2026/07/17 by R. Ianjamasimanana, L. Verdes-Montenegro, K. M. Hess +8
Physics and Astronomy · #astro-ph.GA #astro-ph.CO
The 21 cm line of atomic hydrogen (HI) is a sensitive tracer of the outer disk of galaxies, where environmental signatures are most apparent. The relative extent of HI disks compared to optical disks (D\rm HI vs D25) is thought to provide a quantitative measure of such imprint, yet systematic comparisons between extreme environments remain scarce. We quantify the relative extent of HI disks in Hickson Compact Groups (HCGs) and in the Analysis of the interstellar Medium in Isolated GAlaxies (AMIGA) sample, using AMIGA as a control sample that captures secular evolution with minimal external influence. We calculate HI diameters by directly fitting an ellipse to the 1 M\odot \rm pc-2 iso-density contour. Because D\rm HI and D25 are nonlinearly related, we avoid the traditional D\rm HI/D25 ratio, which carries a size-dependent bias, and instead quantify truncation as the residual from the isolated-galaxy D\rm HI-D25 baseline, which we establish for AMIGA via Bayesian analysis. The full analysis is provided as a reproducible Python package and Snakemake workflow. HCG galaxies lie systematically below the isolated-galaxy baseline in the D\rm HI-D25 plane. When members with HI nondetections are included as upper limits, HCGs have HI disks at least ~71% smaller than expected for isolated galaxies of the same optical diameter. The truncation increases monotonically along the HCG evolutionary sequence, from Phase 1 to Phase 3. A comparison with literature samples places HCGs at the most-truncated end, statistically indistinguishable from the Virgo cluster sample (VIVA). Compared to AMIGA, HI disks are typically smaller relative to the optical disk in loose groups, compact groups, and cluster infall/field environments, and are most strongly truncated in HCGs and in the Virgo cluster sample.