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

Compact galaxies represent a key population for understanding galaxy evolution, as their high stellar densities are closely linked to early formation and/or stripping processes. However, the relative role of internal structure and environment in shaping their origin and evolution 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 < łog(M_⋆/M_⊙)<10.5, comparing compact and control populations as a function of local overdensity by measuring the fraction of galaxies above a given overdensity (δ) threshold. We find that the environmental dependence of compact galaxies is non-monotonic, with two characteristic overdensities, the transition overdensity (δ_̊m t) and the critical overdensity (δ_̊m c), defining three distinct regimes. Below δ_̊m t, compact galaxies are relatively more abundant than the control population, while between δ_̊m t and δ_̊m c their relative abundance decreases significantly. Above δ_̊m c, compact galaxies become increasingly overabundant again, although this behaviour strongly depends on stellar mass. Low-mass (8.2 < łog(M/M_⊙) < 9.0) compact galaxies exhibit the strongest deficit at intermediate overdensities. In contrast, high-mass compact galaxies (9.0 < łog(M/M_⊙) < 10.5) dominate the excess population above δ_̊m c. In addition, we find that the fraction of red galaxies increases with local overdensity in control and compact samples. However, the red galaxy fraction is systematically higher in compact galaxies than in the control sample, indicating that both environment and internal structure (compactness) play a role in galaxy quenching. The relative importance of these effects strongly depends on stellar mass, with environmental quenching being more prominent in low-mass compact galaxies, while internal structural properties may dominate in the high-mass compact population. 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 appear to be closely linked to environmentally driven transformation and quenching processes acting in groups, filaments, and cluster infall regions. In contrast, high-mass compact galaxies are found across all environments, but become increasingly overabundant relative to the control population in the densest regions. Their uniformly high red galaxy fractions and stellar surface mass densities indicate that they survive efficiently in high-density environments as structurally dense systems, similar to compact relic galaxies.