Black hole pair in Mrk 501 could merge within a century

Astronomers have long expected that when galaxies collide, their central black holes eventually pair up and merge. Yet a really close pair of supermassive black holes had never been reliably found. A team led by Silke Britzen of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn now reports what it calls the first direct image of such a system, in the core of the galaxy Markarian 501 (Mrk 501) in the constellation Hercules. The study has been accepted by Monthly Notices of the Royal Astronomical Society.

A second jet gives it away

The black hole at the heart of Mrk 501 fires a powerful jet of particles at nearly the speed of light. Because that jet points almost straight at Earth, it is bright and well studied. By re-analysing radio data collected on dozens of days over about 23 years, including 83 observations with the Very Long Baseline Array (VLBA), the team found a second jet pointing in a different direction. Over a few weeks it appeared to start behind the main black hole and sweep counterclockwise around it, again and again.

"We searched for it for so long, and then it came as a complete surprise that we could not only see a second jet, but even track its movement," Britzen said. The most natural explanation is a second black hole orbiting the first.

Close and heavy

The researchers estimate the black holes are only 250 to 540 times the Earth–Sun distance apart, tiny for objects that each weigh roughly 100 million to a billion Suns. The whole system also wobbles: the orbital plane appears to precess on a cycle of about seven years. Even the Event Horizon Telescope cannot resolve the two objects separately, so astronomers cannot watch them close in directly.

They may be able to listen instead. A pair like this should emit very low-frequency gravitational waves that pulsar timing arrays could detect. "If gravitational waves are detected, we may even see their frequency steadily rise as the two giants spiral toward collision," said co-author Héctor Olivares.

Radio astronomy keeps surprising us, as with the Chandra and radio "treble clef" source we covered recently. Mrk 501 itself is far too faint for backyard optics, but our beginner telescope guide covers what you can see.