Astronomers have studied 70,000 galaxies across cosmic time to find out why some cease generating stars.

The processes that cause galaxies to "quench," (cease star formation) are not well understood and constitute an outstanding problem in the study of the evolution of galaxies.

The international research team includes Behnam Darvish and Bahram Mobasher at the University of California, David Sobral from Lancaster University in the UK and Alessandro Rettura, Nick Scoville, Andreas Faisst and Peter Capak from Caltech.

By exploring the unique COSMOS UltraVISTA survey, astronomers were able to study the role of "nature" (internal processes) and "nurture" (external processes) in the evolution of galaxies over the past 11 billion years with unprecedented detail.

Dr. David Sobral said: "Just like humans, galaxies are affected by both the environment in which they form and evolve, but also by their 'nature' and internal processes; both can have dramatic effects."

"Surprisingly, we find that the external processes are only really relevant in shutting down the production of stars in galaxies over the last eight billion years. At earlier times in the universe, internal processes are the main mechanism for shutting down star formation. In other words: back then, it was all about nature, not nurture, but later on the environment starts to play a major role."

External mechanisms include drag generated from an in-falling galaxy within a cluster of galaxies, which pulls gas away; multiple gravitational encounters with other galaxies and the dense surrounding environment, resulting in material being stripped away from the galaxy; and the halting of the supply of cold gas to the galaxy, thus strangling the galaxy of the material needed to produce new stars.

Internal mechanisms include the presence of a black hole (in which jets, winds, or intense radiation heat up hydrogen gas in the galaxy or blow it out completely, thus preventing the gas from cooling and contracting to form stars) and "stellar outflows" (for example, high-velocity winds produced by massive young stars that push the gas out of the host galaxy).

Dr. David Sobral comments: "Our findings provide important clues towards understanding which process dominates quenching at various cosmic times. It is another big step in understanding the dramatic 'cosmic crisis' that the universe has been experiencing over the last 11 billion years."

Next, the research team will work on to extend this study to the environment of galaxies on much larger scales (in the cosmic web).

"Effects of Local Environment and Stellar Mass on Galaxy Quenching out to z~3," Behnam Darvish et al., 2016 July 10, Astrophysical Journal