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I'm writing creative text and I'm curious to know, just as possibility, can a galaxy by any chance be formed out of a black hole?

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  • $\begingroup$ It can seed air and dust but won't this question be better suited for astronomy.se. $\endgroup$
    – user6760
    May 29, 2020 at 13:46
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    $\begingroup$ It all depends on the intentended meaning of the phrase "be formed out of". $\endgroup$
    – AlexP
    May 29, 2020 at 13:49
  • $\begingroup$ That looks like a great Question yet why does it belong in Worldbuilding, rather than physics or philosophy? If such a Question belongs in Worldbuilding, doesn't that negate any idea of real physics, and insist we run with your imagination? That's no criticism of your imagination, which could be the best there is… the point is, which rules should be accepted and what are those rules? $\endgroup$
    – Robbie Goodwin
    May 30, 2020 at 20:04
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    $\begingroup$ Questions on WB are asked here because, a) you can get away with it and, b) you wouldn't understand it answered anywhere else (myself not the least of which including) $\endgroup$
    – Mazura
    May 30, 2020 at 22:08

3 Answers 3

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Directly? No. Indirectly? Yes.

As Renan indicated in their answer, material that enters a black hole cannot leave; the only way around this is through evaporation. Even then, the Hawking radiation emitted is largely in the form of photons, gravitons and neutrinos. Even neutrinos require a black hole of mass $M\sim10^{19}\text{ kg}$, which is unreasonably low except for a black hole in the late stages of its life. Hawking radiation is not going to help you form a galaxy.

However, black holes can cause surrounding material to form a galaxy. Supermassive black holes have the power to influence their local cosmological environment. Say we have an active galactic nucleus, emitting powerful jets thousands of parsecs long. If there are pockets of gas in nearby intergalactic space, these jets may collide with them, producing shocks that heat up the gas. As you might expect, the pockets could expand and collide with nearby gas, triggering star formation. This isn't conjecture; it's happened around an unnamed radio galaxy at a redshift $z=1.7$ (Gilli et al. 2019). The quasar has triggered star formation in 4-6 nearby areas of gas, ranging from tens to hundreds of kiloparsecs away.

Here's an image of the sky around the system. X-ray emission in the 0.5-7 keV band, as seen by Chandra, is in orange. The white contours show 1400 GHz (21 cm) radio emission, observed by the VLA:

Chandra and VLA image of a section of the J1030 field

It's not clear if these gas pockets already contained stars; it's likely that they did. However, it's certainly quite possible that a similar setup could trigger star formation in gaseous overdensities that had yet to form stars and develop any galactic-like structure. These small galaxies are producing stars at a rate of approximately $\sim8\text{-}60M_{\odot}\text{ yr}^{-1}$ (For comparison, the Milky Way's star formation rate is currently only a couple solar masses per year). At this rate, that AGN could lead to the formation of as many stars as the Milky Way contains in about a couple hundred million years.

We've see, then, that a black hole can cause star formation, presumably also in star-less structures we wouldn't think of as galaxies. This is often considered to be what characterizes a protogalaxy: stars are beginning to form for the first time. By this metric, we could argue that that radio galaxy could indeed trigger the formation of a young galaxy - about half a dozen, in fact.

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No.

The only way black holes lose mass is by evaporating. Yes, that's a technical term. They shed mass by emitting Hawking radiation. This radiation is made of particles that simply get lost to space. Such particles are too fast and too light to coalesce into large bodies. They are also subatomic particles, which would have to go through extreme processes to become atoms.

On top of that, the most massive known black hole is TON 618, with a mass of about 66 billion suns. Galaxies as we usually think of them tend to be more massive by two orders of magnitude: the Milky Way has a mass of about 1.5 trillion suns.

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They could in the form of ejected matter from a quasar.

As the blackhole draws in matter to the accretion disk as it feeds, matter is forced up to the poles and fired off in powerful jets, it is believed these jets can create stars and form new galaxies.

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    $\begingroup$ "Some astronomers have begun labeling black holes of at least 10 billion M☉ as ultramassive black holes. Most of these (such as TON 618) are associated with exceptionally energetic quasars." – Supermassive black hole. What happens when they get too big? They get angry. +1 $\endgroup$
    – Mazura
    May 29, 2020 at 23:56

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