A newly discovered galaxy merged with the Milky Way 11 billion years ago. The encounter with this “Kraken”, which left globular clusters as vestiges, must have been a real shock. What do we know about this galaxy?
By training the Milky Way family tree, a team of scientists made a major discovery: about 11 billion years ago, a galaxy probably collided with ours. Thirteen globular clusters (stellar systems with a high density of stars) would be the memories of this event. Until then, this fusion between the Milky Way and this other galaxy was unknown. The researchers gave it an evocative name: the Kraken.
« The Kraken alludes to the legendary and elusive sea monster. Merging with the Kraken must have been such a defining event in the history of the Milky Way, that it seemed reasonable to name the galaxy after a mythical monster. », Tells Numerama the astrophysicist Diederik Kruijssen, specialist in the formation of stars and galaxies, member of the Center for Astronomy of the University of Heidelberg (Germany) and co-author of the study published in MNRAS.
The name chosen for this galaxy also echoes the difficulty of discovering it: ” It must have been a rarely or never seen monster. There is only one such monster with a short and catchy name, so we quickly decided it had to be the Kraken. », Continues the scientist.
Swallowed up by the Milky Way
What exactly do we know about this Kraken which seems to have played such an important role in the history of the Milky Way? 11 billion years ago, the Kraken must have only been at the beginning of its life, as was the Milky Way. It was similar in size to the (current) Small Magellanic Cloud, a dwarf satellite galaxy in the Milky Way. ” It must have formed fairly quickly, with widespread star formation, spawning around 400 million stars and at least 13 globular clusters. », Describes Diederik Kruijssen. Such clusters, which form compact clusters of around 100,000 stars, have survived to this day, retaining roughly the shape they then had. It was the movements, chemical composition, and ages of these clusters that helped the authors reconstruct the properties of the Kraken, including its mass and the time of its fusion with the Milky Way.
The authors of the study believe that the Kraken must have formed early in the history of the Universe, like other galaxies we see today. But the Kraken differs from them on one point. ” The Kraken formed so close to the Milky Way that the two galaxies ended up colliding and merging. Because the Milky Way was the largest galaxy, the Kraken was engulfed, ‘cannibalized’, if you will, by the Milky Way. », Summarizes the astrophysicist.
Finding the existence of this galaxy was not easy. Diederik Kruijssen compares the work of his team of scientists to “ galactic archeology ”, So much we had to go back in time. When the fusion with the Milky Way occurred, the Universe was only 20% of its current age (about 13.7 billion years), recalls the scientist. Keep in mind that 11 billion years ago, the Milky Way was smaller than it is today – the stars that have formed since have made it bigger. ” The stars and globular clusters that once belonged to the Kraken reside closer to the Galactic Center than the Sun, hidden among tens of billions of other stars, indique Diederik Kruijssen. About 1% of the internal stars in the orbit of the Sun once belonged to the Kraken. Isolating these stars is like looking for a needle in a haystack. »
The Kraken is not the most massive galaxy that the Milky Way has merged with in its existence. The Galaxy of Sagittarius, which played a role of true architect for the Milky Way in a fusion that occurred 7 billion years ago, had a billion stars – 2.5 times more than the Kraken.
” The last time that [la Voie lactée] was shaken by a merger “
Still, the authors of the new study believe the encounter with the Kraken was a real shock. ” The mass of the Kraken was much more similar to that of the Milky Way at that time, more similar than any other galaxy that has merged with the Milky Way since. This makes the encounter with the Kraken the most significant fusion the Milky Way has seen in the past 11 billion years. », According to Diederik Kruijssen. Before that time, the Milky Way resembled the dwarf galaxies we know today: its encounter with other galaxies would not then have been such a major event.
And since then, no other galaxy that could have struck the Milky Way must have had such an impact. ” The Milky Way’s collision with the Kraken represented the last time it was truly rocked by a merger “, Says the astrophysicist. He and his team do not rule out that the fusion played a role in the architecture of the galactic bulb of the Milky Way.
What if the Kraken and the Milky Way hadn’t merged?
What would have become of the Kraken if the Milky Way hadn’t swallowed it up? When it met the ancestor of our galaxy, 400 million stars made up the Kraken – the equivalent of 200 million times the mass of our Sun -, themselves embedded in 100 billion solar masses of black matter, detail Diederik Kruijssen. « If it hadn’t merged with the Milky Way, the Kraken would have become about 20 times as massive, reaching around 8 billion stars today (or 4 billion solar masses of stars). By merging with the Milky Way so early in its life, the Kraken was dissolved. »
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