2025ApJ...993..229S

Compact elliptical (cE) galaxies are compact stellar systems with stellar masses of 10⁸ ≤ M_*/M_⊙ ≤ 10¹⁰ and radii typically <0.6 kpc. Here we investigate the properties of 13 cE galaxies in the Coma cluster, 6 of which are newly identified. Our goal in this paper is to explore whether these cEs form directly in the cluster environment or are preprocessed in small groups before infalling. We find that preprocessing in groups significantly contributes to the cE population in the Coma cluster. We analyze Hyper Suprime-Cam g-band Coma images and validate our photometric measurements through comparison with Hubble Space Telescope data. We also analyze spectroscopic data from the Dark Energy Spectroscopic Instrument. We significantly expand the known cE population in the Coma cluster through joint photometric and spectroscopic selection. We identify a subpopulation of cEs that likely formed in infalling groups, through their association with host galaxies, their positions on the caustic diagram, and their projected phase-space trajectories. We estimate that the central cE population will increase by up to 30% within the next 0.4 Gyr, highlighting the important role of preprocessing in cE evolution.