As a rule of thumb, the more massive a black hole is, the bigger its surrounding galaxy will be. That's why there's something so fundamentally wrong about NGC 1277, a tiny galaxy home to possibly the biggest black hole yet.
Galaxy NGC 1277 is only about a tenth the size of our Milky Way. That isn't ridiculously tiny for a galaxy — our galactic neighbor the Large Magellanic Cloud is only a hundredth our size, for instance — but it does mean that its black hole should be significantly smaller than Sagittarius-A*, the supermassive black hole at the center of our galaxy. And that emphatically isn't the case. The black hole at the center of NGC 1277 is thought to be about 17 billion times as massive as our Sun, compared to just about 4.3 million solar masses for Sagittarius-A*.
What's more, the NGC 1277 black hole takes up a volume whose diameter is 4 light-days across, meaning it spans a distance over 338 times that of the diameter of Earth's orbit around the Sun. Even Neptune's outermost orbit, with a diameter of 8.3 light-hours, is still less than a tenth of the black hole's diameter. And what about Sagittarius-A*? Estimates of its diameter vary, but it's at most about 36 light-hours, perhaps closer to just 12. That's impressive compared to the orbits of planets in our solar system, but it's nothing compared to what's going on in NGC 1277.
But let's return to the seriously impressive thing about any black hole, which is its mass. The average supermassive black hole only makes up about 0.1% of its galaxy's overall mass — not an unimpressive figure, but a good reminder that there's still plenty of galaxy out there that isn't black hole. Not so much the case in NGC 1277, as its black hole makes up a whopping 14 percent of the galaxy's total mass.
The question now is just how this remarkable discovery fits into our overall understanding of black holes and galaxy formation. This is so outside what we've encountered before that our current models can't really account for it, but that may simply mean our models are incomplete. The mass of this particular black hole was calculated as part of research at the University of Texas's McDonald Observatory.
The researchers hope improved data on the mass of black holes and their galaxies — we currently only have confirmed figures for about 100 black holes, which is barely anything in cosmic terms — might help us figure out just why some supermassive black holes are surrounded by equally gargantuan galaxies, whereas something completely different has happened in NGC 1277. Team member Karl Gebhard explains in a statement:
"This is a really oddball galaxy. It's almost all black hole. This could be the first object in a new class of galaxy-black hole systems. When trying to understand anything, you always look at the extremes: the most massive and the least massive. We chose a very large sample of the most massive galaxies in the nearby universe... This study is only possible with [the Hobby-Eberly Telescope]. The telescope works best when the galaxies are spread all across the sky. This is exactly what HET was designed for. The mass of this black hole is much higher than expected. It leads us to think that very massive galaxies have a different physical process in how their black holes grow."
Top image by NASA/ESA/Andrew C. Fabian. Side image by D. Benningfield/K. Gebhardt/StarDate.