The size of the HI disk across different environments: Isolated, compact groups, clusters, and pairs
September 2026 • 2026A&A...713A.233I
Abstract • Context. The 21 cm line of atomic hydrogen (H I) is a sensitive tracer of the outer disk of galaxies, where environmental signatures are most apparent. The relative extent of H I disks compared to optical disks (DHI vs D25) is thought to provide a quantitative measure of such an imprint, yet systematic comparisons between extreme environments remain scarce. Aims. We quantify the relative extent of H I disks compared to stellar disks in Hickson compact groups (HCGs) and in the Analysis of the interstellar Medium in Isolated GAlaxies (AMIGA) sample. We aim to establish how the compact group environment shapes the extent of the H I disk by using AMIGA as a control sample that captures secular evolution with minimal external influence. Methods. We calculated the H I diameters of AMIGA and HCG samples by directly fitting an ellipse to the 1 M⊙ pc-2 iso-density contour. Because DHI and D25 are nonlinearly related, we avoid the traditional DHI/D25 ratio, which carries a size-dependent bias, and instead quantify truncation as the residual from the isolated-galaxy DHI--D25 baseline. We established the DHI--D25 scaling relation for AMIGA via Bayesian analysis. We then quantified how HCGs deviate from this baseline. To maximize the reproducibility of this analysis, we provide a fully reproducible framework via an installable Python package and a Snakemake workflow that generates the full analysis, scientific products, and the manuscript. Results. HCG galaxies lie systematically below the isolated galaxy baseline in the DHI--D25 plane. When members with H I nondetections are included as upper limits, HCGs have H I disks that are at least ~71% smaller than expected for isolated galaxies of the same optical diameter. The H I disk truncation depends strongly on HCG evolutionary sequence, increasing monotonically from phase 1 to phase 3. A comparison with literature samples places HCGs at the most truncated end, which is statistically indistinguishable from the Virgo cluster sample (VIVA). Conclusions. Compared to AMIGA, H I disks are typically smaller relative to the optical disk in loose groups, compact groups, and cluster infall and field environments. They are the most strongly truncated in HCGs and in the Virgo cluster sample.
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