Galaxy discovered dying due to severe collision

The galaxy may be forcing astronomers to rethink their ideas about how galaxies die.

When galaxies stop forming new stars, that galaxy is dying. In that case it will have run out of its supply of interstellar gas. In a new study, however, researchers have stumbled upon something curious. A galaxy appears to be emitting its star-forming gas at breakneck speed. The research team thinks that this spectacular development was triggered by a collision with another galaxy. The observation may force astronomers to change their ideas about how galaxies die.

The galaxy
Using the Atacama Large Millimeter / submillimeter Array (ALMA), researchers observed the galaxy in question – which bears the name ID2299 – for just a few minutes. However, this was long enough for the powerful instrument to detect the galaxy and its gas tail. ID2299 is so distant that it took its light about 9 billion years to reach us. That means we see it when the Universe was only 4.5 billion years old.

The observations reveal that ID2299 is emitting gas at a rate of tens of thousands of solar masses per year. As a result, the system will lose no less than 46 percent of its total amount of cold gas. Because the galaxy is also forming new stars at a very fast rate – hundreds of times faster than our Milky Way Galaxy – the remaining gas will also run out very quickly. As a result, star production in ID2299 will come to a halt in a few tens of millions of years. “It is the first time that we have observed a characteristic mass-rich galaxy in the distant Universe that is beginning to ‘die’ from massive cold gas emissions,” said study leader Annagrazia Puglisi.

The big question is why the gas is released so quickly – with the equivalent of 10,000 suns per year. But the researchers have now found an explanation for this. For example, they think this spectacular gas loss was caused by a collision between two galaxies that eventually merged to form ID2299. Why? The researchers deduce this explanation from the connection of the ejected gas with the so-called ‘tide tail’. These are elongated streams of stars and gas that form when two galaxies merge. However, these currents are usually too weak to be observed in distant galaxies. Because ID2299’s tidal tail has only just ‘launched’ into space – and is therefore relatively clear – the team was able to identify it anyway.

This artist’s impression shows the galaxy ID2299, the result of a galactic collision, and some of its gas expelled from the “tide tail” resulting from this merger. Image: ESO / M. Kornmesser

So it means that the galaxy ID2299 is dying due to a severe collision. And that’s interesting. Until now, astronomers thought that the emission of star-forming material – which causes a galaxy to no longer produce stars – is caused by ‘winds’. These winds are said to be the result of star formation and the activity of the black hole at the core of the galaxy. But the new research now indicates that galaxy collisions may also be responsible for a galaxy’s demise.

This discovery may force astronomers to rethink their ideas about how star formation in galaxies comes to a halt. “Our research suggests that the cause of the gas emissions may be due to the fusion process, and that winds and tidal tails can be very similar,” explains researcher Emanuele Daddi. This means that the winds that some researchers have seen near distant galaxies may actually have been tidal tails. “We may need to adjust our ideas about how galaxies ‘die,'” said Daddi.

All in all, the study makes an important contribution to our understanding of the evolution of distant galaxies. “ALMA has shed new light on the mechanisms that can halt the formation of stars in distant galaxies,” said researcher Chiara Circosta. “The observation of this colossal disruptive event adds an important piece to the complex puzzle of the evolution of galaxies.”

In the future, the team could use ALMA to take a closer look at ID2299. In this way, they could learn more about the dynamics of the gas expelled. In addition, we are waiting for the future Extremely Large Telescope (ELT) that ESO is currently building in the Atacama Desert. Observations with this powerful telescope would allow the team to investigate the relationships between the stars and the gas in ID2299, in order to gain even more insight into the evolution of galaxies.

About the ELT
The Extremely Large Telescope will have a primary mirror with a diameter of about 39 meters. That main mirror consists of 798 segments and together they must form a perfect mirror. If that succeeds, this telescope is capable of great things. There are three main goals. Its main purpose is to try to measure the properties of the atmospheres of Earth-like planets. In addition, the telescope will be able to see further away than ever, so that it can observe even faint points of light at the edge of the visible universe. It is expected that the ELT will be able to tell us more about the first stars and galaxies that arose shortly after the big bang and thus shed a light on the evolution of the universe. Finally, the ELT will be able to split the light from nearby galaxies into the light from individual stars. For example, we can learn more about how galaxies formed in our own backyard.

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