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

TL;DR

The paper addresses the challenge of distinguishing ram pressure stripping (RPS) from gravitational interactions in dense environments by applying the Size-Shape Difference (SSD) measure to spatially resolved stellar populations in real galaxies.Building on simulations, SSD compares the morphology of young versus intermediate-age stellar distributions using SFHs derived with SINOPSIS, showing strong RPS galaxies exhibit significantly larger SSD values than undisturbed or gravitationally interacting systems.The observational test uses 67 GASP galaxies, adapts SSD to SFRD maps, normalizes by galaxy size, and demonstrates robustness to age-bin choices and measurement configurations, including the use of imaging data with Hα and broad-band filters.These results establish SSD as a practical, scalable tool to identify strong RPS candidates for spectroscopic follow-up in upcoming surveys, with implications for efficient targeting in large galaxy samples.

Abstract

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.

Figures (13)

• Figure 1: SSD method applied to GASP galaxies. Top: Spatially resolved maps of star formation rate surface density (SFRD) for the control galaxy with no identified environmental effects, A3376_B_0261, obtained with sinopsis. The top-left frame shows the $t < 20\,$Myr SFRD, while the bottom frame corresponds to the age interval $20\,\mathrm{Myr} \leqslant t < 570\,\mathrm{Myr}$. The color scale is logarithmic with SFRD within the range $-4 \lesssim \log(\mathrm{SFRD}) \,[M_{\odot}\,\mathrm{yr}^{-1}\,\mathrm{kpc}^{-2}] \lesssim -0.5$ . The black line with an arrow indicates the starting point and rotation direction of the varying angle slices (i.e., beginning from South in clockwise rotation). Blue and red contours show the Lagrangian radii enclosing 75%, 90%, 95% and 99% of the total SFRD within each angular slice for the young and intermediate age bins, respectively. The lines grow in a bottom-to-top orientation, following the cumulative order of the Lagrangian radii (i.e, 75%, 90%, 95% and 99%). The black bars on the bottom-right corner of each frame correspond to the mean error of all Lagrangian radii for each case. Angle steps of $\Delta \theta = 20^\circ$ were adopted. Bottom: Same as top, but for the JType = 2 galaxy, JW100. Top and bottom frames share color scales.
• Figure 2: Distribution of $R_e$-normalized SSD values as a function of $R_e$ for JType = 2 (red stars), undisturbed (green triangles), and gravitationally interacting (blue diamonds) galaxies. Empty symbols indicate galaxies for which $R_e$ was estimated using the empirical $R_e$–$M_{\star}$ relation from Franchetto2020. Solid lines represent the median SSD values for each galaxy type, while the shaded regions correspond to the interquartile range (25% -- 75%). Dashed lines correspond to the 25% and 75% of each galaxy type. The SSD value distributions, along with their medians and interquartile ranges, are also shown in the histogram on the right-hand panel.
• Figure 3: Boxplot showing the normalized distribution of SSD values for undisturbed galaxies (green), JType=1 (purple), JType=2 (red), JType=3 (cyan) and gravitationally interacting galaxies (blue). Each point corresponds to the SSD measure of an individual galaxy. A minor shift was applied in the $x$-axis for clarity. The box sizes represent the interquartile range (25% -- 75%) for each galaxy type, while the whiskers indicate the 10% -- 90% range of each distribution.
• Figure 4: SSD values across different galaxy categories, comparing results obtained when different age bins are used to trace the stellar distribution, as indicated in the legend. Thicker lines show out fiducial value, shown in Fig. \ref{['fig:cfr_types']}. Colors and symbols are as in Fig. \ref{['fig:cfr_types']}.
• Figure 5: SSD values as a function of galaxy stellar mass. Galaxy types are represented by different symbols: JType = 2 (red stars), undisturbed (green triangles) and gravitational interactions (blue diamond). If a galaxy hosts an AGN and/or exhibits a bar, an additional square or circle is overplotted, respectively. The vertical black dashed line marks the division between mass regimes at $\log (M_{\star} / M_{\odot}) = 10.5$. Solid lines and shaded areas indicate the median and interquartile range of SSD values for each galaxy type, respectively, computed within their respective mass regime.