A dance of giant black holes (Science Advent 18)

(Every day until December 25, I’m posting a science-related image or video and description.)

Day 18

An image of a pair of possible supermassive black holes in radio light. [Credit: Chao-Wei Tsai et al.]
An image of a possible pair of supermassive black holes in radio light. The system produces two jets of particles from a core (marked A), but one of the jets (C) is noticeably curved and both are zigzagged. A second source of strong emission (E) could be a second black hole. [Credit: Chao-Wei Tsai et al.]

Supermassive black holes are the most massive single objects in the Universe, and they reside at the hearts of most large galaxies, including the Milky Way. While collective bodies — galaxies in particular — are bulkier, these black holes can weigh in at millions or billions of times the Sun’s mass. How they get so huge is still a matter of some conjecture, but one way they can grow even more relates to their location.

Galaxies grow by colliding and merging, with smaller galaxies nommed up by their larger cousins. We see evidence of this in the Milky Way, in the form of populations of stars that obviously formed elsewhere. Astronomers have captured some marvelous and dramatic images of these collisions.

When two galaxies merge, at least in some cases their supermassive black holes also combine. This process starts with a slow gravitational dance as the two black holes orbit each other. According to general relativity, energy is radiated away in the form of gravitational waves, bringing the holes gradually together. Astronomers found an interesting possible binary black hole from its interesting infrared emissions, and given the very memorable name WISE J233237.05−505643.5 . (The galaxy that harbors this system is very distant — 3.9 billion light-years away — so as far as I can tell, it’s never been imaged or given a separate name.)

From infrared, radio, and X-ray observations, the center of this galaxy has two strong sources of light emission, and produces a zigzag wiggly jet of particles. That’s not normal for single black holes! The authors of the research paper are very (wisely) cautious: they suspect the emissions are due to a pair of black holes, but there are other options. Maybe this is an unusual case where the disk of matter falling onto the black hole is misaligned with the axis of the hole. However, the simplest explanation seems to be two supermassive black holes separated by a few light-years.

The evolution of supermassive black holes is intimately connected to that of their host galaxies. Specifically, galaxies with a lot of mass in their central regions have bigger black holes. While we have yet to spot a black hole merger in action (this is a very slow process!), seeing a pair of them dancing the slow waltz of gravity is an important — and exciting! — discovery.

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