Kinematics and simulations of the stellar stream in the halo of the Umbrella Galaxy

TL;DR

This study analyzes the stellar streams in the halo of the Umbrella Galaxy, combining imaging and spectroscopy to model the dynamics of the tidal disruption event and infer the orbit of the merging satellite.

Contribution

It demonstrates that the kinematics of faint halo substructures can be effectively recovered and modeled using discrete tracers, advancing understanding of minor galaxy mergers.

Findings

Identified kinematically cold substructures associated with the stream.

Inferred a highly eccentric orbit with a period of about 0.35 Gyr.

Suggested recent satellite passage through the galaxy's disc.

Abstract

We study the dynamics of faint stellar substructures around the Umbrella Galaxy, NGC 4651, which hosts a dramatic system of streams and shells formed through the tidal disruption of a nucleated dwarf elliptical galaxy. We elucidate the basic characteristics of the system (colours, luminosities, stellar masses) using multi-band Subaru/Suprime-Cam images. The implied stellar mass-ratio of the ongoing merger event is about 1:50. We identify candidate kinematic tracers (globular clusters, planetary nebulae, H ii regions), and follow up a subset with Keck/DEIMOS spectroscopy to obtain velocities. We find that 15 of the tracers are likely associated with halo substructures, including the probable stream progenitor nucleus. These objects delineate a kinematically cold feature in position-velocity phase space. We model the stream using single test-particle orbits, plus a rescaled pre-existing N-body simulation. We infer a very eccentric orbit with a period of roughly 0.35 Gyr and turning points at approximately 2-4 and 40 kpc, implying a recent passage of the satellite through the disc, which may have provoked the visible disturbances in the host galaxy. This work confirms that the kinematics of low surface brightness substructures can be recovered and modeled using discrete tracers - a breakthrough that opens up a fresh avenue for unraveling the detailed physics of minor merging.