Astronomers know that galaxies grow over time through mergers with other galaxies. We can see it happening in our galaxy. The Milky Way is slowly absorbing the Large and Small Magellanic Clouds and the Sagittarius Dwarf Spheroidal Galaxy.
For the first time, astronomers have found evidence of an ancient massive star migration in another galaxy. They spotted more than 7,000 stars in Andromeda (M31), our nearest neighbor, which merged with the galaxy about two billion years ago.
The growth and evolution of galaxies is a hot topic in astronomy and one of the reasons the James Webb Space Telescope has been in the news lately. One of the main scientific objectives of the JWST is to examine in time to the first galaxies of the Universe to understand how they developed and evolved to become what they are today. But it’s not the only telescope that can shed some light on the matter.
“Galaxies like M31 and our Milky Way are built from the building blocks of many smaller galaxies throughout cosmic history.”
Arjun Dey, NOIRLab
These new observations of Andromeda and the internal migration of stars come from the Dark Energy Spectroscopic Instrument (DESI.) It was constructed to measure the effect dark energy a on the expansion of the Universe. To do this, it collects optical spectra on tens of millions of objects, mainly galaxies and quasars, then constructs 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. Data from Gaia has led to a multitude of discoveries about our own galaxy. But this is limited to mapping the stars of the Milky Way.
Now, thanks to DESI, astronomers have for the first time at least a partial map of the stars of Andromeda. And this map, including the movements of nearly 7,500 stars in the inner halo of the Andromeda Galaxy, reveals their history.
These findings can be found in a new paper titled “DESI Observations of the Andromeda Galaxy: Revealing the Immigration History of Our Nearest Neighbor.” It will appear in The Astrophysical Journaland the lead author is Arjun Dey, an astronomer at the National Science Foundation’s NOIRLab, the facility responsible for DESI.
DESI shows that about two billion years ago another galaxy merged with Andromeda. The positions and movements of about 7,500 stars measured by DESI reveal that they came from another galaxy. Theory told us this is how Andromeda and other galaxies got so massive, but there is now a growing body of clear evidence.
“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,” lead author Dey explained. “Although the night sky may seem unchanging, the Universe is a dynamic place. Galaxies like M31 and our Milky Way are built 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 in a different galaxy and joined the Milky Way as a result of ancient merger. Astronomers can learn more about the ancient history of the Milky Way by observing this similar and more recent merger event at Andromeda up close.
“We have never seen this so clearly in the motions of stars, nor had we seen some of the structures resulting from this merger,” said Sergey Koposov, an astrophysicist at the University of Edinburgh and co- author of the article. “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 that formed as a result of the merger. “Expected observational signatures of galactic migration include debris flows, shells, rings, and plumes, the expected results of merger interactions between large galaxies and their companions,” the authors said. write in their paper.
“We find clear kinematic evidence for shell structures in the Giant Stellar Stream, Northeast Shelf and West Shelf regions,” the paper said. “The kinematics are remarkably similar to predictions of dynamical models constructed to explain the spatial morphology of the inner halo. The results are consistent with the interpretation that much of the substructure in the inner halo of M31 is produced by a single galactic immigration event 1 –2 Gyr ago.”
“While hints of coherent structures have already been detected in M31, this is the first time they have been seen in such detail and clarity in a galaxy beyond the Milky Way,” the authors write in their article. “Observations reveal a consistent and complex kinematic structure in the positions and velocities of individual stars: fluxes, corners and chevrons.”
Although the positions and velocities of the 7,500 stars play a major role in these discoveries, stellar metallicity also played a role. The team found stars with high metallicity in all the substructures resulting from the merger. “We find a significant number of metal-rich stars in all detected substructures, suggesting that the progenitor galaxy (or galaxies) had a long history of star formation, perhaps more representative of galaxies more massive”, explain the authors in their conclusion.
The study highlights similarities between Andromeda and the Milky Way, reinforcing 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 says. “Indeed, a recent study of the kinematics of Milky Way stars near the Sun reports chevron-shaped kinematic substructures reminiscent of those reported here.”
The power of DESI is on full display in this research. The results stem from DESI’s ability to simultaneously collect the spectra of 5,000 objects. This complex instrument is the most powerful multi-object spectrograph in the world and can reconfigure its 5,000 distinct focal planes in just two minutes as it moves from target to target.
It was designed to measure the spectra of more than 40 billion distant galaxies and quasars to map the large-scale structure of the Universe and how dark energy is fueling its expansion. Along the way, he shows us how galaxies merge over time.
“This science could not have been done in any other facility in the world. DESI’s incredible efficiency, throughput and field of view make it the best system in the world for surveying stars in the Andromeda Galaxy. “, said Dey. . “In just a few hours of observation, DESI was able to surpass more than a decade of spectroscopy with much larger telescopes.”
“It’s amazing that we can look up into the sky and read billions of years of another galaxy’s history as written in the movements 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 now hope to investigate the entire M31 halo with DESI. Who knows what new discoveries lie ahead!”
This article was originally published by Universe today. Read it original article.