I'm trying to build a story around a supermassive black hole, which is ejected from a merger of two galaxies, that is hurtling our own way. What is the smallest realistic distance at which the black hole could sneak upon us with our current technology?

For extra dramatic effects I would like us to notice it as late as possible. The black hole is coming from depth of the intergalactic space toward our Solar System.

The black hole doesn't have any accretion disc around it; my assumption is that it swallowed everything originally around it, if that is possible, and so the only effect would be gravitational.

I don't care under which angle it enters our galaxy - whatever one is stealthiest, so long as there is the least mass to interact with. Maybe it could travel perpendicular to the galaxy disc.

Speed is not important to me too, as long as it is a realistic speed for an ejected black hole following a galaxy merger.

The assumptions with my limited knowledge gained from reading articles and watching documentaries are that:

  • A black hole without an accretion disc doesn't emit radiation.
  • There isn't much matter in intergalactic space to swallow.
  • A black hole's magnetic field is weak, according to this article.
  • A black hole could be discovered only by gravitational effect such as lensing, at least until it enters the galaxy.

Please correct me if my assumptions are wrong.

  • 1
    $\begingroup$ Welcome to worldbuilding, please take the tour and visit the help center to make yourself familiar with this community and its standards. A similar question to your is this one. $\endgroup$
    – L.Dutch
    Dec 7, 2018 at 16:03
  • $\begingroup$ @L.Dutch From what I see in that question neutron star is mainly detected either through its gravitational field, or through its glow. $\endgroup$
    – bantu
    Dec 7, 2018 at 16:22
  • $\begingroup$ Wouldn't that apply to your black hole, too? $\endgroup$
    – L.Dutch
    Dec 7, 2018 at 16:23
  • $\begingroup$ @L.Dutch Forgive my lack of knowledge, but black hole without accretion disc, matter to swallow doesn't glow. And magnetic field should be weak. space.com/… 500 Gauss according to article $\endgroup$
    – bantu
    Dec 7, 2018 at 16:28
  • $\begingroup$ I'm not an astrophysicist, but I'd imagine it could develop an accretion disk, depending on the density of the of the matter it was passing through and its speed. Speaking of speed, how fast is is going? $\endgroup$ Dec 7, 2018 at 16:46

4 Answers 4


This sort of scenario is quite possible, and would likely be the result of the merger of two supermassive black holes during the collision of the galaxies. We have evidence of this in the quasar 3C 186 (see Chiaberge et al. 2017). Over the course of about two billion years, two supermassive black holes circled around each other, emitting gravitational waves. The final burst, as they combined, was likely anisotropic, emitted in a particular direction. This propelled the resulting black hole the opposite way, ejecting it from the galaxy (although it's still nearby; it's only been about 5 million years since the merger).

I pick 3C 186 because we're fairly sure it's been ejected from its host. That's because it is spatially offset from the host galaxy's center by 10-11 kpc, and because it has a velocity offset, traveling towards us at about 2000 km/s, although its overall velocity vector does not point directly at us. Other candidates simply have only spatial or velocity offsets - not both.

If we use 3C 186 as a model, we have some parameters we can look at and analyze:

  • Radial velocity: $\sim$2000 km/s
  • Mass: $\sim10^9M_{\odot}$
  • Emission: Mainly from the broad line region around the black hole
  • Luminosity: $2.6\times10^{13}L_{\odot}$

What's notable is that the active galactic nucleus stayed active. The supermassive black hole was ejected along with high-velocity clouds orbiting close to it. That's why were able to still observe it, and compare its redshift with that of its former host galaxy. It's unclear how long this emission can continue, of course, but if the black hole approaches us fairly soon after it's ejected, we should still see emission from the broad line region and possibly from relativistic jets.

Let's say that it's been a long time since the black hole was ejected, and the gas and dust around it has long since been depleted. In this case, we have a compact object with the mass of a small dwarf galaxy headed our way. We should be able to observe it via gravitational microlensing. Since the angular size of an Einstein ring scales with the square root of the mass of the lens, we should observe lenses about $\sim10^4$ times larger than those created by stellar-mass black holes: $$\theta_E=\sqrt{\frac{4GM}{c^2}\frac{d_O-d_L}{d_Od_L}}=\sqrt{\frac{4GM}{c^2}\left(\frac{1}{d_L}-\frac{1}{d_O}\right)}$$ where $d_O$ and $d_L$ are the distance to the lensed object and the distance to the lens, respectively. Say we observe the lens while the black hole is in intergalactic space - maybe between us and Andromeda. The lensed object, presumably a star in Andromeda, would have $d_O\approx780\text{ kpc}$. If we pick a resolution of $\theta_E\approx0.4$ arcseconds, then we find $d_L\approx768\text{ kpc}$. In other words, if the black hole was coming at us from Andromeda, we could see it from pretty far away!

