Distinguishing ram pressure from tidal interactions: Size-shape difference measure

¹ Departamento de Física, Universidad Técnica Federico Santa María, Vicuña Mackenna 3939, San Joaquín, Santiago de Chile, Chile
² Millenium Nucleus for Galaxies (MINGAL), Valparaíso, Chile
³ Center for Computational Astrophysics, Flatiron Institute, 162 5th Ave, New York, NY, 10010, USA
⁴ Observatoire Astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, F-67000 Strasbourg, France
⁵ INAF-Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5 122, Padova, Italy
⁶ Departamento de Fisica, Universidad Tecnica Federico Santa Maria, Avenida España 1680, Valparaíso, Chile

^⋆ Corresponding authors: rorysmith274@gmail.com, stonnesen@flatironinstitute.org

Received: 19 February 2025
Accepted: 16 June 2025

Abstract

Aims. In dense environments, disk galaxies can be subjected to tidal interactions with other galaxies and/or ram pressure stripping. Some morphological features are clearly associated with one or the other interaction (e.g., tidal bridges versus long one-sided linear gas tails). However, under certain circumstances, both of these mechanisms can result in morphological features that might be easily confused, such as lopsided or asymmetric disks and unwinding spiral arms.

Methods. Our aim is to develop new measures that can be applied to asymmetric galaxies of this type, distinguishing gravitational-only tidal interactions from ram pressure stripping cases. Such measures could then be applied directly to simulations and potentially to observations as well.

Results. We defined a new measure for galaxies called the size-shape difference (SSD). This measure is sensitive to differences in the size and shape of a younger stellar population (< 200 Myr) compared to that of an intermediate-age stellar population (200–400 Myr). To test the SSD measure, we used numerical simulations of galaxies undergoing gravitational-only tidal interactions and/or undergoing ram pressure stripping.

Conclusions. Because ram pressure tends to directly alter the gas distribution, the younger stellar population (which serves as the best tracer of the gas distribution) tends to change shape and morphology with respect to the intermediate-age population. The SSD measure is sensitive to this change and we find it can effectively distinguish between ram pressure and gravitational-only tidal encounters. In fact, we find it is even more effective at identifying cases where a combination of a tidal interaction and ram pressure has occurred together, as may arise in dense environments. As tidal interactions tend to enhance the spiral structure in disk galaxies, the effectiveness of the SSD measure is further enhanced when combined with a measure of the spiral arms’ strength.