Through a standard telescope, the nearby galaxy NGC 7727 looks like a gossamer tumbleweed drifting in the night sky. But within it are two supermassive black holes beginning a dance that will end with their violent merger. As a team of astronomers recently found, these objects are closer to Earth than any other supermassive pair.
One of the black holes is 6.3 million times the mass of the Sun, while the other is a whopping 154 million solar masses. The duo is located 89 million light-years from Earth in the constellation Aquarius. The team determined the objects’ masses by studying how their gravitational pulls affected stars in their vicinity.
Supermassive black holes lurk at the center of galaxies—our own galaxy hosts Sagittarius A*, a roughly 4 million solar mass black hole 26,000 light-years from Earth. When two galaxies merge, the black holes end up circling one another and eventually merging themselves. These black hole mergers are some of the most violent astrophysical phenomena in the universe, and they generate the gravitational waves famously predicted by Einstein and first observed by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015.
The nearness of the NGC 7727 pair blew the previous record-holding pair out of the interstellar water—that pair was 470 million light-years from us. The team’s research is set to publish in Astronomy & Astrophysics.
“Once the black holes get much closer to each other, they will become gravitationally bound and orbit each other,” said lead study author Karina Voggel in an email to Gizmodo. “This is in theory observable, but this stage in black hole evolution lasts only a short time over a cosmic timescale, and so far we have not observed it.” Voggel, an astronomer at the University of Strasbourg in France, said that unknown galaxy merger relics like it could increase the total number of supermassive black holes by up to 30%.
“Currently, LIGO can detect gravitational wave events from black holes that merge that have a couple times the mass of our Sun,” Voggel added. “When the LISA space mission will come online in a few years, we will be able to also detect the gravitational wave events from the merging of such supermassive black holes.”
While the galaxy is visible through a normal telescope, when seen through the European Southern Observatory’s Very Large Telescope one can make out little orbs of light within the galaxy that mark where the black holes are. (The gravitational pull of black holes is so strong that light famously cannot escape from them, but the objects are often surrounded by superheated plasma that glows brightly.)
“The small separation and velocity of the two black holes indicate that they will merge into one monster black hole,” said study author Holger Baumgardt, an astrophysicist at the University of Queensland, Australia, in an ESO release.
Black hole astronomy is about to get a boost, as the ESO’s Very Large Telescope is set to be succeeded by the Extremely Large Telescope by the end of the decade. The new telescope will sit high in Chile’s Atacama Desert, an attractive spot for astronomers for its altitude, clear skies, and lack of light pollution.
“This detection of a supermassive black hole pair is just the beginning,” said co-author Steffen Mieske, an astronomer at ESO in Chile, in the same release. “We will be able to make detections like this considerably further than currently possible. ESO’s ELT will be integral to understanding these objects.”
Modern gravitational wave observatories are able to detect the ripples in space-time created by collisions of black holes as well as black holes and neutron stars. But we probably won’t get a chance to see this pair finally embrace, since the researchers’ best guess for their merger date is simply “within the next 250 million years,” according to Baumgardt.
This article has been updated to include comments from Karina Voggel.