Size-Shape Difference for 67 GASP galaxies

In our fiducial model, we compare stellar populations in two age bins (t<20Myr and 20Myr<=t<570Myr) to calculate SSD values. The sample includes confirmed cases of RPS with different stripping intensities, as well as undisturbed and gravitationally interacting galaxies. We find that the extreme cases of RPS show SSD values ~3.5x higher than undisturbed and gravitationally interacting galaxies (56^+24^_-15_) as compared to 16^+6^_-2_) and 16^+6^_-3_, respectively), confirming 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. SSD 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.