Low-mass Quiescent Galaxies Are Small in Isolated Environments: Environmental Dependence of the Mass-Size Relation of Low-mass Quiescent Galaxies

We study the mass-size relation of quiescent galaxies across various environments, with a particular focus on its environmental dependence at the low-mass part of log ( M star / M ⊙ ) ≲ 10.0 . Our sample consists of 13,667 quiescent galaxies with log ( M star / M ⊙ ) ≥ 9.4 and 0.01 < z < 0.04 from the Sloan Digital Sky Survey. We discover that the mass-size relation of low-mass quiescent galaxies (LQGs) with log ( M star / M ⊙ ) ≲ 10.0 depends on their environment, with LQGs in the highest-density environments exhibiting an average size ∼70% larger than those in isolated environments. Moreover, the slope of the mass-size relation for LQGs in high-density environments is significantly shallower than that of their counterparts in isolated environments. This is in contrast with high-mass quiescent galaxies with log ( M star / M ⊙ ) ≳ 10.5 that show a nearly identical mass-size relation across all environments. Combined with additional discoveries that the mass-size relation slopes of LQGs and star-forming galaxies are similar to each other in high-density environments, and that LQGs in higher-density environments exhibit more disk-like structures, our results support the idea that LQGs in high-density environments have evolved from star-forming galaxies through environmental effects, which are capable of causing their quenching and transformation into quiescent galaxies. With the aid of an analysis of merger rates for simulated galaxies from a cosmological galaxy formation simulation, we suggest that the steep slope and low normalization of the mass-size relation of LQGs in the lowest-density environments may originate from recent gas-rich mergers, which occur over 10-30 times more frequently in the progenitors of LQGs in the lowest-density environments than in their counterparts in high-density environments at low redshifts.