![\"X-ray](\"https:\/\/cdn.sci.news\/images\/enlarge13\/image_14094e-NGC-6099-HLX-1.jpg\")
<\/a><\/p>\nX-ray and infrared images of NGC 6099 HLX-1. Image credit: NASA \/ CXC \/ Inst. of Astronomy, Taiwan \/ Y-C Chang \/ ESA \/ STScI \/ HST \/ J. DePasquale.<\/p>\n
NGC 6099 is located about 450 million light-years away in the constellation of Hercules.<\/p>\n
Astronomers first saw an unusual source of X-rays in an image of this galaxy taken by NASA\u2019s Chandra X-ray Observatory in 2009.<\/p>\n
They then followed its evolution with ESA\u2019s XMM-Newton space observatory.<\/p>\n
\u201cX-ray sources with such extreme luminosity are rare outside galaxy nuclei and can serve as a key probe for identifying elusive intermediate-mass black holes,\u201d said Dr. Yi-Chi Chang, an astronomer at the National Tsing Hua University.<\/p>\n
\u201cThese objects represent a crucial missing link in black hole evolution between stellar mass and supermassive black holes.\u201d<\/p>\n
X-ray emission coming from NGC 6099 HLX-1 has a temperature of 3 million degrees consistent with a tidal disruption event.<\/p>\n
Using the NASA\/ESA Hubble Space Telescope, the astronomers found evidence for a small cluster of stars around the black hole.<\/p>\n
This cluster would give the black hole a lot to feast on, because the stars are so closely crammed together that they are just a few light-months apart (about 500 billion miles).<\/p>\n
The suspected intermediate-mass black hole reached maximum brightness in 2012 and then continued declining to 2023.<\/p>\n
The optical and X-ray observations over the period do not overlap, so this complicates the interpretation.<\/p>\n
The black hole may have ripped apart a captured star, creating a plasma disk that displays variability, or it may have formed a disk that flickers as gas plummets toward the black hole.<\/p>\n
\u201cIf the intermediate-mass black hole is eating a star, how long does it take to swallow its gas?\u201d said Dr. Roberto Soria, an astronomer at the Italian National Institute of Astrophysics.<\/p>\n
\u201cIn 2009, HLX-1 was fairly bright. Then in 2012, it was about 100 times brighter. And then it went down again.\u201d<\/p>\n
. \u201cSo now we need to wait and see if it\u2019s flaring multiple times, or there was a beginning, there was peak, and now it\u2019s just going to go down all the way until it disappears.\u201d<\/p>\n
The authors emphasize that doing a survey of intermediate-mass black holes can reveal how the larger supermassive black holes form in the first place.<\/p>\n
There are two alternative theories. One is that intermediate-mass black holes are the seeds for building up even larger black holes by coalescing together, since big galaxies grow by taking in smaller galaxies. The black hole in the middle of a galaxy grows as well during these mergers.<\/p>\n
Hubble observations uncovered a proportional relation: the more massive the galaxy, the bigger the black hole. The emerging picture with this new discovery is that galaxies could have satellite intermediate-mass black holes that orbit in a galaxy\u2019s halo but don\u2019t always fall to the center.<\/p>\n
Another theory is that the gas clouds in the middle of dark matter halos in the early Universe don\u2019t make stars first, but just collapse directly into a supermassive black hole.<\/p>\n
Webb\u2019s discovery of very distant black holes being disproportionally more massive relative to their host galaxy tends to support this idea.<\/p>\n
However, there could be an observational bias toward the detection of extremely massive black holes in the distant Universe, because those of smaller size are too faint to be seen.<\/p>\n
In reality, there could be more variety out there in how our dynamic Universe constructs black holes.<\/p>\n
Supermassive black holes collapsing inside dark matter halos might simply grow in a different way from those living in dwarf galaxies where black-hole accretion might be the favored growth mechanism.<\/p>\n
\u201cSo if we are lucky, we\u2019re going to find more free-floating black holes suddenly becoming X-ray bright because of a tidal disruption event,\u201d Dr. Soria said.<\/p>\n
\u201cIf we can do a statistical study, this will tell us how many of these intermediate-mass black holes there are, how often they disrupt a star, how bigger galaxies have grown by assembling smaller galaxies.\u201d<\/p>\n