted: 26 August 2025

Abstract

Understanding the quenching mechanisms in dwarf galaxies is crucial for constraining models of galaxy formation and evolution. Isolated dwarf galaxies offer valuable insight by helping disentangle the relative roles of internal and environmental processes in shutting down star formation. Here we report the discovery of a quiescent early-type dwarf galaxy (dE), SDSS J011754.86+095819.0 (hereafter dE01+09), located in a nearly isolated environment at a projected distance of approximately one megaparsec from its most likely host group, the NGC 524 group. dE01+09 has M_r = −15.72 and g − r = 0.67 mag and its light profile is well described by a Sérsic function with an index n = 1.1, consistent with typical dEs. Using optical spectroscopy from the DESI survey, we derived its simple stellar population properties, and found an intermediate luminosity-weighted age of 8.3 ± 1.4 Gyr and a subsolar metallicity of −1.19 ± 0.21 dex, characteristics comparable to those of classical quiescent dEs. We propose that NGC 524 may represent an extreme example of group dynamics, in which a member galaxy, dE01+09, was ejected from its host group and subsequently evolved as an isolated system in the field.