Astronomers discover mass migration of stars converging on Andromeda: ScienceAlert

Astronomers know that galaxies grow over time through mergers with other galaxies. We can see it in our galaxy. The Milky Way is slowly absorbing the Large and Small Magellanic Clouds and the Sagittarius Dwarf Galaxy.

For the first time, astronomers have found evidence of an ancient mass migration of stars to another galaxy. They discovered over 7,000 stars in Andromeda (M31), our nearest neighbor, which merged with the galaxy about two billion years ago.

The giant halo of gas is outlined in this image of the Andromeda Galaxy. (NASA, ESA and E. Wheatley (STScI))

The growth and evolution of galaxies is a hot topic in astronomy and one of the reasons why the James Webb Space Telescope has been making a lot of headlines lately. One of the main scientific goals of the JWST is to look back in time at the earliest galaxies in the universe to understand how they grew and evolved into what they are today. But it’s not the only telescope that can bring light into the darkness.

“Galaxies like M31 and our own Milky Way have been built from the building blocks of many smaller galaxies throughout cosmic history.”

Arjun Dey, NOIR Lab

These new observations of Andromeda and stellar immigration come from the Dark Energy Spectroscopic Instrument (DESI). It was built to measure the effect of dark energy on the expansion of the universe. It does this by collecting optical spectra from tens of millions of objects, mostly galaxies and quasars, and then creating a 3D map of the results.

DESI is similar to the better-known Gaia spacecraft. Gaia has the ambitious goal of accurately mapping the positions and movements of billions of stars in the Milky Way. Gaia data led to a wealth of discoveries about our own galaxy. But it’s limited to mapping stars in the Milky Way.

Now, thanks to DESI, astronomers have at least a partial map of the stars in Andromeda for the first time. And this map, including the movements of nearly 7,500 stars in the Andromeda Galaxy’s inner halo, reveals their history.

These findings are included in a new paper entitled DESI Observations of the Andromeda Galaxy: Revealing the Immigration History of our Nearest Neighbor. It appears in The Astrophysical Journaland the lead author is Arjun Dey, an astronomer at the National Science Foundation’s NOIRLab, the entity responsible for DESI.

DESI shows another galaxy merged with Andromeda about two billion years ago. The positions and motions of about 7,500 stars measured by DESI show they are from another galaxy. Theory told us that Andromeda and other galaxies grew so massively, but now there’s a growing body of unequivocal evidence.

“Our new observations of the Milky Way’s nearest large galactic neighbor, the Andromeda galaxy, reveal detailed evidence for a galactic immigration event,” explained lead author Dey. “Although the night sky appears unchanging, the universe is a dynamic place. Galaxies like M31 and our own Milky Way have been constructed from the building blocks of many smaller galaxies throughout cosmic history.”

The Milky Way experienced a similar merger 8 to 10 billion years ago. Most of the stars in our galaxy’s halo originated from another galaxy and joined the Milky Way as a result of the ancient merger. Astronomers can learn more about the ancient history of the Milky Way by closely observing this similar, more recent merger event in Andromeda.

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

For the first time, we get a glimpse of the structures created by the merger. “Expected observational signatures of galactic migration include debris flows, shells, rings and feathers, the expected results of merger interactions between large galaxies and their companions,” the authors write in their article.

Diagram showing the distribution of colors representing the velocities of stars and their relative positions
Stars in Andromeda’s inner halo and their velocities. (Dey et al., ApJ, 2023)

“We find clear kinematic evidence for shell structures in the Giant Stellar Stream, in the Northeast Shelf and Western Shelf regions,” the paper states. “The kinematics are remarkably similar to predictions from dynamic models constructed to explain the spatial morphology of the inner halo. The results are consistent with the interpretation that much of the substructure in M31’s inner halo is generated by a single galactic immigration event of 1–2 Gyr.”

“While evidence of coherent structure has already been discovered in M31, this is the first time it has been seen in such detail and clarity in a galaxy beyond the Milky Way,” the authors write in their paper. “The observations reveal a complicated coherent kinematic structure in the positions and velocities of individual stars: streams, wedges and chevrons.”

Diagram showing colors representing velocities of stars and their distances from Andromeda
Velocities vs. distances of some stars from Andromeda by color. (Dey et al., ApJ, 2023)

Although the positions and velocities of the 7,500 stars play an important role in these results, so does stellar metallicity. The team found stars with high metallicity in all of the substructures resulting from the fusion. “We find a significant number of metal-rich stars in all discovered substructures, suggesting that the progenitor galaxy (or galaxies) had a long star-forming history that is perhaps more representative of more massive galaxies,” the authors explain in their conclusion.

The study highlights similarities between Andromeda and the Milky Way and reinforces the theoretical idea that mergers play a key role in galactic evolution and growth. “M31 is remarkably similar to the Milky Way in that the inner halos of both galaxies are dominated by stars from a single accretion event,” the paper states. “Indeed, a recent study of the kinematics of stars in the Milky Way near the Sun reports chevron-shaped kinematic substructures reminiscent of those reported here.”

The power of DESI comes into its own in this research. The results come from DESI’s ability to collect spectra from 5,000 objects simultaneously. The world’s most powerful multi-object spectrograph, this complex instrument can reconfigure its 5,000 separate focal planes in just two minutes while slewing between targets.

“It is amazing that we can look up at the sky and read billions of years of history of another galaxy as written in the movements of its stars.”

Joan R. Najita, NOIRLab.

It was designed to measure the spectra of over 40 billion distant galaxies and quasars to map the large-scale structure of the Universe and how dark energy is driving its expansion. As an aside, it shows us how galaxies are merging over time.

“This science could not have been performed at any other facility in the world. DESI’s amazing efficiency, throughput and field of view make it the best system in the world to perform a survey of the stars in the Andromeda galaxy,” Dey said. “In just a few hours of observing time, DESI was able to surpass more than a decade of spectroscopy with much larger telescopes.”

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

This article was originally published by Universe Today. Read the original article.

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