Black Holes Wrapped in a Gas Cocoon express
Water plunges into an abyss, but part of it shoots back up — churning and foaming at the very edge. That's roughly how gas behaves as it flows onto a supermassive black hole when its stream becomes too dense. In a new study, scientists traced this picture using computer simulations. They accounted for the effects of general relativity — how a black hole warps space and time around itself.
The calculations considered black holes with masses from 100,000 to 10 million solar masses. Gas fell onto them at a rate close to the Eddington limit — the threshold beyond which the pressure of light should push matter away. The fate of this gas depends on how heavy the central black hole is. If it's not too massive, up to a million solar masses, radiation trapped in the inner region accelerates some of the gas and ejects it through the poles in narrow jets. This creates a layered structure: streams of matter flow both inward and outward.
For more massive black holes, over three million solar masses, the picture is different. Radiation can no longer punch through the thick infalling gas, and no outflows occur. Instead, an extended shell gradually forms around the hole. It's like a boiling pot with a tight lid: the gas inside constantly churns but barely reaches the event horizon. In both cases, only about ten percent of what would fall in under free-fall actually reaches the black hole itself. The rest is either carried away or gets stuck in the shell.
This result is important for understanding how supermassive black holes grow in the centers of galaxies. If a massive shell accumulates around a black hole, it could hide it from our telescopes for a long time. Then, when heating exceeds a critical threshold, it might flare up and eject the accumulated material. Such processes may explain the mysterious synchronization of growth between black holes and their host galaxies.
The shell around a black hole can weigh several times more than the black hole itself before it begins to dissipate, like a snowball that grows faster than it melts.
In Greg Bear's novel "The Light of Distant Stars," intelligent clouds around black holes use their radiation to sustain life — an idea that echoes the convective cocoon from this work.