Environmental Sculpting of Galaxy Structure at Fixed Stellar Mass: A Multi-Scale Analysis Across Cosmic Time using 3 Million HSC Galaxies

Abstract

The extent to which galaxy structure is shaped by environment beyond the local universe, once stellar mass is controlled, remains an open question in galaxy evolution. We address this challenge using an unprecedentedly large sample of $\sim$3 million galaxies from the Hyper Suprime-Cam Subaru Strategic Program spanning $0.3\le z<0.7$ with $\log (M/M_{\odot })\ge 8.9$. We correlate a mass-independent bulge-to-total ratio statistic with large-scale overdensity maps and cluster catalogs, propagating structural parameter posteriors through a Monte Carlo framework to robustly assess significance. We confirm with $>5\sigma$ confidence that galaxy structure depends on environment at fixed stellar mass, but this dependence is secondary to stellar mass and varies with redshift, mass, and environmental scale. At $z<0.5$, we detect no significant structural correlation with large-scale overdensity, but cluster galaxies show statistically significant bulge enhancement compared to mass-matched field galaxies, indicating cluster-specific processes such as ram-pressure stripping and cumulative tidal interactions dominate structural transformation at these epochs. At $z\ge 0.5$, massive galaxies exhibit bulge-enhancement across both cluster- and large-scale environments, while lower-mass systems show enhancement only in cluster environments. This indicates that environmental mechanisms operate across broader spatial scales at earlier cosmic epochs, and enhanced merger rates, group preprocessing, and cosmic web stripping augment cluster-specific processes. By separating into star-forming and quiescent subsamples, we find nearly flat trends within each, demonstrating that the observed environmental effects arise from coupled morphological and star formation transformations. These results collectively reveal the multi-scale, epoch-dependent nature of environmental effects on galaxy structure.

25 pages, 15 figures, published in ApJ, C. Igel and A. Ghosh contributed equally to this work

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