That said, such an alignment would be unlikely. It's more probable that the supermassive black hole would be coming from another direction - say, from the Virgo Cluster, 18 Mpc away. This means we would see the black hole from 13.3 Mpc away at the most. In general, the distance to the lens at which the ring would have a radius of $\theta_E$ at the critical value is $$d_L=\frac{d_O}{\frac{\theta_E^2c^2}{4GM}d_O+1}$$ and you can check my calculations for the given figures. It's even more likely that the black hole would not be in front of any source even mere tens of megaparsecs away. This of course would make it harder to detect, as the lensed object might appear dimmer, and the ring might be smaller.

The optimal direction for the black hole to sneak up on us from would be from a region of the sky we can't easily observe. I would recommend the Zone of Avoidance, where much of the sky is obscured by gas and dust in the Milky Way. This makes it very hard to perform observations of background galaxies, let alone detect lensing. We would likely need to see lensing from the IC 342/Maffei group, which lies about 3.3 Mpc away. Within 3 Mpc, the lensing would show up, but at that distance, the images would likely be blocked by the Zone of Avoidance.

I don't know how close it would be before we could make that detection; I'm not sure how to calculate it. I assume, though, that the distance would be greater than the distance at which the black hole would gravitationally affect the Milky Way (recall that its mass is comparable to a middling dwarf galaxy). I will work on calculating that range, if I can. But I suspect strongly that microlensing is the best detection method, and that the Zone of Avoidance is the optimal approach. I just need to determine how to combine extinction with lensing.

  • $\begingroup$ That's too far away to work for my story. Are we looking in all directions? Or alternatively is there a chance for us to not look at that part of that sky, or the the sky to be empty behind it. Maybe all bright objects behind are very far away. $\endgroup$
    – bantu
    Dec 7, 2018 at 18:12
  • $\begingroup$ @bantu - the distance to the stars behind it is somewhat irrelevant. Olbers' Paradox is an excellent illustration of why - there is radiation coming from everywhere in the sky. Even if no stars are visible, radio telescopes would see gravitational lensing from the incoming black hole. $\endgroup$
    – jdunlop
    Dec 11, 2018 at 17:23
  • $\begingroup$ Wasn't the OP's question asking for the minimum distance at which detection would become inevitable? This answer seems to be more the maximum detection range. $\endgroup$
    – Monty Wild
    Jan 7, 2020 at 3:13
  • $\begingroup$ @MontyWild Rereading the question, yes, you're correct. Let me work on some edits. $\endgroup$
    – HDE 226868
    Jan 7, 2020 at 3:14

First of all, a Super Massive Black Hole would most likely never be ejected in the merge of two galaxies. These Black Holes are in the galactic centers for a reason. And even if something similiar would happen - two galaxies merge, but only one SMBH stays, then there would certainly be a few stars, that would follow this fleeing Black Hole. But for now, lets just ignore that.

There are two cases we have to evaluate:

  1. The SMBH is coming from the the side of the milky way (in the galactic plane)
  2. The SMBH is coming from the flat side of the milk way (from 'above' or 'below')

In the first case, lets assume its heading straight to us, shortest distance. It is estimated, that the edge is about 20 k LY away from us. That means, the star furthes away from us is about 20.000 years older, than we see him now. And there are stars, maybe far and few, but there are.

That said, we would notice that something is wrong with the stars in that direction - dimming, redshifting, changing its movement - all because they move away from us into the direction of the black hole.

It is hard to assume a distance when this Black Hole would be noticably effect those stars, mostly depending on the mass of the object, but lets assume it is only noticable when the Black Hole is already there. This would mean that if the stars in question would change noticeably now, that was in fact be 20.000 years ago.

Now the next question would be, how fast is this SMBH? I found an article stating that the fastest stellar object we know of - in our galaxy of course - is a white dwarf traveling with around 2400 m/s. Thats about 0.008% the speed of Light. That means, while the light would take 20.000 years to reach us, if this black hole had that speed, it would take about 2.498 x 10^9 years to reach us. Pretty long time for preperations, even if you substract the 20000 years the light traveled.

The second case is a bit more promissing. As it is estimated, that the milky way is about 2 k lightyears thick, we would notice it at max a thousand lightyears away. Under the same speed assumptions, the black hole would only need 124 913 524 years to reach us minus a thousand for the distance.

In both cases, a detection would be almost certain at the earliest points. SMBH arent a thing to joke about an most likely we would notice is much sooner that those 20000 and 1000 years respectivly

But why does it have to be a super massive Blach Hole. In fact, if it was a stellar sized Black Hole, it could sneak up on us, without even noticing. If it only had five solar masses, we maybe only could detect it 1 or 2 lightyears away, if we get lucky. In that case we would feel the effects much sooner.

