Distinguishing ram pressure from gravitational interactions: Applying the Size-Shape Difference method to real galaxies

In dense environments, mechanisms like ram pressure stripping (RPS) and gravitational interactions can induce similar morphological features in galaxies, distinguishable only through detailed study of their stellar properties. While RPS affects recently formed stars by displacing the gas disk from which they form, gravitational interactions perturb stars of all ages rather similarly. We present the first observational test of the Size-Shape Difference (SSD) measure, a novel approach validated for simulated galaxies, that quantifies morphological differences between young and intermediate-age stellar populations to distinguish RPS from gravitationally interacting galaxies. We analyze 67 galaxies from the GASP survey using spatially-resolved star formation histories derived using SINOPSIS. In our fiducial model, we compare stellar populations in two age bins (t < 20 Myr and 20 Myr <= t < 570 Myr) to calculate SSD values. The sample includes confirmed RPS cases with different stripping intensities, as well as undisturbed and gravitationally interacting galaxies. We find that 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: youngest stars are compressed along the leading edge and/or displaced into the extended tails of cold gas, while older populations remain undisturbed. In contrast, gravitational interactions perturb all stars uniformly, producing lower SSD values. SSD robustly distinguishes strong RPS cases, even adopting different age bins. 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 surveys.

💡 Research Summary

This paper presents the first observational test of the Size‑Shape Difference (SSD) metric, a novel quantitative tool originally devised and validated on simulated galaxies, for distinguishing ram‑pressure stripping (RPS) from gravitational interactions in real galaxies. The authors apply SSD to a carefully selected subsample of 67 galaxies from the GASP (GAs Stripping Phenomena in Galaxies) survey, for which spatially resolved star‑formation histories (SFHs) have been derived with the SINOPSIS spectral fitting code.

In the “fiducial” implementation, the stellar population is split into two age bins: a “young” component (t < 20 Myr) that traces stars formed from the currently stripped gas, and an “intermediate‑age” component (20 Myr ≤ t < 570 Myr) that records the stellar distribution prior to the onset of stripping. For each component, the two‑dimensional stellar mass map is fitted with an ellipse, yielding an effective radius (R_e) and an ellipticity (e). The SSD parameter combines the normalized differences in radius and ellipticity between the two age bins into a single scalar: SSD = √