BEWILDERING BLACK HOLES
(“Tuning in to the ‘Great Hum,’ ” Jan/Feb 2024)
is article states that astronomers weren’t sure that supermassive black holes in binary systems could close in on each other. I’m sure I’m not the only one wondering why that would be — I would assume that objects with such massive gravity would absolutely attract each other over time and eventually merge. What am I missing?
Bill Ziegler
ALISON KLESMAN, SENIOR EDITOR OF OUR SISTER MAGAZINE, ASTRONOMY, RESPONDS:
All massive galaxies are believed to host supermassive black holes. When galaxies merge, it seems evident that their supermassive black holes (SMBHs) should also merge. But the physics involved in the way SMBHs approach each other gets a little tricky. is conundrum is o en called the nal parsec problem.
Let’s start with two merging galaxies. Each has an SMBH. Over time, a single galaxy forms, made up of the combined stars, gas, and black holes. As things settle, the two SMBHs, once in the middle of their respective galaxies, begin to work their way toward the center of the nal galaxy as well. ey do this through dynamical friction. Sometimes instead of falling into the SMBH, the stars or gas clouds the black holes encounter get slingshotted away from the SMBH. Giving that energy to a star or gas robs the SMBH of a little momentum each time.
Eventually, dynamical friction brings both SMBHs to the galactic center and they begin to orbit each other. ey continue to lose momentum for several billion years, until they get to a distance about 1 parsec (3.26 light-years) apart.
en the process stalls out. To reach this point, the black holes will have cleared the area of stars and gas, leaving nothing for further interactions. It would take the black holes essentially the age of the universe to ever get any closer!
e NANOGrav result detecting a background hum of low-frequency gravitational waves indicates there is a way for SMBHs to merge. is is because to generate gravitational waves, the merging SMBHs must get closer than 1 pc. Astronomers are in the process of solving this conundrum, but the pieces are yet to be fully uncovered.