2026A&A...706A..85L

Context. In dense environments, mechanisms such as ram pressure stripping (RPS) and gravitational interactions can induce the formation of similar morphological features in galaxies that are only distinguishable through a detailed study of the stellar properties. While RPS affects recently formed stars through the displacement of the gas disk from which they are formed, gravitational interactions perturb stars in a similar way.
Aims. We present the first observational test of the size-shape difference (SSD) measure. This novel approach, which was originally designed and validated for simulated galaxies, quantifies morphological differences between young and intermediate-age stellar populations to distinguish RPS from gravitationally interacting galaxies.
Methods. We analyzed 67 galaxies from the GASP survey using spatially resolved star formation history derived from the SINOPSIS spectral fitting code. In our fiducial model, we compared stellar populations in two age bins (t < 20 Myr and 20 Myr ≤ t < 570 Myr) to calculate SSD values. The sample includes confirmed cases of RPS with different stripping intensities, as well as undisturbed and gravitationally interacting galaxies.
Results. We find that the extreme cases of RPS show SSD values ∼3.5× higher than undisturbed and gravitationally interacting galaxies ($ 56^{+24}_{-15} $ as compared to $ 16^{+6}_{-2} $ and $ 16^{+6}_{-3} $, respectively), which confirms simulation predictions. This enhancement reflects RPS-induced asymmetries: the youngest stars are either compressed along the leading edge or displaced into extended tails of cold gas from which they are formed (or both), while older populations remain undisturbed. In contrast, gravitational interactions perturb all stars uniformly, producing lower SSD values.
Conclusions. Size-shape difference robustly distinguishes strong RPS cases, even when different age bins are used. This holds even without correcting for disk inclination, or when single-band imaging are used to trace stellar distributions. This makes SSD a promising tool to select RPS candidates for spectroscopic follow-up in upcoming large-scale surveys.