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    $\begingroup$ A couple comments: It is quite possible for supermassive black holes to be ejected from host galaxies, usually through galaxy mergers and interactions with other supermassive black holes. Second, I think you mean 2400 km/s, not meters per second - that gets you a much different answer for your timescale! $\endgroup$
    – HDE 226868
    Dec 7, 2018 at 17:45
  • $\begingroup$ Well if the super-massive is detectable from too far away I would be forced to switch to stellar one $\endgroup$
    – bantu
    Dec 7, 2018 at 18:18
  • $\begingroup$ I would give a bit more time to the Stellar sized Hole as Black Holes aren't invisible. There is actually a pheomena associated with them called the Gravitational Lensing Effect which distorts the light of any object that passes behind them. This is caused by the photons that cross near to the event horizon but not close enough to be pulled in past the EH will exhibit a sort of slingshot effect around the black hole and escapes on the other side and continues on creating a distorted image to the observer opposite the Black Hole. They also would have an accretion disk $\endgroup$
    – hszmv
    Dec 7, 2018 at 18:46

I must disagree on the detection range of a SMBH, approaching from above or below it could get a lot closer before being detected if it's behind another star. If the proper motion of the black hole and the star are the same it could stay in hiding until it got close enough that we noticed it tugging other stars, we would not get a lensing observation until it was no longer behind the other star.

Obviously, it can't stay behind another star forever because the star is orbiting the galaxy but the black hole is not, but it doesn't need to stay hidden all that long. You don't have to go back too far to find a time we wouldn't have noticed it and if it's heading straight for us it doesn't come out of the shadow until it passes (but that will be a spectacular event if it's close enough.) Note that the star should be a loner, if it has companions the wobble will offer opportunities to note the spectrum from the accretion disk.

As for the ejection velocity--Sag A* is something like 4 million masses, but some of them are in the billions. Lets look at a spectacular merger--galaxies A (4 million solar mass black hole) and B (40 million solar mass black hole) merge, then merge with C (400 million solar mass black hole.) The A and B black holes are in a close orbit, they then pass C--but B goes in. A could have a relativistic ejection velocity.

  • $\begingroup$ There'd be an obvious gravitational lensing which would vastly exceed that which would be expected from the star. Further, you vastly overestimate how long it could be hidden. The sun has a velocity of 220 km/s in its galactic orbit, or, to put it another way, it moves its own diameter every 1.76 hours. If someone was hidden behind it, it wouldn't be hidden for long. $\endgroup$ Jan 7, 2020 at 21:04
  • $\begingroup$ @KeithMorrison You can't see the lensing past the glare of the star in front. And note that I'm assuming the intruder has the same apparent lateral velocity as the star in front of it--it remains hidden until the curvature of the orbit changes the lateral velocity. We see this on Earth occasionally in deadly car/bike accidents in the countryside--both vehicles are on straight roads that intersect, they're arriving at the same time and so the bike remains at a fixed angle to the car--if that's behind the passenger side A pillar... $\endgroup$ Jan 7, 2020 at 21:52
  • $\begingroup$ Think about the geometry for a minute. If the SBH is a long way away (which it would have to be in order to be undetected by the gravitational effects), it's transverse velocity would have to be many multiple times more than that of the star it's hidden behind, directly proportional to the distance. If a star is at x light years, an object at 10x light years has to have relative transverse velocity velocity 10 times higher in order to maintain the alignment. $\endgroup$ Jan 8, 2020 at 1:39
  • $\begingroup$ @KeithMorrison Of course. Note that I showed you can have it coming at relativistic velocity. $\endgroup$ Jan 8, 2020 at 1:45
  • $\begingroup$ To give an example, Barnard's Star, at about 6 light years away, has a transverse velocity relative to us of 90 km/s. If a SBH was 600 light years away, its transverse velocity (which is, remember, on part of its true velocity) would have to be 9,000 kilometers per second. That's 7.5 times faster than the total velocity of the fastest star we've ever found so far. And again, geometry. If it's approaching us at a relatively fast rate, but has that much transverse velocity, then it's true velocity would be freaking ridiculous. So yeah. $\endgroup$ Jan 8, 2020 at 1:47

A supermassive black hole would almost certainly be noticed at least a few thousand years in advance. However, these aren't your only option. It is reported that several smaller, stellar-mass black holes orbit Sagittarius A* at the center of our galaxy, so I think it's safe to assume that there are similar conditions in other galaxies. In a galaxy merger, these stellar-mass black holes could also be ejected. If one of these were coming at us, we would still likely detect it earlier, but it isn't unthinkable that we would have only a few months of warning as we notice orbital irregularities in Oort cloud, or maybe it's too difficult to monitor the Oort cloud(I honestly don't know) and you would notice the orbital irregularities in things a bit close, such as Pluto. In any case, likely a few months of warning before we got to the really bad things, like humanity dying. (see this Kurzgesagt video-https://www.youtube.com/watch?v=gLZJlf5rHVs)

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    $\begingroup$ Hello Gabriel, welcome to Worldbuilding. When you have a moment, please take our tour and review our help center to better understand us. Could you elaborate about how quickly we'd know about the BH? I believe the fastest information can travel is the speed of light, and the BH itself must physically travel intergalactic distances. $\endgroup$
    – JBH
    Dec 7, 2020 at 23:15

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