A supermassive black hole has been found that might be called super-supermassive, up to thirty times larger than it should be. To put it another way, the galaxy in which it lies is thirty times too small to host this monster. The discovery is reminder that for all we have learned about black holes recently, abundant mysteries remain.
The supermassive black holes that lie at the center of galaxies normally have a strong relationship with the mass of the galaxy's central bulge. So a black hole that doesn't fit the pattern is intriguing, and possibly a pointer to something important.
This is what Jacco van Loon and Anne Sansom found in galaxy SAGE0536AGN. Around 2 billion light-years away and with a moderate mass of 25 billion solar masses, a fortieth of the Milky Way, SAGE0536AGN isn't the sort of thing that would normally attract a lot of attention.
However, SAGE0536AGN's nucleus was observed in 2011 to be astonishingly bright for a galaxy this size, having the brightest silicate emissions of any known galactic nucleus. This is an indication of a ravenous black hole, which in turn suggests it is probably large. Using the Southern African Large Telescope, Van Loon and Sansom report in Monthly Notices of the Royal Astronomical Society that the hydrogen gas around the galactic center is moving at a speed consistent with orbiting a 350 million solar mass black hole. A black hole with more than 1% of its galaxy's mass in extraordinary.
"Galaxies have a vast mass, and so do the black holes in their cores. This one though is really too big for its boots – it simply shouldn't be possible for it to be so large," Van Loon, of Keele University, U.K., said in a statement.
The discovery raises the question of whether SAGE0536AGN is a one-off, or represents a type of galaxy that bucks the usual trend in terms of black hole formation. Either way the finding challenges the idea that large galaxies evolve faster, creating what the paper calls an “Intimate link between the mass of the black hole and that of the host galaxy's spheroidal component.”
The question is complicated by the fact that there is still debate about the relationship between galaxy and black hole mass. For larger galaxies things are simple – double the galactic mass means double the black hole, give or take a bit.
However, we know less about the black holes at the core of smaller galaxies, and one paper this year argued that the relationship is quadratic at smaller sizes. The authors of that theory proposed that the size at which black holes stop growing four times for each doubling of bulge size is 4 million solar masses, almost 100 times smaller than SAGE0536AGN.
Van Loon and Sansom explore several explanations for the anomaly, such as a recent galactic merger, but find none to be convincing.
The eliptical object at the center of the image, SAGE0536AGN doesn't look like much. Credit: Vista Magellenic Clouds Survey.