Viewing a single comment thread. View all comments

etherified t1_j13nkyw wrote

Reply to comment by Weed_O_Whirler in What assumptions are made about gravity when calculating the gravity within a galaxy? by bizzehdee

I wonder, has it been completely ruled out that the extra mass necessary could be inside the black hole center? That is to say, that there could be a lot more mass in black holes than we have thought, and the stars closer to the center haven't yet fallen in because of [... some possible reason...]? Well certainly the intelligent people working on the problem have ruled this out, but I wonder how.

2

DoctorWho984 t1_j148hfw wrote

Yes, for a couple reasons.The majority of mass in the galaxy does not orbit the central black hole, but orbits the all mass interior to its position in the galaxy. The strongest effects are from the stuff closest to it, as the force due to gravity decreases as a inverse square law. Adding mass to the central black hole would increase the gravitational forces felt by the stars that directly orbit the black hole, changing their orbital motion significantly, but the rest of the stars further away wouldn't feel these effects. You need some relativity spread out distribution of mass (dark matter) to explain why all the stars in the galaxy orbit faster than they "should", and not just the ones around the supermassive black hole.

Additionally, there are galaxies without supermassive black holes at their centers and we still see these "anomalies" in their orbital velocity, so you'd have to come up with some other explanation for these galaxies.

And finally, on a case by case basis, we have measurements of stellar orbits around SagA* constraining it's mass, and independent mass measurements from the event horizon telescope for SagA* and M87, so in these two cases we have even more evidence for it to not just be a bigger supermassive black hole.

Edit: Changed confusing pronouns

5

etherified t1_j14t9nz wrote

Thank you,

"... majority of mass in the galaxy does not orbit the central black hole but orbits the mass interior to it"

a little confused because, isn't orbiting the mass interior to the black hole the same as orbiting the black hole...?

Ah, your second point clears something up for me. I think I had read some time ago that some galaxies are found to actually lack dark matter, and by coincidence or not, also don't have black hole centers (were just start clusters?). But if galaxies exist without black holes and yet still have the velocity anomalies, then that's pretty definitive indeed.

2

DoctorWho984 t1_j14vtko wrote

Sorry, that's bad use of pronouns on my part. What I was trying to get across is that the effects of gravity a star, or gas cloud, or whatever feels is mostly not due to the central black hole. The star feels forces of gravity from all the mass interior to its position in the galaxy. For example, our Sun is 8 kpc away from the central black hole, SagA*. Even though SagA* is massive, we're just so ridiculously far away that the force of gravity exerted by SagA* on the Sun is tiny. Instead, what keeps the Sun orbiting is the force of gravity applied from stars and gas closer to the Sun. So increasing the mass of the central black hole would have almost no effect on the Sun's orbit, and the same goes for most the stars in the galaxy.

3