“Where there is an observatory and a telescope, we expect that any eyes will see new worlds at once.” –Henry David Thoreau
This past weekend, the Astronomy Picture of the Day was a remarkable shot of the giant spiral galaxy NGC 6872, taken by the Gemini Telescope. (It’s not Hubble, but Gemini is pretty impressive in its own right!)
One look at this galaxy should tell you that it’s an interacting galaxy in the process of a collision! How should you be able to tell?
The distorted shape is a big clue; normal galaxies don’t have extra-long tails stretched out in a line through space! That’s something normally caused by strong gravitational interactions from a galaxy interacting with another one.
There’s also, if we look closely, the big clue of the disrupted tail of stars on one side!
It looks like, in the wake of that smaller galaxy, there’s a stream of stars that’s sort of been “dragged” out of the larger galaxy’s tail, further evidence of a collision and (likely) a future merger!
But how big is this galaxy we’re talking about? Believe it or not, that galaxy in the picture is the size of the Milky Way. Not that big galaxy, though. This one.
Say hello to IC 4970, the small galaxy in this interacting pair. The large one, NGC 6872, is about four times the diameter of the Milky Way. For scale, that’s about the same size comparison as the Earth to the Earth’s Moon.
So if these are two colliding galaxies, there should be more than just the visible light that’s interesting. So I went to do a search to see what else was out there on these interacting galaxies, and wouldn’t you know it? The Chandra X-ray telescope has taken a look at these guys! What does it see?
Interesting! There are some strong point sources of X-rays here, which are usually caused by cool gas getting fed to a massive black hole, which accelerates the matter, spits it out at an incredible speed, and causes the emission of X-ray light. Pay particular attention to the double source at the top, as pointed out to me by my student, Rye. If we rotate and zoom in on our new optical image to line it up with the X-ray image, what do we find?
That normal-sized galaxy, IC 4970, looks like it has two strong X-ray sources coming from it.
I got curious, so I started looking for any scientific papers on the topic. I came across this one by Machacek et al. (pdf here), that shows that the nucleus of this galaxy doesn’t have enough gas on its own to produce this X-ray emission, but is being fueled by the larger galaxy’s gas!
That’s kind of amazing in and of itself. But what do they have to say about the fact that there are two X-ray sources?
Well, they definitely see two of them, and while they focus on the brighter, variable one at the nucleus, the other one is about 40% as bright and located a pretty hefty 2,000 light years away.
Your first thought and my first thought are likely the same: could this be evidence of binary supermassive black holes in a Milky Way-sized galaxy?
Until now, we’ve only seen binary black holes in galaxies like this one:
(Image credit: Hubble Space Telescope & Spitzer composite.)
NGC 6240, which is a galaxy in the midst of a major merger! Yes, but those are two galaxies, each with their own black hole, that are currently emitting significant amount of X-rays due to the violence of the collision.
Could binary supermassive black holes in normal, relatively quiet galaxies simply be another normal occurrence? And could this X-ray shot of IC 4970 be evidence of a major (or semi-major) merger in the distant past? (The responsible answer is “we don’t know,” but if I were a betting man, I’d say it sure seems likely to me.) The above Chandra image — of NGC 6280 — is only 10 years old, and was the first ever observation of two supermassive black holes in the same galaxy, regardless of circumstance!
If that’s the case, it would mean that every time we either gobble up another galaxy or merge together with another large one, the black holes could merge together to form a much more larger one, or they could simply find a nice, mostly stable orbit in near the center of the galaxy, and hang out there for an indefinitely long amount of time! We just keep realizing that the Universe is more and more amazing that we’d ever imagined, don’t we?