The Hubble Space Telescope has detected the best-ever candidate for an intermediate- mass black hole (IMBH) —- a medium-sized black hole some 50,000 times the mass of our Sun in the act of gobbling up a passing star. If this discovery holds, it will provide astrophysicists with the long-sought after missing link in theories of black hole evolution. 

Stellar mass black holes and supermassive black holes have been detected and are relatively well understood. Our own Milky Way galaxy has a relatively quiescent supermassive black hole at its galactic center. But intermediate-mass black holes represent an astrophysical puzzle. Researchers are still questioning how these IMBHs actually assemble and whether they are the building blocks of super massive black holes.  

Hubble honed in on the putative IMBH identified by its x-ray moniker 3XMMJ215022.4−055108 inside the star cluster that it calls home. Astrophysicist Dacheng Lin of the University of New Hampshire in Durham and colleagues have just published their analysis of the observations in The Astrophysical Journal Letters. The authors note that the object is “best explained as the tidal disruption of a star by an intermediate-mass black hole in a massive star cluster at the outskirts of a large barred, lenticular galaxy.” 

The x-ray glow from the star shredded by this intermediate mass black hole allowed astronomers to estimate the black hole’s mass of 50,000 solar masses, says NASA. It betrayed its existence by tearing apart a wayward star that passed too close to it, says the agency.

This IMBH is smaller than the supermassive black holes (at millions or billions of solar masses) that lie at the cores of large galaxies, but larger than stellar-mass black holes formed by the collapse of a massive star, says NASA. 

The researchers were surprised to realize that the source of this object’s x-ray and optical emission was not located within our Milky Way. Instead, says NASA, it was found to be located within a distant, dense star cluster on the outskirts of another galaxy —- just the sort of place astronomers expected to find an IMBH.  

NASA says that these new observations also suggest that the star cluster that is home to 3XMMJ215022.4-055108 could be the stripped-down core of a lower-mass dwarf galaxy. That is, one that has been gravitationally and tidally disrupted by close interactions with its current larger galaxy host.

Why are such intermediate mass black holes so difficult to actually detect?

IMBHs are particularly difficult to find because they are smaller and less active than supermassive black holes, says NASA. The agency says that these intermediate mass black holes typically do not have readily available sources of fuel. NASA says that they also don’t have a gravitational pull that is strong enough for them to be constantly drawing in stars and other cosmic material to produce such tell-tale x-ray glows.

To detect them, astronomers first have to catch an IMBH red-handed in the relatively rare act of gobbling up a star, says NASA. Thus, the team combed through the XMM-Newton data archive, searching hundreds of thousands of sources to find this one IMBH candidate.

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