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Might there be some roaming black galaxies out there eating up other galaxies?

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The black holes at the centre of galaxies are tiny compared to the galaxy around them. For example the black hole at the centre of our galaxy has a radius of about 1.25 micro light years and the Milky way has a radius of around a hundred thousand light years. So the Milky Way is about eleven orders of magnitude larger than the black hole.

Even a black hole with the mass of the while Milky Way (about $10^{12}$ Solar masses) would have a radius of only a third of a light year, which is still about a million times smaller than the galaxy.

So the black hole isn't going to eat the whole Milky Way any time soon for the simple reason that it's tiny compared to the galaxy. In fact it's likely the Milky Way will mostly evaporate due to dynamical friction long before Saggitarius A$^*$ eats more than a small fraction of the stars in it.

This makes it very unlikely there are galaxy sized black holes wandering around snacking off other galaxies.

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  • $\begingroup$ probably will get the check mark from me, but i'll see if there are other answers. thanks, John. $\endgroup$ – robert bristow-johnson Oct 4 '17 at 7:23
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    $\begingroup$ How does dynamical friction cause evaporation of the galaxy? $\endgroup$ – Count Iblis Oct 4 '17 at 7:37
  • $\begingroup$ @CountIblis: that would be a new question. There is a related question here $\endgroup$ – John Rennie Oct 4 '17 at 7:43
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  1. Angular momentum conservation prevents Sagittarius A* from capturing anything of the Milky way but a tiny fraction, which happens to be on collision course.

  2. Although it is a totally unrealistic scenario, it is nevertheless fun to estimate the extremality of a Kerr black hole with$^1$ $$ \text{mass } M~\sim~ 10^{11} M_{\odot}\qquad\text{and} \qquad\text{angular momentum } J~\sim~ 10^{67}Js\tag{A}$$ of the Milky Way. The relevant dimensionless fraction is $$ f~=~\frac{cJ}{GM^2}.\tag{B} $$ The value $f=1$ corresponds to an extremal Kerr black hole. If we plug in the numbers (A) for the Milky Way, we get $$ f~\sim~ 10^3, \tag{C}$$ i.e. a naked singularity, which violates the cosmic censorship hypothesis.

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$^1$The numbers (A) are extracted from the Wikipedia page. We exclude dark matter, partly because its angular momentum is not known experimentally.

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No one so far seems to have touched on the long term future of the universe. Between the end of the stelliferous era when stars are still forming, and the dark era or true heat death of the universe, there is a long era of cooling matter and black holes. https://en.m.wikipedia.org/wiki/Future_of_an_expanding_universe#Stellar_remnants_escape_galaxies_or_fall_into_black_holes NB: only 1-10% of galaxies are ever consumed by their central blackhole. Still, a significant fraction.

Black holes evaporate in proportion to their surface area, but their energy scales with mass. So while small blackholes like proposed small 'primordial blacholes' would have evaporated, all other types wouldn't, and supermassive ones will be among the longest lived atructures, when most baryonic matter has degeneeated

Its worth saying, there is a lot we don't know about blackholes https://www.independent.co.uk/news/science/black-hole-nasa-supermassive-size-big-bang-gemini-mass-universe-beginning-a8095801.html Gravitational astronomy may hold key insights

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yes, a super duper massive black hole with mass nearly 90000 solar masses can engulf a medium sized galaxy. As we know that when a black hole engulfs matter in large quantity then it grows in size and tends to engulf more matter and also produces quasars.

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    $\begingroup$ A black hole with a mass of $90,000$ Solar masses has a radius of about half the distance from the Earth to the moon. You'll need to explain how something that small can eat an object $100,000$ light years across. $\endgroup$ – John Rennie Oct 4 '17 at 7:30

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