Interacting and Merging Galaxies

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Observed galaxies exhibit a wide range of morphologies, traditionally arranged in a sequence from almost featureless elliptical galaxies through disk-like objects to magnificent grand design spirals, which are among the most beautiful objects in the sky. There are also various kinds of peculiar galaxies, which do not fit into this scheme. One subset of these shows long tails of stars and gas, extending to distances as large as several hundred kiloparsec from the main bodies of the galaxies.

Such tails can be ejected by tidal forces during close encounters between spirals. Usually, these collisions lead to a final coalescence of the galaxies, turning them into a single pile of stars in which they lose their original identity.

In Figure 1, we show a typical example of the time evolution of such a galaxy merger, as seen in one of our computer simulations. Here, two equal disk galaxies fall towards each other under their mutual gravitational attraction. The time (in billions of years since the beginning of the simulation) is indicated by the number in each picture. When the galaxies reach closest approach for the first time, tidal forces quickly transform the disks into a pair of open bisymmetric spirals. Simultaneously, stars and gas on the outside of the encounter are ejected into arcing trajectories which later form the tidal tails. Material from the near side is drawn towards the companion, giving rise to bridges between the separating galaxies. These bridges are destroyed when the galaxies come back together for a second encounter, but the tails survive and grow for a longer time in the quiet outer regions. The disks are destroyed as the centres of the galaxies coalesce to form a spheroidal merger remnant - an elliptical galaxy?

Figure 1

Recently, interest in tidal tails has been stimulated by numerical work that showed that disk galaxies of differing structure can differ strongly in their susceptibility to tail formation. This suggests that tidal tails may provide important clues about the amount of dark matter in galaxies, and about its distribution relative to that of the visible light.

We have simulated many collisions between disk galaxies to study this point further. From our work it appears that in the context of current cold dark matter (CDM) theories many interacting systems should be capable of tail formation, i.e. the abundance of observed systems with tidal tails can be explained relatively easily. This conclusion is practically independent of the cosmological parameters of the CDM models. It thus seems unlikely that tidal tails will be useful to constrain cosmological parameters.

Figure 2

Tidal tails nevertheless remain a very useful tool to probe the structural properties of galaxies. Here, they can potentially provide powerful constraints, especially if detailed dynamical modeling of observed interacting systems is combined with observations of their velocity field. In collaboration with Pierre-Alain Duc (University of Cambridge), we have started such a detailed study. In Figure 2 we show first results from a study of the interacting pair of galaxies NGC2992/3. The panel on the top left shows the optical image of NGC2992/3, while its color-coded HI velocity field is displayed on the top right. On the lower left we show the time evolution of the gas distribution in one of our computer models of this system, and the corresponding velocity field is seen on the bottom right.

Figure 3

Mergers of galaxies are a common and inevitable process according to current theories for the growth of cosmic structure, and for the formation of the luminous galaxies within it. In such theories, galaxy formation proceeds hierarchically; small galaxies form first, and merging and collapse processes build-up ever more massive systems. Current numerical work at MPA follows these merging processes directly in the correct cosmological background. For example, in Figure 3 we show a picture of a high-resolution simulation of a cluster of galaxies. Each lump of dark matter in this image presumably corresponds to an individual galaxy. The analysis of the detailed merging history of the dark matter in this simulation will lead to more accurate predictions for the galaxy population expected in CDM cosmologies.

V. Springel, S. D. M. White

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Last modified: Wed May 19 13:27:42 MDT 1999 by Markus Rampp
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