The Andromeda galaxy (Messier 31), which is located 2.5 million light-years away, is the Milky Way’s nearest large galactic neighbor. By measuring the motions of 7,438 stars in the Andromeda’s inner halo, astronomers discovered telltale patterns in the positions and motions of stars that revealed how these stars began their lives as part of another galaxy that merged with the Andromeda about 2 billion years ago.
“Our new observations of the Milky Way’s nearest large galactic neighbor, the Andromeda galaxy, reveal evidence of a galactic immigration event in exquisite detail,” said Dr. Arjun Dey, an astronomer at NSF’s NOIRLab.
“Although the night sky may seem unchanging, the Universe is a dynamic place.”
“Galaxies like the Andromeda and our Milky Way are constructed from the building blocks of many smaller galaxies over cosmic history.”
“We have never before seen this so clearly in the motions of stars, nor had we seen some of the structures that result from this merger,” added Dr. Sergey Koposov, an astrophysicist at the University of Edinburgh.
“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.”
Most of the stars in the Milky Way’s halo were formed in another galaxy and later migrated into our own in a galactic merger 8-10 billion years ago.
Studying the relics of a similar, but more recent, galaxy merger in the Andromeda galaxy gives astronomers a window onto one of the major events in the Milky Way’s past.
Dey et al. uncovered striking new evidence for a mass migration of stars into the Andromeda galaxy. Image credit: KPNO / NOIRLab / AURA / NSF / Local Group Survey Team / T.A. Rector, University of Alaska Anchorage / D. de Martin / M. Zamani.
To trace the history of migration in the Andromeda, the study authors turned to the Dark Energy Spectroscopic Instrument (DESI) on the Nicholas U. Mayall 4-m telescope at Kitt Peak National Observatory.
DESI was constructed to map tens of millions of galaxies and quasars in the nearby Universe in order to measure the effect of dark energy on the expansion of the Universe.
It is the most powerful multi-object survey spectrograph in the world, and is capable of measuring the spectra of more than 100,000 galaxies a night.
DESI’s world-class capabilities can also be put to use closer to home, however, and the instrument was crucial to the team’s survey of the Andromeda.
“This science could not have been done at any other facility in the world,” Dr. Dey said.
“DESI’s amazing efficiency, throughput, and field of view make it the best system in the world to carry out a survey of the stars in the Andromeda galaxy.”
“In only a few hours of observing time, DESI was able to surpass more than a decade of spectroscopy with much larger telescopes.”
The researchers now plan to use the unparalleled capabilities of DESI and the Mayall telescope to explore more of Andromeda’s outlying stars, with the aim of revealing its structure and immigration history in unprecedented detail.
“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 Dr. Joan Najita, an astronomer at NSF’s NOIRLab.
“Our initial observations exceeded our wildest expectations and we are now hoping to conduct a survey of the entire Andromeda halo with DESI. Who knows what new discoveries await!”
The findings appear today in the Astrophysical Journal.
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Arjun Dey et al. 2023. DESI Observations of the Andromeda Galaxy: Revealing the Immigration History of our Nearest Neighbor. ApJ, in press; doi: 10.3847/1538-4357/aca5f8
