Star cluster or devoured galaxy? Expelled stars hint toward origins of Andromeda object

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Mayall II, also known as Andromeda's Cluster, is a globular cluster located on the outskirts of the Andromeda Galaxy and is believed by some to be the remnants of a cannibalized dwarf galaxy. (Michael Rich, Kenneth Mighell, and James D. Neill (Columbia University), and Wendy Freedman (Carnegie Observatories), and NASA)

Newly discovered stars, tugged away from the Andromeda Galaxy's largest globular cluster by tidal forces, lend credence to the theory that the cluster was once a dwarf galaxy that is being digested by its much larger neighbor.

Astronomers at the University of California, Davis, who led a paper published April 13 in Monthly Notices of the Royal Astronomical Society, said that determining the origins of the cluster would inform how both spiral and dwarf galaxies formed and continue to evolve.

Globular clusters are tightly packed collections of stars; the one in the sights of the study's authors was G1 in the Andromeda Galaxy. Also known as Mayall II or Andromeda's Cluster, G1 is about 130,000 light-years away from the center of Andromeda, beyond the galaxy's spiral arms and instead residing in its relatively sparse galactic halo.

With a mass about 10 million times larger than the sun's, the cluster is the most massive globular cluster in the region of space shared by Andromeda and our own Milky Way Galaxy. 

G1 might also be the remnant core of a dwarf galaxy, swallowed by the massive gravity of Andromeda. Astronomers have been considering the cluster's possible galactic history since at least 2001, pointing to its unusual features such as its flattened shape and possible intermediate-mass black hole.

Michael Gregg, a research astronomer at UC Davis and lead author of the recent study, said, "Someone suggested that in fact, it's not really a globular cluster, but it's the remaining nucleus of a satellite galaxy that used to orbit M31 [Andromeda] that's been sort of whittled down in size from tidal interactions over the last few billion years."

To better understand G1's history, Gregg and other astrophysicists searched for tidal debris stars, which would be pulled out of the cluster by tidal forces between the Andromeda Galaxy and the cluster — similar to the forces from the moon and sun that cause tides on Earth. 

Their analysis determined the velocity of 351 stars in the sky around G1 with spectroscopy data, which was used to determine how their light was being shifted via the Doppler effect. After filtering out the Milky Way stars, 225 in the Andromeda Galaxy were identified, and 13 of them had a velocity similar to G1 and were aligned in a diagonal on either side of the globular cluster.

These 13 "tidal debris stars" are the first to be discovered emanating from G1. A similar search for tidal debris stars was conducted in 2004, but it was inconclusive because its spectroscopy data was less precise and covered a much smaller part of the sky, according to Gregg.

The new findings led him and his co-authors to conclude that the stars recently departed from G1, and in a way that would not be expected from globular clusters. By extrapolating from the tidal debris stars and assuming a constant outflow, the scientists estimated that G1 would completely dissipate in 3 billion to 6 billion years. That is much quicker than the tens or hundreds of billions of years that it is expected to persist under the assumption that it is a standard globular cluster, according to the researchers.

"That runs completely counter to the theoretical understanding of globular clusters, which is that they're tightly bound gravitational systems and it's very hard to take them apart," Gregg said.

He and his co-authors note that there are scenarios in which the speedy loss is achievable by G1 — if it is near the farthest point from Andromeda in its orbit, or the unlikely occurrence of recently being struck by a massive unidentified object. 

But they prefer the explanation that G1 is the core of a small galaxy, and that the tidal debris stars are from the galaxy's small remaining "envelope" of stars around that core. That explanation would not require the group of stars to be at a specific point in its orbit, they said.

"We think that this is good evidence — or it's certainly consistent evidence — with G1 being the nucleus of a former galaxy, probably a dwarf elliptical galaxy," Gregg said.

The 13 stars and others tidally stripped stars from G1 will populate the Andromeda Galaxy's halo, a sign that the cluster and "compact stellar systems in general" were important to the halo's formation, the astronomers wrote.

Confirming G1's nature as a dwarf galaxy, and specifically as an "ultra-compact dwarf galaxy," would help explain the origins of ultra-compact dwarfs, which are hypothesized to be formed by similar tidal stripping.

Gregg, who helped discover ultra-compact dwarfs two decades ago, said G1 would also inform the evolution of larger spiral galaxies like Andromeda and the Milky Way, which seem to form in part from mergers of many smaller galaxies.

But 13 tidal debris stars are not enough to confidently conclude that G1 was once the core of a dwarf galaxy, the astronomers said. Gregg said he and his colleagues are analyzing more stars that will at least double the study's sample size, and they will also determine the amount of metals and other heavy elements present in the stars to further elucidate their relationship with G1. They also plan to enlist theorists to model tidal interactions between dwarf and large galaxies to see whether it is consistent with their data.

The study, "Discovery of tidal debris stars from G1/Mayall-II in M31," published April 13 in Monthly Notices of the Royal Astronomical Society, was authored by Michael Gregg and Brian Lemaux, University of California, Davis; Michael West, Lowell Observatory; and Andreas Küpper, QuantCo.

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