EXtragalactic Astronomy Research Group   
of
Thorsten Lisker
at the
Zentrum für Astronomie der Universität Heidelberg (ZAH)

Our independent research group has been established in May 2007 within the framework of the Excellence Initiative by the German Research Foundation (DFG) as part of the Heidelberg Graduate School of Fundamental Physics. It focuses on the origin of the various galaxy types that we see in the Universe today. These include the types that are described by the famous Hubble-sequence, but also the smaller dwarf galaxies, which constitute the most numerous type of galaxies in the Universe. We aim at understanding the characteristics of these galaxies, their evolution, and the processes that gave them their present-day appearance. To this end, several local collaborations have already been established, in order to benefit from the wide-ranging expertise among the different Heidelberg astronomy institutes.

Deep space-based imaging surveys like the Hubble Ultra Deep Field reveal a variety of distant galaxies that can hardly be described by standard classification schemes: many objects consist of several bright clumps, are highly distorted, or appear to be galaxy pairs in a phase of heavy interaction. Some of these might end up as massive elliptical galaxies, while others might form spiral galaxies like our Milky Way. We thus need to bridge the gap between the distant, younger galaxies and the nearby, further evolved ones, in order to understand the mechanisms that shape them.

As for the dwarf galaxies, it is still difficult to observe them at larger distances because of their very low surface brightnesses. However, the studies of dwarf galaxies in nearby clusters are sufficient to conclude that many of them cannot have been in place since long, but must have formed through a structural transformation of other types of galaxies. For example, a spiral galaxy that falls into a galaxy cluster might experience several close encounters with massive cluster galaxies, during which it gets stripped of its gas, and is finally transformed into an elliptical dwarf galaxy. The attempt to trace back these and other formation mechanisms faces the primary challenge of astronomical research: to follow the evolution of stars and galaxies over billions of years, solely through the information provided by the photons that arrive today.