New study at UCR explains why galaxies stop creating stars

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Galaxies stop creating stars at some point during their existence. Does this cessation come from within the galaxy or from external influences? This is one very important unsolved mystery in astrophysics in which Dr. Behnam Darvish, from Caltech/UC Riverside, sheds light in his latest research.



Galaxies are broadly distributed into two populations: blue and red. Blue galaxies are still actively forming stars, whereas red galaxies are almost quenched (not presenting current star-formation) and “dead”. The processes responsible for ceasing the star formation activity in galaxies (transforming them from blue/active to red/dead) are still one of the most important unsolved problems in galaxy formation and evolution. The results of this new research led by Dr. Behnam Darvish [Caltech/UC Riverside] studies the effects of both internal and external processes and their role in quenching star-formation in galaxies have been investigated.

The team of researchers  used a large sample of galaxies (around 70,000) from the COSMOS/UltraVISTA survey. The available data give accurate distance estimates allowing the study of galaxies over the past 11 billion years. This sample allows them to investigate both the external and internal quenching mechanisms. External mechanisms may be:

  • Ram pressure stripping: drag generated from an infalling galaxy within a cluster of galaxies, which pulls the gas away;
  • Galaxy harassment: multiple gravitational encounters with other galaxies and the dense environment, which results in material being stripped away from the galaxy;
  • Strangulation: the supply of cold gas to the galaxy is halted, thus ‘strangling’ the galaxy of the material needed to produce new stars over a prolonged period of time.

While internal mechanisms might be:

  • Active galactic nuclei: a black hole’s activity in the form or jets, winds, or intense radiation can heat up the hydrogen gas in the galaxy or blow it out completely, therefore preventing the gas from cooling and contracting to form stars;
  • Stellar outflow: high-velocity winds produced by young stars and supernovae that push the gas out of the host galaxy.


By using observable properties of the galaxies and statistical methods, the authors’ results show that on average, external processes are only relevant to quenching during the last 8 billion years, whereas the internal processes are the dominant mechanism for shutting-off star-formation before this time, and closer to the beginning of the Universe. Moreover, they found that the external processes act in a relatively short time-scale (around 1 billion years) and can more efficiently quench galaxies that are more massive, and that internal effects are also more efficient in dense clusters of galaxies.

The paper was co-authored by Prof. Mobasher [UC Riverside], Prof. Sobral [Lancaster University], Dr. Rettura [Caltech], Prof. Scoville [Caltech], Dr. Faisst [Caltech] and Prof. Capak [Caltech].


The research paper can be found here.
Full news release by UCR Today.


ESO 137-001 is a perfect example of a spiral galaxy zipping through a crammed cluster of galaxies. Gas is being pulled from its disc in a process called ram pressure stripping. The galaxy appears to be losing gas as it plunges through the Norma galaxy cluster. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Behnam Darvish & Mario De Leo-Winkler
Darvish graduated from UCR with a Ph.D. in astronomy in September 2015. The bulk of the research was done while he was working toward his doctoral degree. He is now a postdoctoral scholar at Caltech.