Two of the nodes, they concluded, were likely small galaxies with small internal motions that cannibalize from the large galaxy. Measurements of the third node had error bars so large that it could not yet be excluded or excluded as the location of the black hole.
The fourth, very compact knot near the bottom edge of the core was too weak even for the Hubble, reported Dr. Burke-Spolaor. “Observing this node would have required excessive time (hundreds of hours) to observe with the Hubble Space Telescope,” she said in an email, and so it remains a candidate for hiding the black hole.
The galaxy’s core also sends out radio waves, but they didn’t help the search, said Dr. Burke-Spolaor.
“We had originally hoped the radio emission would be some sort of literal smoke weapon showing an active jet pointing straight back to the location of the black hole,” she said. But the radio relic was at least 50 million years old by its spectral properties, which meant that the great black hole would have had enough time to move elsewhere since the jet was switched off.
The next stop was NASA’s Orbiting Chandra X-ray Observatory. Kayhan Gultekin of the University of Michigan, another veteran Nuker who was not part of the original discovery team, pointed the telescope at the cluster core and those suspicious nodes. No dice. The suspected black hole would have to feed at a millionth of its potential rate if it were there at all, said Dr. Gultekin.
“Either a black hole in the middle is very weak or it isn’t there,” he wrote in an email. The same is true in the case of a binary black hole system, he said; it would have to eat very little gasoline to stay hidden.
Meanwhile, Imran Nasim of the University of Surrey in the UK, who is not a Dr. Postman’s team published a detailed analysis of how the merger of two supermassive black holes could transform the galaxy into what astronomers found.
“The recoil of the gravitational wave” simply throws the supermassive black hole out of the galaxy, “explained Dr. Nasim in an email. After losing its supermassive anchor, the star cloud spreads around the black hole and becomes more diffuse. The density of stars in this 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 appears anemic compared to our own.