Memory in Mergers Major and Minor

Joshua E. Barnes

Institute for Astronomy, University of Hawai`i,
2680 Woodlawn Drive, Honolulu, Hawai`i 96822

An invited review presented at
Formation of the Galactic Halo...Inside and Out,
Tucson, Arizona, October 9-11, 1995.


Evolution of stellar-dynamical systems is reversible; given enough information on the phase-space structure of a merged system, its past history may be recovered. In mergers of equal-mass galaxies, kinematic memories of the progenitor disks partly survive violent relaxation. Dissipation introduces an irreversible element into galactic mergers, but in practice many stellar memories are left intact. Unequal-mass mergers are less disruptive of primary disks; in 3:1 encounters up to half the remnants retain disk-like kinematics. Accretion of satellite galaxies seems a viable process for producing the stellar halo, and also for heating the thick disk, of the Milky Way galaxy.

Selected Images

Retrograde 3:1 Merger. This sequence shows the parabolic encounter and subsequent merger of two disk galaxies with mass ratios of 3:1.


A compressed postscript copy of this paper is available. The text will be published in Formation of the Galactic Halo...Inside and Out, eds. H. Morrison & A. Sarajedini (Astronomical Society of the Pacific).

Joshua E. Barnes (

Last modified: December 12, 1995