Early universe’s ‘little red dots’ may be black hole stars

From the article:

Since NASA’s JWST observatory began peering into the distant universe in 2022, it has discovered a rash of “little red dots”—hundreds of them, shining within the first billion years of the 13.8-billion-year-old universe, so small and red that they defied conventional explanation. Only in the past few months has a picture begun to emerge. The little red dots, astronomers say, may be an entirely new type of object: a colossal ball of bright, hot gas, larger than the Solar System, powered not by nuclear fusion, but by a black hole.

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In March, teams led by de Graaff and Naidu each posted analyses of the light emitted by a particular little red dot. Broken up into its spectrum, the light displayed the broad optical emission typical of a star peaking in the red, punctuated by narrow absorption lines where hydrogen in the stellar atmosphere absorbs certain wavelengths. In addition, the spectrum contained bright emission lines, characteristic of the superheated gas around an active black hole, which are broadened by Doppler shifts because the gas particles are moving so fast.

So both teams arrive at a bold new picture: The little red dot is a thick gaseous cocoon, not unlike a stellar atmosphere, surrounding an active black hole. Radiation from the black hole heats and props up the gas shell, which is so dense that it absorbs the most energetic UV and x-rays. “Basically, the gas is opaque and so it radiates like a star,” Greene says.

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Given how common little red dots appear to be in the early universe, theorists are beginning to wonder whether this giant-ball-of-gas phase is an essential part of black hole growth and the evolution of galaxies. “We’re probably looking at kind of a new phase of black hole growth that we didn’t know about before,” de Graaff says. Greene agrees: “I can totally imagine that the Milky Way was a little red dot that got its black hole started and then kind of piddled along for the rest of cosmic time.”

The dots’ great distance and small size make it extremely hard to flesh out this picture. “We know basically nothing about their host galaxies because we can’t see them,” Casey says.

That may change with a report this month of a handful of objects much closer to Earth that look like little red dots. Xiaojing Lin of Tsinghua University and her colleagues found several good candidates just 2.5 billion light-years away in a scan of data from an earlier observation campaign, the Sloan Digital Sky Survey. One source had been imaged by the Hubble Space Telescope several years ago. “We do see a compact, relatively faint galaxy there,” Lin says. Her team has recently been granted more time on Hubble for a closer look. They hope to see “whether there are a lot of interactions, activities, outflow, inflow, whatever is happening” that might reveal the workings of a black hole star, says team member Xiaohui Fan of the University of Arizona.