A Dynamical Model of the M101 / NGC 5474 Encounter

TL;DR

This paper presents a dynamical simulation of the M101 galaxy's encounter with NGC 5474, explaining its asymmetries, starburst regions, and structural features resulting from a close, retrograde passage 200 million years ago.

Contribution

First detailed dynamical model of M101's interaction with NGC 5474 that reproduces observed features and explains their origins.

Findings

Reproduces M101's lopsidedness and extended features

Links starburst populations to galaxy collision points

Shows long-term structural changes due to interactions

Abstract

We present the first dynamical simulation that recreates the major properties of the archetypal nearby spiral galaxy M101. Our model describes a grazing but relatively close (14 kpc) passage of the companion galaxy NGC 5474 through M101's outer disk approximately 200 Myr ago. The passage is retrograde for both disks, which yields a relatively strong gravitational response while suppressing the formation of long tidal tails. The simulation reproduces M101's overall lopsidedness, as well as the extended NE Plume and sharp western edge of the galaxy's disk. The post-starburst populations observed in M101's NE Plume are likely a result of star formation triggered at the point of contact where the galaxies collided. Over time, this material will mix azimuthally, leaving behind diffuse, kinematically coherent stellar streams in M101's outer disk. At late times after the encounter, the density profile of M101's disk shows a broken "upbending" profile similar to those seen in spiral galaxies in denser environments, further demonstrating the connection between interactions and long-term structural changes in galaxy disks.