Observational Problems for Collision/Merger Scenarios
Infrared observations have shown interacting galaxies to be sites of enhanced star formation. The most extreme starbursting galaxies, however, tend to be smaller companions or isolated galaxies. It is argued that it is the recent formation of stars and newer galaxies from the internal activity of older galaxies, not the collision of previously separate galaxies, which is responsible for the current starbursts. It is proposed that material outflowing from active nuclei cools and condenses under the constraint of jets. The strong correlation between the intensity of non thermal radio emission and the infrared emission from star formation supports this model. Multi-wavelength observations of specific galaxies such as NGC 2777 are used to illustrate a recent ejection origin for starburst companions.
The general hypothesis of collision/mergers is considered. It is argued that collisional interaction generally inhibits rather than promotes star formation. It is pointed out that real peculiar velocities of galaxies are not sufficient to produce appreciable collisions and moreover that the necessary reservoir of potential inieractors in groups is observationally absent.
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