Single black hole may be masquerading as a pair

One telescope giveth, another taketh away. Astronomers may have been seeing double when they recently announced the discovery of a pair of supermassive black holes at the heart of a galaxy collision. New observations of the same galaxy suggest that the duo may instead be one lonely black hole blasting the surrounding space with two jets of charged particles. 

At the center of the controversy are two blobs about 450 light-years apart that emit radio waves – light at the low-frequency end of the electromagnetic spectrum. Roger Deane, an astrophysicist at the University of Cape Town in South Africa, and colleagues reported that the radio waves came from two supermassive black holes locked in a gravitational embrace (SN: 7/26/14, p. 10).

Another team of astronomers, led by Joan Wrobel of the National Radio Astronomy Observatory in Socorro, N.M., decided to take a closer look with the Very Long Baseline Array, a network of 10 radio telescopes stretching from Hawaii to the Virgin Islands. “We didn’t know what we would find,” says collaborator Hai Fu, an astrophysicist at University of Iowa in Iowa City.

What they found was something that didn’t turn up in Deane’s observations: faint tails of radio waves connected to the two bright spots, each pointing to a spot halfway between them.

The new images, the researchers report July 28 on arXiv.org, show the subtle signature of two fountains of charged particles — electrons, positrons and protons — driven by a single black hole and slamming into dense clouds of interstellar gas.

Deane, who originally thought a dual jet was unlikely, agrees — but with reservations. Jets from black holes should pump out more energy at lower radio frequencies, he says. But these spots parcel out their energy equally across a range of frequencies, which makes them look more like the stew of particles that closely surround a black hole.

While puzzling, Fu says the dual black hole hypothesis doesn’t fare much better. The alignment of the faint tails, he says, means the black holes would have to be shooting at each other across 450 light-years of space. “That’s too much coincidence,” he adds, preferring to choose the simpler explanation of two jets blasting from a central point.

The new data don’t confirm or deny anything; they just raise questions. “This is not the final nail in the coffin,” Fu cautions. “Science is not like that. We constantly go back and forth.”

To solve the mystery, the two teams have joined forces. They propose using the VLBA in concert with other radio observatories to tease out very faint structures that any telescope on its own wouldn’t see. Fu also says that if the two bright spots have moved since the last observations, then they’re probably jets. Deane adds that further clarification may come from measurements of the magnetic field, which should be strong if the radio waves originate close to two black holes.

Both outcomes are intriguing. Binary black holes should be sources of gravitational waves, theoretical ripples in the fabric of space that no one has directly detected. But jets might help unravel the curious link between the mass of a galaxy and the black hole at its core. “A black hole seems to know about its galaxy,” Fu says, and vice versa. But galaxies are hundreds of thousands of light-years across, and the gravitational reach of a black hole — even a supermassive one — is relatively puny. Astronomers suspect that jets may be the key to understanding how a tiny speck can control the fate of an entire galaxy.