SISSI: Supernovae in a stratified, shearing interstellar medium

I. The geometry of supernova remnants

¹ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr. 1, D-81679 München, Germany
² Max-Planck-Institut für extraterrestrische Physik, Giessenbachstr. 1, D-85741 Garching, Germany
³ Excellence Cluster ORIGINS, Boltzmannstr. 2, D-85748 Garching, Germany

^⋆ Corresponding author: lromano@usm.lmu.de

Received: 17 March 2025
Accepted: 6 August 2025

Abstract

Aims. We introduce the Supernovae In a Stratified, Shearing Interstellar medium (SISSI) simulation suite, which aims to enable a more comprehensive understanding of supernova remnants (SNRs) evolving in a complex interstellar medium (ISM) structured by the influence of galactic rotation, gravity, and turbulence.

Methods. We utilized zoom-in simulations of 30 SNRs expanding in the self-consistent ISM of a simulated isolated disk galaxy, as the first such simulation achieving sub-parsec resolution in a galactic context. The ISM of the galaxy was resolved down to a maximum resolution of ∼12 pc and we achieved a zoom-in resolution of ∼0.18 pc in the vicinity of the explosion sources. We computed the time evolution of the SNRs’ geometry and compared it to the observed geometry of the Local Bubble (LB).

Results. During the early stages of evolution (≲1 Myr), SNRs are aptly described by existing analytical models. Afterward, SNRs depart from spherical symmetry, within ∼1% of an orbit, earlier than galactic shear alone predicts, with deformation timescales correlating strongly with local density variations. The minor axis of oblate SNRs is preferably aligned with the galactic poles, while the major axis of prolate SNRs is aligned with galactic rotation, with a pitch angle in the range of 10 − 60°. This result is in agreement with the expectation from galactic shear, suggesting a shear-related origin, such as interactions with shear-deformed substructure. A comparison with the geometry of the LB reveals that it might be slightly younger than the previously estimated ∼14 Myr; otherwise, it exhibits a standard morphology for a SNR of its age and size.

Conclusions. Studying the geometry of SNRs can reveal valuable insights about the complex interactions shaping their dynamical evolution. Future studies targeting the geometry of Galactic SNRs can use these insights to obtain a clearer picture of the processes shaping the Galactic ISM.