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What happens to a black hole-less galaxy?

analog31 10y ago
My guess is that it gains a new black hole as stuff coalesces at the center. This hunch comes from not much more than knowing what Newton predicted, which is that any system with more than two bodies is unstable, with outcomes that include ejecting a body, or two bodies colliding. The formation of stars in the first place is the result of such coalescence, so it's not unthinkable that the same thing will happen at the center of a galaxy full of stars.
M_Grey 10y ago
Truthfully, most if not all large galaxies probably do have central supermassive black holes. Dwarf galaxies might not, but they're more like drifting clouds (and tend to be gobbled up by larger galaxies over time). It's just that there is no way to say with certainty that a BH is in the center of a galaxy when it isn't actively feeding, unless it's close enough for us to resolve the motions of individual stars in the core. Remember also that there are large swaths of the sky we basically cannot observe in the 'Zone of Avoidance'.
- sandworm101 10y ago
  If a galaxy is in a position to be lensing, or we can otherwise get a grip on its mass, then we can guess the BH if it isnt emitting enough. If the galaxy is dim but massive, black holes are likely.
gliese1337 10y ago
Not much. Relative to a star, central supermassive black holes are, well, super-massive. But relative to the total mass of the galaxy, or even just the core region, they aren't that big. The rest of the galaxy will just keep orbiting around its center of gravity, regardless of whether or not there happens to be a black hole there.
- saagarjha 10y ago
  In what kind of galaxy is a 1 billion solar mass black hole not a significant portion of the mass?
  - idlewords 10y ago
    Pretty much any galaxy. The Milky Way has something like 300 billion stars, with the "average" star weighing 1/4 as much as the sun. So even the stellar mass of a galaxy like ours outweighs such a black hole by two orders of magnitude, and most of a galaxy's mass is dark matter.
    - saagarjha 10y ago
      For whatever reason I had the number "300 million" in my head for the size of the Milky Way; I see your point.
  - LordDragonfang 10y ago
    Well, for reference, the lower bound on estimates of the mass of the milky way is ~5×10^11 solar masses, so a billion solar mass black whole would account for less than half of a percent of that.
idlewords 10y ago
Made fun of by the other galaxies

My guess is that it gains a new black hole as stuff coalesces at the center. This hunch comes from not much more than knowing what Newton predicted, which is that any system with more than two bodies is unstable, with outcomes that include ejecting a body, or two bodies colliding. The formation of stars in the first place is the result of such coalescence, so it's not unthinkable that the same thing will happen at the center of a galaxy full of stars.

M_Grey 10y ago

Truthfully, most if not all large galaxies probably do have central supermassive black holes. Dwarf galaxies might not, but they're more like drifting clouds (and tend to be gobbled up by larger galaxies over time). It's just that there is no way to say with certainty that a BH is in the center of a galaxy when it isn't actively feeding, unless it's close enough for us to resolve the motions of individual stars in the core. Remember also that there are large swaths of the sky we basically cannot observe in the 'Zone of Avoidance'.

sandworm101 10y ago
If a galaxy is in a position to be lensing, or we can otherwise get a grip on its mass, then we can guess the BH if it isnt emitting enough. If the galaxy is dim but massive, black holes are likely.

gliese1337 10y ago

Not much. Relative to a star, central supermassive black holes are, well, super-massive. But relative to the total mass of the galaxy, or even just the core region, they aren't that big. The rest of the galaxy will just keep orbiting around its center of gravity, regardless of whether or not there happens to be a black hole there.

saagarjha 10y ago
In what kind of galaxy is a 1 billion solar mass black hole not a significant portion of the mass?
- idlewords 10y ago
  Pretty much any galaxy. The Milky Way has something like 300 billion stars, with the "average" star weighing 1/4 as much as the sun. So even the stellar mass of a galaxy like ours outweighs such a black hole by two orders of magnitude, and most of a galaxy's mass is dark matter.
  - saagarjha 10y ago
    For whatever reason I had the number "300 million" in my head for the size of the Milky Way; I see your point.
- LordDragonfang 10y ago
  Well, for reference, the lower bound on estimates of the mass of the milky way is ~5×10^11 solar masses, so a billion solar mass black whole would account for less than half of a percent of that.

idlewords 10y ago

Made fun of by the other galaxies