Astronomy Now - January 2021

T

Galactic fossil found as Milky Way’s family

tree is assembled

he history of our Milky Way Galaxy has been reconstructed for the rst time, showing when it

merged with other galaxies, and how massive those other galaxies were, all based on the motions

of globular clusters.

Globular clusters are ancient, having formed alongside the galaxies themselves. Our Galaxy has

about 150 globulars, but not all of them formed in situ. Rather, the Milky Way has acquired about

half of its globular clusters while colliding and merging with other galaxies. Now, an international

team of scientists led by Diederik Kruijssen at the University of Heidelberg and Joel Pfeffer of

Liverpool John Moores University has been able to trace which globular clusters came from which

galaxies, and when. In a paper published in the October 2020 issue of Monthly Notices of the Royal

Astronomical Society, they describe how their starting point was software called E-MOSAICS, which

simulates the formation and evolution of globular clusters, determining their chemical composition,

age and orbital motions. ey then applied a machine-learning algorithm that can relate the

observed properties of the Milky Way’s globular clusters to the simulations, and gure out the mass

and chemical composition of the progenitor galaxies around which they formed, and when they

must have merged with the Milky Way.

“e main challenge of connecting the properties of globular clusters to the merger history of their

host galaxy has always been that galaxy assembly is an extremely messy process, during which the

orbits of the globular clusters are completely reshuffled,” says Kruijssen. “To make sense of the

complex system that is left today, we therefore decided to use articial intelligence. We tested the

algorithm tens of thousands of times on the simulations and were amazed at how accurately it was

able to reconstruct the merger histories of the simulated galaxies, using only their globular cluster

populations.”

Kruijssen and Pfeffer’s results show that the Milky Way Galaxy has cannibalised ve other galaxies

that were greater than 100 million solar masses, and about 15 galaxies of at least 10 million solar

masses. All of these galaxies are small. For comparison, the Sagittarius Dwarf Spheroidal Galaxy,

which is a current satellite galaxy of the Milky Way, contains 400 million solar masses; the Large

Magellanic Cloud has ten billion solar masses. e most massive of these mergers took place

between six and eleven billion years ago. While traces of most of these galaxies had previously been

found, in terms of streams of stars left behind as the cannibalised galaxy was ripped apart, there was

a dearth of evidence (aside from the globular clusters) for one in particular. Kruijssen’s team

nicknamed the missing galaxy ‘the Kraken’, but it wasn’t missing for very long.

Another team of astronomers, led by Danny Horta and Ricardo Schiavon of Liverpool John Moores

University, used the Sloan Digital Sky Survey’s Apache Point Observatory Galactic Evolution

Experiment (APOGEE) in New Mexico, USA, to identify thousands of stars with a distinct

chemical composition that matches them to a hitherto unknown galaxy that was cannibalised 10

billion years ago by the Milky Way. In a paper describing their ndings in the November 2020 issue

of Monthly Notices of the Royal Astronomical Society, they refer to this new galaxy as an ‘inner Galaxy

structure’, or IGS, although in their press material they name it ‘Heracles’. Regardless of what it’s

called, it is the same galaxy as Kraken.

“eir paper is very exciting and independently conrms our results,” Kruijssen tells Astronomy Now.

“e group of stars that they have identied are indeed the remnants of Kraken.”

Kruijssen says that both teams are in agreement and sure of the galaxy’s identity based on the

number of globular clusters. Adding Kraken, which would have brought with it about 13 globular

clusters, to the other known galaxy mergers brings the total number of globular clusters up to the

observed number. If IGS/Heracles was a separate galaxy to Kraken, we would expect the Milky Way

to have more globular clusters than we see.

e stars stripped away from Kraken/IGS/Heracles now lie close to the centre of the Galaxy, in the

galactic halo, where they make up a third of the stars there. Whereas most of the stars in the Milky

Way’s spiral disc formed from local star formation, it is believed that most of the stars in the halo

come from galaxy mergers.

As such, the size of the halo, and particularly the number of globular clusters, tells us how many

mergers our Milky Way Galaxy has experienced.

“Our simulations predict that the number of globular clusters is strongly correlated with the number

of mergers that a galaxy has experienced,” Kruijssen tells Astronomy Now. Galaxies that possess more

globular clusters have probably had more mergers, although, Kruijssen adds, this correlation holds

only for smaller, ‘minor’ mergers with dwarf galaxies, as opposed to ‘major’ mergers between two

large galaxies. However, there is no evidence that our Milky Way Galaxy has experienced a major

merger – in fact, it appears to have experienced very few mergers at all in the past ten billion years.

is will change, of course, within the next ve billion years, when the we will collide with the

Andromeda Galaxy.

Halo stars found not to be where they should be
Some stars that should be in the Milky Ways’ halo – and therefore possibly originate from cannibalised dwarf
galaxies – have been found orbiting in the plane of the disc of our Galaxy. The discovery raises questions as to how
these stars ended up in the spiral arms.

Halo stars typically have low abundances of heavy elements, and are generally older, than stars born in the gas-rich
environs of the spiral disc. However, astronomers led by Giacomo Cordoni of the University of Padova, Italy, using
data from several surveys, including the Gaia satellite, have found stars with the same characteristics as halo stars,
but orbiting on circular orbits in the plane of the Galaxy.

“This discovery is not consistent with the previous galaxy-formation scenario and adds a new piece to the puzzle that
is the Milky Way,” says Cordoni. “Their orbits are very much like that of the Sun, even though they contain just a
tiny fraction of its iron. Understanding why they move in the way that they do will likely prompt a significant
reassessment of how the Milky Way developed over many billions of years.” The research is published in the
November issue of Monthly Notices of the Royal Astronomical Society.

The ‘Galaxy Merger Tree’ that reconstructs the history of the Milky Way Galaxy in terms of when it collided and merged with other

galaxies. The bold lines indicate galaxy mergers for which the remnants have already been identied, while the dotted lines are

mergers that have taken place but for which stellar remnants (other than globular clusters) have yet to be identied. The ‘trunk’ of

the tree represents the Milky Way, its colour changing as its mass increases. Image: D. Kruijssen/Heidelberg University.

The contour lines represent the extent of the stars in the galactic halo, centred on the Milky Way’s bulge, that have come from an

ancient dwarf galaxy known as Kraken, or IGS.

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Galactic fossil found as Milky Wa...

January 2021

Astronomy Now