Galaxy Clusters from the DESI Legacy Imaging Surveys. II. Environmental Effects on the Size-Mass Relation

To investigate the environmental effects on the growth of galaxies, we study the size-mass relation across a broad range of environments with a vast sample of approximately 32 million galaxies at z < 0.5 from the DESI Legacy Imaging Surveys. This sample is divided into three subsamples representing galaxies within three different environments: brightest cluster galaxies (BCGs), other cluster galaxies, and field galaxies. The BCGs in our large sample are dominated by quiescent galaxies (QGs), while only a minority (similar to 13%) of BCGs are star-forming galaxies (SFGs). To demonstrate the influence of the environment on size growth, we attempt to observe the difference in size-mass relation for these three subsamples. In general, the slope differences between QGs and SFGs within various environments are significant and tend to be greater at higher redshifts. For the mass-complete subsamples at z < 0.5, BCGs are found to have the highest slope of size-mass relation, and no difference in size-mass relation is found between cluster members and field galaxies. To assess whether the observed slope differences stem from the variations in environment or mass distribution, we construct the mass-matched subsamples for QGs and SFGs. As a result, both QGs and SFGs show negligible differences in the slope of the size-mass relation among the galaxies within three distinct environments, indicating that stellar mass is the most fundamental factor driving the size evolution at z < 0.5, though the mass growth mode for QGs and SFGs may have been affected by galaxy environment.