• Hubble finds best evidence for elusive mid-sized black hole (Hubble Space Telescope)
• NASA: Hubble finds best evidence for elusive mid-sized black hole (Phys.org)
• Images: Hubble finds best evidence for elusive mid-sized black hole (Hubblesite)

Astronomers have finally, and pretty incontrovertibly, found an intermediate mass black hole; an object too big to be the result of a single giant star dying, and too small to have formed at the heart of a galaxy. This new object, name 3XMM J2150022.4-055108 is estimated to be 50,000 times bigger than our Sun, which may seem big, but for perspective, our Milky Way’s central black hole is 4.1 million solar masses. 

Since intermediate black holes can’t be part of a binary star system, and don’t have massive galaxies pointing at their location, trying to find them has been a challenge. Black holes don’t give off any light, and the only ways we can pinpoint their locations are to look for the tell tale light of a surrounding disk of material, catch the high energy flash of something falling past their event horizon, or get lucky and see them gravitationally lensing the light of a more distant object. So far, gravitational lensing surveys have failed us, and constantly bright accretion disks haven’t been a thing we could find. Coming to the rescue was an X-Ray flash caught by Chandra X-Ray observatory and followed up on with the Hubble Space Telescope. The actual image of the area where this object is located is kind of boring: it’s nothing more than a barely resolved star cluster hanging out in the outskirts of a not too distant galaxy. Amusingly, the boringness of the actual image seems to have inspire Hubble Europe, HEIC, and Hubble USA, STScI, to each create their own art’s renditions of this system, with the European graphic being, in my opinion, a truly magnificent graphic. 

This flare of X-Ray that is consistent with an intermediate black hole coming from a star cluster is exactly what folks would expect of an intermediate blackhole. We can’t say exactly how you form one of these mid-sized relativity machines, but we suspect it is either through the chaotic processes that lead to the formations of dwarf galaxies, or through the collisions and mergers of many objects in dense environments. The distant star cluster where this object was seen could easily be a dwarf galaxy caught up and shredded a bit by the galaxy it now orbits. 

This work is published in a new paper in the Astrophysical Journal Letters led by Dacheng Lin of the University of New Hampshire. My favorite part of this story is this black hole was only discovered because Chandra was pointed in exactly the right direction when the BH ate a star or something that flared up in X Rays as it was destroyed. For all we know, the myriad dwarf galaxies around our galaxy could be rich in these same kinds of intermediate mass objects, but we haven’t been able to find them because they just haven’t been eating while we’ve been looking.