Just a correcting FYI…

Still, with the count of 1 SMBH, and the possibility of at least 1 smaller, though unobservable partner, this opens the question as to whether there might be several equally unobservable black holes viciously spinning around out there, all of which falls to the fanciful realm of speculative sci-fi — or theoretical astronomy and black hole mechanics.

Do black holes eat black holes, and how messy is that? Are they violently repelled? Are they violently attracted?

Hm-m-m. Just a few million years to find out.

I have no evidence of multiple black holes at Sagittarius Sgr A, ER. Sorry ’bout that.

But though stars may not interact much, the large gas and dust clouds in the spiral arms DO interact, colliding, giving rise to shock fronts thousands of light years across, pressure waves that precipitate bursts of star formation along their edges, like foam bubbles on a whitecap at sea. Most stars formed this way are run-of-the-mill, dim and long-lived dwarfs and main sequence stars that quickly settle down to long, uneventful lives, but a tiny minority are massive, blue supergiants that put out extravagant luminosity until they burn out and go supernova after just a few million years.

On a galactic time scale, these collisions are violent and spectacular events, but to real-time human eyes, its about as exciting as watching grass grow or paint dry. We have to speed up these proceedings mentally to grasp just how incredibly violent and disruptive they are. Galactic studies are like forestry or glaciology. There’s a lot going on, but it is spread out over unimaginable deep time.

I would expect the galactic nucleus would be generously populated with stellar black holes, the end result of big supernova events. This is the oldest part of the galaxy, and the most populated part. However, I wasn’t aware that there were multiple supermassive black holes (several million solar masses)lurking in galactic nuclei, evidence of previous encounters. (Any references?). There is at least one supermassive black hole present in most galactic nuclei, but I wasn’t aware that there were “several”. We can see truly catastrophic long term events at multi-megaparsec distances which might be evidence of the mergers of such objects (AKA “active galactic nuclei”, and there are occasional transient events even more powerful in nature we can see all the way out to the edge of the observable universe. Let’s hope nothing like this ever happens in our own galaxy.

Odds are, the two colliding galaxies would simply pass through each other, with mild stellar and planetary system perturbation. The oddity would therefore be when a galaxy eats another galaxy. We have some possible samples available via Hubble for this phenomenon, but also a local view. The Large and Small Magellanic Clouds are two arms that used to form a single spiral galaxy in their own right, and are now apparently the aftermath of the Milky Way’s last big supper.

…What a glorious sight that must have been…

The Milky Way Also has multiple black holes at its core, the number of which might even be a count of galaxies devoured, or otherwise disrupted.