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.