Two of the knots, they concluded, were probably small galaxies with small internal motions being cannibalized by the big galaxy. Measurements of the third knot had such large error bars that it could not yet be ruled in or out as the black hole’s location.
The fourth, very compact knot near the bottom edge of the core was too faint even for the Hubble, Dr. Burke-Spoloar reported. “Observing this knot would have required an overblown amount of time (hundreds of hours) observing with Hubble Space Telescope,” she said in an email, and so it also remains a candidate for the black-hole hiding spot.
The galaxy core also emits radio waves, but they didn’t help the search, Dr. Burke-Spolaor said.
“We were originally hoping the radio emission would be some kind of literal smoking gun, showing an active jet that points directly back to black-hole location,” she said. But the radio relic was at least 50 million years old, according to its spectral characteristics, which meant, she said, that the large black hole would have had ample time to move elsewhere since the jet turned off.
Next stop was NASA’s orbiting Chandra X-ray Observatory. Kayhan Gultekin of the University of Michigan, another veteran Nuker who was not on the original discovery team, aimed the telescope at the cluster core and those suspicious knots. No dice. The putative black hole would have to be feeding at one-millionth of its potential rate if it were there at all, Dr. Gultekin said.
“Either any black hole at the center is very faint, or it isn’t there,” he wrote in an email. The same goes for the case of a binary black-hole system, he said; it would need to be eating very little gas to stay hidden.
In the meantime, Imran Nasim, of the University of Surrey in the U.K., who was not part of Dr. Postman’s team, has published a detailed analysis of how the merger of two supermassive black holes could reform the galaxy into what the astronomers have found.
“Simply, gravitational wave recoil ‘kicks’ the supermassive black hole out of the galaxy,” Dr. Nasim explained in an email. Having lost its supermassive anchor, the cloud of stars around the black hole binary spreads out, becoming more diffuse. The density of stars in that region — the densest part of the entire giant galaxy — is only one-tenth the density of stars in our own neighborhood of the Milky Way, resulting in a night sky that would appear anemic compared with our own.