Research: Interactions and Mergers


The galaxy sequence above is constructed to study the evolution of spiral galaxy mergers. A sample of interacting/merging systems at different stages during the interaction is ordered into a time sequence. This comprises: (i) early (top row left) and evolved (top row right) pre-mergers where the two spirals are clearly separated, (ii) systems at nuclear coalescence where the participant galaxies have just merged into a single body (middle row) and (iii) young (bottom row left) and evolved (bottom row right) post-mergers. Following the evolution of various galaxy properties along this sequence can provide valuable information on the nature of tidal interactions between spiral galaxies and their significance in galaxy formation and evolution.

Tidal interactions and mergers are believed to play a key role in the evolution of galaxies. Such phenomena can not only enhance the activity in and around the nuclear region (star-formation, AGN) but are also believed to irreversibly alter the morphological appearance of the participant galaxies.

Of particular significance among galaxy interactions are mergers between spiral galaxies. These are thought to be responsible not only for the spectacular starbursts revealed by IRAS but also and most importantly for the formation of elliptical galaxies. Indeed, this formation scenario, called the ``merger hypothesis'', has gained significant impetus over the past few years by both observational and theoretical studies.

However, despite the extensive work on disc galaxy mergers, little is known about the evolution of these systems during the various stages of the interaction (pre-mergers, nuclear coalescence, post-mergers). Moreover, it is not yet clear how the properties of post-mergers compare with those of ``normal'' (old/relaxed) ellipticals. Such a study has the potential to address the following points:

  • investigate whether post-mergers evolve with time to resemble ``normal'' ellipticals and hence, test the ``merger hypothesis'',
  • explore the conditions necessary to trigger powerful starbursts similar to the far-infrared luminous systems observed by IRAS (e.g. cold gas content, interaction geometry etc).
  • investigate the relevance of mergers to the study of distant active galaxies revealed by observations at various wavelengths (e.g. X-ray, radio, UV)
  • constrain and refine models on galaxy formation and evolution.

To study disc galaxy mergers a sample of merging systems at different stages during the interaction has been compiled. This sample is then ordered in chronological order, starting from early interacting spirals, systems close to nuclear coalescence and finally post-mergers spanning a wide range of ages after the merger event. The evolution of galaxy properties (e.g. cold gas content, radio-emission, dust proprties; Georgakakis et al. 2000; 2001; Xilouris et al. 2004) along this sequence is then studied to address the points above.