What would happen if the black hole at the center of a galaxy was removed?

The answers so far have assumed that the galaxy in question is a spiral galaxy - and if we're talking about the Milky Way, then that's all well and good. But galaxies are pretty diverse, both in shape, size, mass and composition. Most look nothing like our own. It turns out that if you're willing to set your story in a different galaxy, you can get some pretty interesting effects from the removal of a large black hole.

I'll look at the ratios between the mass of a certain black hole in a galaxy/star cluster and the mass of the galaxy itself: $M_{\text{BH}}/M_{\text{galaxy}}$. For reference purposes, the black hole at the center of the Milky Way, Sagittarius A*, has a mass of $\sim4\times10^6$ solar masses, while the Milky Way itself has a mass of $\sim1\times10^{12}$ solar masses, giving us $M_{\text{BH}}/M_{\text{galaxy}}\approx0.000004$. That's small; removing Sagittarius A* from the Milky Way won't do squat.

Globular clusters and intermediate-mass black holes

Globular clusters are dense, gravitationally bound sets of stars, gas and other objects, usually of around . They're usually quite old - in the case of the Milky Way's globular clusters, as old as the galaxy itself. Now, what's interesting for our purposes is that there's not really a firm dividing line between certain globular clusters and dwarf galaxies, which may contain up to $\sim10^8$-$10^9$ solar masses. In fact, a few globular clusters, such as Mayall II and Omega Centauri, may contain intermediate-mass black holes, a putative class of objects with masses of up to $\sim10^6$ solar masses.¹

In the case of Omega Centauri - where the existence of the black hole is disputed - the maximum mass is $\sim10^4$ solar masses. The mass of the globular cluster itself is $\sim4\times10^6$ solar masses, meaning $M_{\text{BH}}/M_{\text{galaxy}}\approx0.0025$. Mayall II gives a ratio that's roughly the same, maybe a bit lower. If the black hole in one of these two globular clusters was removed, it would influence the orbits of the innermost stars. This is perhaps more dramatic than in the case of a normal galaxy, because globular clusters have density distributions strongly peaked towards the center. In other words, yes, many orbits would be disrupted, although I doubt that it would be enough to disrupt the cluster. Remember, the mass ratio is still less than 1%.

Massive elliptical galaxies

Some supermassive black holes have masses on the order of $\sim10^9$ to $10^{10}$ (1 billion to 10 billion) solar masses, three of four orders of magnitude greater than Sagittarius A*. These black holes yield much better mass ratios than smaller supermassive black holes. One issue, unfortunately, is that some of these ultra-high mass supermassive black holes are found in very massive elliptical galaxies, which can be up to several trillion solar masses in size.

Consider NGC 1600. Its central supermassive black hole likely has a mass of $\sim2\times10^{10}$ solar masses, while the galaxy itself has a mass of $\sim10^{12}$ solar masses. That's not bad; we get a mass ratio of $M_{\text{BH}}/M_{\text{galaxy}}\approx0.02$. NGC 4889, a supergiant elliptical, has a central black hole of similar mass; its total mass is $\sim10^{13}$ solar masses, yielding $M_{\text{BH}}/M_{\text{galaxy}}\approx0.002$ - possibly smaller, if non-luminous matter exists there in large quantities.

Dwarf galaxies and supermassive black holes

Omega Centauri (and certain other high-mass globular clusters) may be the cores of dwarf galaxies, stripped apart by tidal forces from the Milky Way. As I said before, the dividing line doesn't really exist. However, a high-mass dwarf galaxy is certainly different from a low-mass globular cluster.

Now, consider a set of dwarf galaxies called ultra-compact dwarfs (UCDs). Their masses are on the order of $\sim10^8$ solar masses. One UCD that particularly excites me is M60-UCD1. This galaxy has a mass of $\sim10^8$ solar masses, and might house a supermassive black hole of $\sim2\times10^{7}$ solar masses - five times the mass of Sagittarius A*! This leads to a mass ratio of $\sim0.15$, which is enormous! The orbits of many stars in the galaxy - which is only about 200 light-years across - are quite strongly influenced by the black hole. Removing it would certainly disrupt a number of orbits.

There ultra-compact dwarf population continues to grow, as does the population of supermassive black holes in UCDs. It was recently announced that UCD-3, a galaxy with a mass of $\sim9\times10^7M_{\odot}$, likely contains a black hole of $3.5\times10^6M_{\odot}$, giving us $M_{\text{BH}}/M_{\text{galaxy}}=0.038$. This is lower than M60-UCD1 by a factor of four, but that's not much, and it's quite encouraging.

I will say that I don't think you can get any better than this. Compared to the Milky Way, M60-UCD1 is an excellent candidate for this sort of setting. It's also extremely dense, and quite massive for an ultra-compact dwarf. The high density means that, just like in a globular cluster, you can probably find plenty of exotic objects inside, from blue stragglers to Thorne-Żytkow objects.

¹ As of July 2018, no intermediate-mass black holes have been confirmed, but there are a number of candidates:

• The possible black holes at the centers of the globular clusters 47 Tucanae, Mayall II and Omega Centauri (all disputed)
• The central black hole in the spiral galaxy NGC 4395
• Possible black holes in Messier 82 (Messier 82 X-1), Messier 74, and ESO 243-49 (HLX-1)
• The central object in the star cluster GCIRS 13E
• An object in the high-velocity cloud CO-0.40-0.22
• A number of other ultraluminous X-ray sources (ULXs)

If some of these exist, they could be reasonable decent choices for you. Also, a recent search of Chandra data indicates that there may be a substantial population. I'll update this list if any of these are verified in the future.

posted about 6 years ago

HDE 226868‭

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