dv said:
Scientists have identified the corpse of a galaxy that fell into the Milky Way about 10 billion years ago in what was likely the last major overhaul to our home galaxy during its development.
That’s the conclusion of new research based on 2 billion measurements of how stars within the Milky Way are moving. Those measurements let scientists identify about 33,000 stars that live in our galaxy but were born elsewhere, carried here during a giant galactic collision.
“The Milky Way is a cannibal. It has eaten many dwarf galaxies in the past, and we’ve just found a major one that it ate in the past,” Kathryn Johnston, an astronomer at Columbia University in New York who wasn’t involved in the new research, told Space.com. “This is like a police investigation — this one in particular, because it’s not a galaxy that we can see today. It’s a dead galaxy, so that makes it kind of fun.”
https://www.space.com/42305-milky-way-absorbed-giant-dwarf-galaxy-gaia-enceladus.html
> Old news
Galaxy collisions with the Milky Way are old news. But this one is “comparatively huge”. In fact it’s so huge that it may account for all of the stars in the Milky Way’s inner halo.
From link:
Helmi and her colleagues drew on data from a European project called Gaia, which is mapping the location of 1 billion stars in the Milky Way with unprecedented precision. The team identified a set of about 33,000 of those stars that are moving in a completely different manner compared to the bulk of the Milky Way. The team also studied the chemistry of nearly 600 of those stars using ground-based telescope data, which confirmed that these stars had come from somewhere beyond the Milky Way. And the sheer amount of data at the team’s fingertips let the scientists estimate the size and age of the colliding galaxy. Given those numbers, the researchers said, the former galaxy was likely about a fifth the size of the modern Milky Way. That means that while this is hardly the only galactic collision scientists have pinpointed, it’s a comparatively huge one.
Does “size” here refer to number of stars or to diameter? I think they have to mean diameter, don’t they, because 33,000 is nowhere near a fifth of a billion.
party_pants said:
Interesting. How do they know the ages of these remnant stars? Seeing as the Solar system is only about half that age and is expected to expired before it gets to 10 billion years old, I interested to know how old these stars are.
The standard method of determining the age of a cluster of stars is the turn-off point at the hot end of the main sequence. Hotter stars have shorter lifetimes, so the presence of a hot star means that it was formed comparatively recently. Yes, that’s shown in Figure 2 of the linked article in Nature magazine.
What interests me is how they know from the stars’ chemistry that they were not formed along with the Milky Way. I see, that’s also shown in Figure 2. These stars have a much lower iron concentration than those in the original Milky Way.
The original identification of the stars is based on them having a retrograde velocity, a rotation around the Milky Way that is opposite the direction of all the other stars.