Quantification of the star-formation and black-hole growth rate in galaxies
Until the middle of the last decade, mergers of galaxies and the star formation episodes triggered by these phenomena had been considered as a major, even dominant, phase in the cosmic history of the formation of stars. This view of galaxy evolution stemmed from the increasing (i) fraction of interacting galaxies as a function of redshift (measured by the density of galaxy pairs or morphological analysis) and (ii) bolometric luminosity of distant galaxies whereas, in the local Universe, such high luminosities are only observed in merging systems. The latest progress in this field of astrophysics, particularly by our group, however, strongly suggest that the role of galaxy mergers in the global star-formation history of galaxies has been overestimated and that star-formation may instead be regulated by the combination of infalling extragalactic matter together with internal physics such as turbulence and feedback from both stars and active galactic nuclei.
Morphological transformation of galaxies over cosmic history
If the overall contribution of galaxy mergers in the history of star formation appears to have been fairly small, interaction phenomena have yet played a fundamental role in the mass growth of galaxies, including their morphological transformation in the course of cosmic evolution. Numerical simulations revealed the importance of continuous accretion of filaments of cold gas toward the heart of dark matter halos, and how this could help building disks of gas, and later one stars, around spheroids at lower redshift. This accretion of cold gas therefore appear to have played a significant role in the evolution of the morphology and properties of galaxies, with the possibility for them to evolve from an early-type stage to a star-forming phase.
Statistical properties of galaxy clusters and their implication on cosmology
Galaxy clusters form through hierarchical gravitational collapse driven by the merging of dark matter haloes. They are excellent laboratories for the study of dark matter and baryonic physics in structure formation, and are potentially powerful cosmological probes because they are sensitive to both geometry (luminosity distance) and the rate of expansion of the Universe (growth of structure). As clusters are dark matter dominated objects, and since gravity is a scale-free process, the galaxy cluster population is expected to exhibit structural similarity and scaling laws in mass and redshift. These scaling laws are fundamental to constrain the physics of cluster formation and for exploitation of the cluster population as a cosmological probe.