The Morphological Origin and Transformation of Dwarf Galaxies
Gerhard Hensler
Department of Astrophysics
University of Vienna
From the predictions of CDM cosmology low-mass dark matter sub-halos are expected to form at first and to feed the major galaxies hierarchically. Also the baryonic mass fraction of dwarf galaxies (DGs) should have been assembled early in the universe. Because of their morphological appearance DGs have been traditionally classified in analogy to the massive Hubble-type galaxies. Comprehensive and detailed studies of DGs, however, have spotted a much larger variety and complexity of their substructures to exist and much stronger environmental effects to act continuously during their evolution. Since DGs are very sensitive to energetic influences due to their low gravitation, they are exposed to internal processes, as e.g. starburst-driven galactic mass loss, and to external influences, exerted e.g. in galaxy groups and clusters. Both effects can account for morphological transitions. This presentation sheds light on the various evolutionary effects of DGs from a possible birth path in tidal tails of merger galaxies to changes of their observable structure and morphology:
1. The enhanced star formation in some DGs seems to be triggered by gas accretion, but, on the other hand, leads to strong galactic outflows. How do gas infall and outflow interact and how do they affect the evolution?
2. Ram pressure by intergalactic gas can evacuate DGs from their gas content and transform them morphologically. But how can their morphological variety in galaxy clusters be understood?
3. The Milky Way and probably other Hubble-type spirals are surrounded by a system of satellite DGs which form the faintest population and must evolve much different from isolated DGs in the field. Which characteristics can identify their formation scenario?
4. DG-like objects are formed in tidal tails of mergers. Whether such DGs can contribute a significant fraction to the present-day DG population is yet unexplored and depends on their survival probability as dark matter-free objects.