Are Recently Quenched Ellipticals Truly Isolated Centrals?

Abstract

Recently Quenched Ellipticals (RQEs) provide a valuable test case for disentangling intrinsic and environmental quenching, particularly because they are commonly classified as isolated central galaxies in low-mass halos. However, central/satellite assignments and isolation labels can vary across group catalogs, and such misclassifications can strongly bias physical interpretations. We present a uniform, physically motivated reassessment of the environments of 155 RQEs previously identified as centrals in an SDSS-based group catalog. We construct value-added neighbor catalogs via KD-tree searches and apply consistent thresholds in projected separation and line-of-sight velocity to (i) verify centrality, (ii) quantify isolation using a mass-ratio-based companion criterion, and (iii) identify potential pseudo-centrals via proximity to massive clusters. We find that 132/155 (85.2\%) RQEs remain true centrals, while 23/155 (14.8\%) are better interpreted as misidentified centrals with a more massive neighbor within $R_{\rm proj}\leq0.8$~Mpc and $\Delta V\leq250$~km~s$^{-1}$. Among the true centrals, 110/132 (83.3\%) satisfy our isolation criterion, and only one system meets our pseudo-central definition, indicating that direct cluster-scale preprocessing is rare for RQE centrals. Using projected number density and surface stellar mass density, we show that misidentified and non-isolated systems occupy systematically denser regimes than isolated true centrals. These results imply that while most RQEs ($\sim$71\%) are consistent with predominantly internal quenching in genuinely isolated centrals, a non-negligible minority ($\sim$29\%) likely experienced environmentally influenced pathways at group scales.