From relics to stripped systems: the environmental origin of compact galaxies

Abstract

Compact galaxies represent a key population for understanding galaxy evolution, but the relative role of internal structure and environment in shaping their origin remains unclear. We aim to investigate how the abundance and quenching of compact galaxies depend on environment, and to determine whether their formation is governed by a single evolutionary pathway or multiple channels. We analyse a large sample of galaxies in the nearby Universe ($z<0.05$) with stellar masses in the range $8.2<\log(M_\star/M_\odot)<10.5$, comparing compact and control populations as a function of local overdensity. We find that the environmental dependence of compact galaxies is non-monotonic, with two characteristic overdensities, the transition overdensity ($δ_t$) and the critical overdensity ($δ_c$), defining three distinct regimes. Below $δ_t$, compact galaxies are relatively more abundant than the control population, while between $δ_t$ and $δ_c$ their relative abundance decreases significantly. Above $δ_c$, compact galaxies become increasingly overabundant again. The environmental trends show a strong stellar-mass dependence, with low-mass galaxies exhibiting the strongest deficit at intermediate overdensities and high-mass galaxies dominating the excess population above $δ_c$. In addition, the red galaxy fraction increases with local overdensity and is systematically higher in compact galaxies. Our results support a unified evolutionary framework in which compact galaxies do not arise from a single evolutionary channel, but instead reflect multiple pathways whose relative importance depends strongly on stellar mass and environment. Low-mass compact galaxies are closely linked to environmentally driven transformation, whereas high-mass compact galaxies occur across all environments and become increasingly overabundant in the densest regions.