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Cougar
2019-Mar-07, 03:59 PM
Today's Inbox Astronomy informed me that astronomers have recently gotten a good estimate for the mass of the Milky Way by carefully
studying the motions of globular star clusters that orbit our galaxy. Estimates are that there are 200 billion stars in the galaxy, but that the mass is 1.5 trillion solar masses. Dark matter accounts for the "excess" between solar masses from stars and the total mass.

My question is, the globular clusters whose movement allowed for this determination are not situated right at the edge of the galaxy disk, but are some not-insignificant distance away. That means that the clusters are being gravitationally affected by a lot more dark matter than the dark matter that resides within the galaxy.

Now, I know that astronomers are super-clever folks. But can anyone confirm that the dark matter exterior to the galactic disk but interior to the globular clusters' orbits was subtracted from the recent estimate?

Hornblower
2019-Mar-07, 06:41 PM
Today's Inbox Astronomy informed me that astronomers have recently gotten a good estimate for the mass of the Milky Way by carefully
studying the motions of globular star clusters that orbit our galaxy. Estimates are that there are 200 billion stars in the galaxy, but that the mass is 1.5 trillion solar masses. Dark matter accounts for the "excess" between solar masses from stars and the total mass.

My question is, the globular clusters whose movement allowed for this determination are not situated right at the edge of the galaxy disk, but are some not-insignificant distance away. That means that the clusters are being gravitationally affected by a lot more dark matter than the dark matter that resides within the galaxy.

Now, I know that astronomers are super-clever folks. But can anyone confirm that the dark matter exterior to the galactic disk but interior to the globular clusters' orbits was subtracted from the recent estimate?


We can define what is part of the galaxy as needed for scientific usefulness. I see no compelling need to exclude matter (halo stars, globular clusters, dark matter) that is gravitationally bound but outside the visible edge of the disk. All of it adds to the gravitational signature as seen from nearby galaxies.

Grey
2019-Mar-07, 07:00 PM
I'd agree with Hornblower. We generally consider a galaxy to consist of the entire gravitationally bound system, including not just the visible portion, but the matter that extends beyond that. The orbiting globular clusters often seem like they end up forming an outer boundary.

Ken G
2019-Mar-07, 07:26 PM
In fact, since the disk is rather narrow but the dark matter is all spread out, the amount of dark matter within the disk itself would be less than the amount of matter in stars. The galaxy includes the halo, that's why the globular clusters are viewed as part of our galaxy.

You do raise an interesting point of nomenclature, though. When the "Milky Way" was originally named, it just meant the stars in the nearby spiral arms, that look like a swath of light across the sky. But later the entire galaxy took the name "Milky Way galaxy." That latter term is used so much that when someone says "Milky Way," they normally mean "Milky Way galaxy," not swath of light across the sky. So when they say "the Milky Way is 90% dark matter," they mean the whole galaxy is, not the swath of light is.

Cougar
2019-Mar-07, 08:00 PM
Ah, OK. Yes, I'm well aware the dark matter extends well beyond the disk. So most of the mass in our galaxy is undetected! Except for its gravitational effect.

kzb
2019-Mar-08, 12:35 PM
MW mass estimates vary quite significantly, from about 0.6 trillion to 3 trillion solar masses roughly (working from memory here).

It seems to be highly method dependent.

There was a calculation some months back, based on another method, which gave a much lower estimate for the mass.

Usually they calculate a mass within a certain radius from the MW centre. I've not seen the article in question here but they might have calculated the mass inside the GC orbits. Alternatively they may have used models to estimate how much additional mass there would be outside the orbits, given the result they got for mass inside.

A popular definition of the total mass is the mass within the "virial radius" of the MW.

Cougar
2019-Mar-08, 02:31 PM
....I've not seen the article in question here but....

Oh yeah, I might have provided the link. :o Here you go. ==> link (http://hubblesite.org/news_release/news/2019-16)