GALAXY MERGING · At which rate do galaxies merge in the universe? Unveiling galaxy evolution through two-dimensional kinematic studies

Final Activity Report Summary - GALAXY MERGING (At which rate do galaxies merge in the Universe? Unveiling galaxy evolution through two-dimensional kinematic studies)

Galaxies are not isolated systems in our Universe. On the contrary, they experience encounters of all kinds. It is now known that galaxy encounters, or interactions, play a very important role in the formation and evolution of these systems. When galaxies 'meet' their structure changes, and very often strong bursts of star formation take place changing the face of the galaxies involved. Most of the time, galaxies that interact become eventually a single system, often called a 'galaxy merger'. Observations showed that galaxy interactions and mergers were more common in the past. Knowledge of the number and the way galaxies interact with time is very important to understand the evolution of the Universe and test the present theories on structure formation. If merging is at work during the lifetime of galaxies, then their space density, mass, luminosity and morphology must change with epoch. The main goal of this project was therefore to determine the merger rate with time, that is how often and how soon do galaxies merge according to epoch in the Universe. The evolution of the merger rate with time was a key observable that could be used to test galaxy formation models. Nevertheless, the time scales associated with merging were not straightforward to constrain, since there was a considerable uncertainty in any transformation from an 'instantaneous' merger rate, computed directly from observations at a given look-back time, to a 'global' merger rate indicating the likelihood for a galaxy to experience merging from any epoch to the present. Transforming the observed merger fraction into a merger rate required knowledge of the average time-scale over which a merger was completed, i.e. the time over which morphological traces of the merger were gone at the resolution at which one was observing a particular sample. An upper limit for this time scale of 400 Myr to 1 Gyr could be inferred from both data and numerical simulations. Nevertheless, there were still considerable uncertainties in the value of this rate. For this project, the uncertainties in the average time-scale over which a merger was complete were to restrain through the detailed observations of the kinematics of particular interacting pairs at different epochs, or redshifts, and comparing these results with simulations of interacting pairs done with computers. Observations of interacting pairs encompassed distant galaxies using the new three-dimensional multi-object spectroscopy instrument GIRAFFE at the European Southern Observatory (ESO) Very large telescope (VLT), as well as interacting pairs in the nearby Universe using the CIGALE scanning interferometer at ESO's 3.6 m telescope and the PUMA scanning interferometer at the 2.1 m telescope at the Observatorio Astronomico Nacional in San Pedro Martir, Mexico. We demonstrated the importance of kinematical information to constraint any model of an encounter between galaxies. Morphological information, i.e. the way a galaxy looked, could give misleading information on the configuration of the encounter and its nature. We saw that galaxies could be perturbed by much smaller galaxies that were barely detected, but they could still induce an important formation of stars. We also studied distant interacting galaxies combining different observation techniques, such as long-slit spectroscopy, direct imaging and integral field units, and instruments, e.g. the FORS2 spectrograph and the FLAMES-GIRAFFE spectrograph on the VLT, as well as the Hubble Space Telescope. We finally showed the importance and relevance of combining different sorts of information, i.e. kinematics, broad image and spectroscopy, in order to determine the stage and nature of an encounter at large redshifts.

Data: CORDIS, © European Union