Mergers and acquisitions among galaxies

Homo sapiens have been on the move almost from the start, migrating out of Africa and shaping the course of human history. The stars, astronomers found, are no different.

Galaxies grow over time through mergers with other galaxies. The Milky Way is a known example that slowly absorbs the Large and Small Magellanic Clouds and the Sagittarius Dwarf Spheroidal Galaxy.

But for the first time, astronomers have found evidence of an ancient mass migration of stars into another galaxy. They spotted over 7,000 stars that merged into Andromeda (M31)—our nearest galactic neighbour—about two billion years ago. For comparison, the age of the universe is 13.7 billion years.

"Galaxies like M31 and Milky Way are constructed from the building blocks of many smaller galaxies over cosmic history," said Arjun Dey, a Bengalurean working at USA National Science Foundation’s NOIRLab (National Optical-Infrared Astronomy Research Laboratory) in Tucson, Arizona.

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The Andromeda galaxy is similar to the Milky Way and contains roughly 100 billion stars. Most of them reside in the disk and central bulge, but about 10 per cent of the stars (nearly 10 billion) lie in the halo of the galaxy, which was the focus of the study carried out by Dey and his collaborators.

“So far, we have only managed to obtain spectroscopic observations of a minuscule fraction of these stars, and that too only for the brightest ones. Nevertheless, this is significantly more than what anyone has been able to do until now, and provides a much more ‘galaxy-wide’ perspective on the immigration of stars than ever before," Dey, who studied at St. Joseph’s College and was a member of the Association of Bangalore Amateur Astronomers, told DH.

He is the lead author of the study that shows at least a partial map of the stars in Andromeda for the first time. The map which includes the motions of nearly 7,500 stars in the inner halo of Andromeda reveals an interesting history.

Providing evidence

About two billion years ago, another galaxy, whose identity is uncertain, merged with Andromeda. The positions and motions of stars that the scientists measured demonstrate they came from that galaxy. While how Andromeda and other galaxies grew so massive was known in theory, the team provided the evidence.

The new observations come from a sophisticated equipment named Dark Energy Spectroscopic Instrument (DESI) that was built to measure the effect dark energy has on the expansion of the universe. It does so by gathering optical spectra on tens of millions of objects, mostly galaxies and quasars, and then constructing a 3D map of the results. Observing Andromeda was one of the supplementary tasks for DESI attached to the 50-year-old Mayall telescope at Kitt Peak.

"We have never before seen this so clearly in the motions of stars, nor had we seen some of the structures that result from the merger," said Sergey Koposov, an astrophysicist at the University of Edinburgh and one of the co-authors. "The emerging picture is that the history of the Andromeda galaxy is similar to that of the Milky Way. The inner halos of both are dominated by a single immigration event."

Current estimates suggest that the Milky Way’s stellar halo is less massive than that of Andromeda’s, and contains maybe a billion stars though the total mass of the two neighbouring galaxies is comparable. Most of the stars in the Milky Way’s halo were also formed in other galaxies.

Tracing the history

“We can trace the Milky Way’s evolutionary history in a way similar to what we did with Andromeda. But because of our location within the Milky Way, it is sometimes more challenging to study the ‘big picture’, since the halo of our own galaxy is spread out over the entire sky. Since Andromeda is located nearly 2.5 million light-years away, we have a bird's-eye view of its structure,” said Dey.

Also in the Andromeda stars, the astronomers measured high metallicity compared to what is normally found in other galaxies, suggesting M31 constituents were cooked in different, fairly massive galaxies that were in the process of being assimilated.

Because of the similarities between Andromeda and Milky Way, the scientists say that the observational evidence would back up the theories of how mergers play a role in galactic evolution and growth.

"M31 is remarkably similar to the Milky Way. The inner halos of both are dominated by stars from a single accretion event. For the first time, we get a glimpse of the structures that were formed as a result of the merger," the team reported in the Astrophysical Journal.

As DESI continues with its main task of measuring the spectra of more than 40 million galaxies, Dey and his colleagues hope to use the instrument to sample nearly 100,000 stars over the entire halo of Andromeda for a better insight into the fate of Andromeda, which is moving towards the Milky Way and would embrace the Milky Way about five billion years from now.

"It's amazing that we can look out at the sky and read billions of years of another galaxy's history as written in the motions of its stars — each star tells part of the story," said co-author Joan R Najita, also at NOIRLab. "Our initial observations exceeded our wildest expectations and we are now hoping to conduct a survey of the entire M31 halo with DESI. Who knows what new discoveries await!"