March 30, 2017

The dominant processes that stop the formation of stars in galaxies is currently still unknown. Similarly, we do not know what grows galaxies after they stop their star formation and therefore should not change their size and mass anymore. Likely the life of very massive galaxies is very different than that of low-mass galaxies.

This new study on COSMOS targets to answering these questions for the most massive galaxies in our Universe. It uses the COSMOS/UltraVISTA near-infrared data to measure the correlation between size and stellar mass for a sample of 400 of the most massive star-forming and quiescent galaxies over the past 10 billion years. The large 2-deg² contiguous area of COSMOS is crucial to find these rare galaxies.
With an empirical model, the authors follow star-forming galaxies over time and eventially stop their star formation assuming different processes of quenching. By comparison of the modeled size vs. stellar mass relation of quiescent galaxies with the observed one, this study draws the conclusion that collisions between galaxies play an important role in the final parts of the life of these most massive galaxies. A possible path of quenching could be a major collision triggering a central starburst that consumes quickly the gas in the galaxy and likely expells and heats parts of it.
Furthermore, while the average size growth of less massive quiescent is due to the addition of newly quenched larger galaxies over time, this study makes clear that this is not enough for massive galaxies, which individually have to experience a substantial growth via collisions with less massive galaxies.

The paper is accepted for publication in ApJ and can be downloaded here!