Behnam Darvish, California Institute of Technology – Why Galaxies Stop Forming Stars

Galaxies stop forming stars, and scientists are working on finding out how.

Behnam Darvish, postdoctoral scholar in physics at the California Institute for Technology, delves into this scientific quandary.

Postdoctoral Scholar in Physics at California Institute of Technology. Research interests:  formation and evolution of galaxies in dense environments (clusters, groups, filaments).  Behnam Darvish was a Graduate Student at the University of California Riverside where this research was conducted.

Why Galaxies Stop Forming Stars

Galaxies are divided into two populations based on their colors – blue and red. Blue galaxies are actively forming stars whereas the red ones are passive with little or no star formation activity. A long lasting question in astronomy is how do galaxies stop forming stars? This process, sometimes called “quenching,” is not well understood.

The main sites of star formation in galaxies are cold and dense molecular gas clouds. Therefore, any physical process that is able to remove the gas, halt the gas supply to galaxies or heat it up can potentially stop the formation of new stars. This can be a result of the environment of galaxies, external mechanisms, and/or internal processes. Galaxies located in dense environments are stripped of their gas because of multiple gravitational encounters with other systems and the dense environment. Also, the drag force generated by infalling galaxies within clusters can effectively remove their gas content. Further, the presence of a black hole or stellar outflow, can deplete the galaxy of its gas content and/or heat it up.

Using one of the largest and most comprehensive datasets from space and ground-based observatories, our team studied with unprecedented detail the role of internal and external processes in the evolution of galaxies over the past 11 billion years.

We showed that, on average, external processes are only relevant to quenching galaxies during the last eight billion years, whereas internal processes were the dominant mechanism for shutting down star¬ formation before this time. This confirms that the nature of the quenching process varies at different cosmic epochs. We found that, on average, external processes act in a relatively short time-scale, and internal and external processes do not act independently of each other in quenching star formation.