If we were to divide the galaxies in the universe by color, we would notice that they are either bluish or yellow/orange/red, and the color depends on the number of new stars they are producing. You see, some galaxies stop forming stars, but the reason why is not completely clear.

So an international group of astronomers studied 70,000 galaxies across cosmic time to find out why and how star formation is quenched. The research, published in the Astrophysical Journal, highlights how mergers and interactions with other galaxies both big and small can profoundly affect massive galaxies in the recent universe. Collisions can initiate star-formation episodes, but also rob intergalactic space of cold gas and create the right conditions in a galaxy for the internal processes to quench star formation.

“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,” said study co-author Dr David Sobral in a statement. “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.”

The internal episodes are either generated by a lot of new stars quickly going supernova (heating and dispersing interstellar gas) or by the activation of the supermassive black hole at the center, which can generate powerful galaxy winds that quench star formation.

Galaxies in the early universe were producing bigger, hotter stars, and their black holes were a lot more active, so it’s not surprising that the internal mechanisms were more efficient then.

“Our findings provide important clues towards understanding which process dominates quenching at various cosmic times,” said Sobral. “It is another big step in understanding the dramatic `cosmic crisis’ that the universe has been experiencing over the last 11 billion years.”

This is just the first step in the research. The team plans to expand this analysis to a significantly larger number of galaxies, providing stronger confirmation of what has been observed so far.