​Four to five billion years ago, the cosmic noon was characterized by very significant star-forming activity in most galaxies. Curiously, however, nearly a third of the most massive galaxies of that time no longer form stars. This anomaly is often attributed to gas flows caused by feedback from supermassive black holes located within these galaxies.

Using an astronomical interferometer, the Atacama Large Millimeter/Submillimiter Array (ALMA), astrophysicists have detected an exceptional gas ejection from a massive galaxy, ID2299. No less than half of the galactic cold gas has escaped into the surrounding space. At this rate, the galaxy will be deprived of the material needed to form new stars in just a few million years, i.e., moments on the cosmological time scale. This ejection somehow "kills" the galaxy by depriving it of matter to "make" new stars.

ID2299, an intriguing "ancestor" of elliptical galaxies

The family of elliptical galaxies is seemingly simple to describe but actually remains quite mysterious. Most of the stars in the near Universe are located in these large, extremely massive, spheroidal galaxies, which are made up of very old stars and no longer form new ones. Astrophysicists have long tried to understand the mechanisms responsible for the inactivity of these galaxies: the discovery of the massive ejection of cold gas from ID2299 may well shed some light on them.

Until now, scientists have attributed the origin of massive gas ejections to supermassive black holes, located in the heart of galaxies. The accretion of matter around the black hole is in fact accompanied by the emission of large amounts of energy and the appearance of powerful winds, capable of sweeping away the gas in the galaxy. This feedback is an essential ingredient for reproducing galaxy observations through numerical simulations. It does not however explain the extreme case of ID2299.

"Tidal tails" resulting from the merging of galaxies

The ejection of ID2299 can only be interpreted by the effect of "tidal forces" that occur whenever galaxies merge.

The final states resulting from these tidal forces are regularly observed in the form of "tidal tails" in galaxies close to us undergoing gravitational interaction, such as in the Antenna Galaxies. However, it is much more difficult to detect them in distant galaxies because of their low light emission.

According to researchers, an exceptional tidal force would have been produced in ID2299 by the ongoing collision of the two massive galaxies of which it is composed. At the same time, half of the cold gas in ID2299 would have been compressed in a dense core at the center of the galaxy and very quickly converted into new stars (starburst), at a rate about 500 times faster than in our galaxy. This hypothesis is supported by detailed numerical simulations consistent with the observation of gas ejection.

Astrophysicists have calculated that the frequency of these events is sufficient to explain in principle most of the inactive galaxies that formed during the cosmic noon.

According to their study, violent ejection phenomena are not necessarily due to black hole feedback but can be explained by the merging of galaxies. However, it is very difficult to distinguish these mechanisms, which often coexist within the same galaxy, because they produce similar effects. The researchers recommend reconsidering the interpretation of certain observations in light of their findings.

The discovery of the ID2299 ejection was possible due only to the large statistical sample of cold gas observations in more than one hundred galaxies of the cosmic noon using the ALMA interferometer in Chile. 

Gas distribution in a colliding galaxy based on numerical simulations