ASTRONOMERS have looked nine billion years into the past to find evidence that galaxy mergers in the early universe could shut down star formation and affect galaxy growth.

New research led by Durham University shows that a huge amount of star-forming gas was ejected into the intergalactic medium by the coming together of two galaxies.

The researchers say that this event, together with a large amount of star formation in the nuclear regions of the galaxy, would eventually deprive the merged galaxy – called ID2299 – of fuel for new stars.

This would stop star formation for several hundred million years, effectively halting the galaxy’s development.

Astronomers observe many massive, dead galaxies containing very old stars in the nearby Universe and do not exactly know how these galaxies have been formed.

Simulations suggest that winds generated by active black holes as they feed, or those created by intense star formation, are responsible for such deaths by expelling the gas from galaxies.

Now the study, by a team from the university, the French Alternative Energies and Atomic Energy Commission (CEA) – Saclay and the University of Paris-Saclay, offers galaxy mergers as another way of shutting down star formation and altering galaxy growth.

The Northern Echo: This artist’s impression of ID2299 shows the galaxy, the product of a galactic collision, and some of its gas being ejected by a “tidal tail” as a result of the merger. New observations made with ALMA, in which ESO is a partner, have captured the earliest stages of this ejection, before the gas reached the very large scales depicted in this artist’s impressionThis artist’s impression of ID2299 shows the galaxy, the product of a galactic collision, and some of its gas being ejected by a “tidal tail” as a result of the merger. New observations made with ALMA, in which ESO is a partner, have captured the earliest stages of this ejection, before the gas reached the very large scales depicted in this artist’s impression

Observational features of winds and “tidal tails” caused by the gravitational interaction between galaxies in such mergers can be very similar, so the researchers suggest that some past results where galactic winds have been seen as the cause of halting star formation might need to be re-evaluated.

Researchers used the European Southern Observatory’s Atacama Large Millimeter Array (ALMA) telescope, in northern Chile, to observe the galaxy.

The findings are published in the journal Nature Astronomy.

Lead author Dr Annagrazia Puglisi, in Durham University’s Centre for Extragalactic Astronomy, said: “We don’t yet know what the exact processes are behind the switching off of star formation in massive galaxies.

“Feedback driven winds from star formation or active black holes are thought to be the main responsible for expelling the gas and quenching the growth of massive galaxies.

“Our research provides compelling evidence that the gas being flung from ID2299 is likely to have been tidally ejected because of the merger between two gas rich spiral galaxies. The gravitational interaction between two galaxies can thus provide sufficient angular momentum to kick out part of the gas into the galaxy surroundings.

The Northern Echo: Ejected gasEjected gas

“This suggests that mergers are also capable of altering the future evolution of a galaxy by limiting its ability to form stars over millions of years and deserve more investigation when thinking about the factors that limit galaxy growth.”
The researchers now hope to obtain higher resolution images of ID2299 and other distant galaxy mergers and carry out computer simulations to further understand the effect galaxy mergers have on the life cycle of galaxies.