Astronomers have discovered a rather odd discrepancy in the heart of a distant galaxy - it contains two supermassive black holes, but one of those black holes is "naked", with few stars surrounding it. In stark contrast, its black hole sibling is buzzing with stars.
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This discovery, using observations from the Hubble Space Telescope and NASA's Chandra X-ray Observatory, has gotten astronomers thinking: why do some black holes possess a population of stars surrounding them whereas others do not? Also, how does this contrast influence black hole evolution and, indeed, how does it impact the evolution of galaxies?
As reported by Discovery News earlier this week, the first clue to the odd disparity in SDSS J1126+2944 is that this particular galaxy is the result of a galactic merger. Mergers occur when two (or more) galaxies collide and get trapped in their mutual gravitational well. Some stars are scattered in the dramatic upheaval, but for the most part, the stars mingle and then settle. (It is thought that massive galaxies like the Milky Way are in fact the cannibalized remains of many smaller galaxies.)
Eventually the supermassive black holes, which are thought to be hiding in the cores of most galaxies in the universe, may themselves merge, creating a super-supermassive black hole. This is an inevitable part of black hole growth in galactic evolution.
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But in the case of SDSS K1126+2944, the two black holes are still some distance apart and in new research presented this week at the American Astronomical Society (AAS) meeting at Kissimmee, Fla. and published in the Astrophysical Journal, researchers have taken a stab at understanding why one of the black holes is lacking a population of stars, a feature that will undoubtedly impact its supply of in-falling matter.
"One black hole is starved of stars, and has 500 times fewer stars associated with it than the other black hole," said lead investigator Julie Comerford, of Colorado University, Boulder. "The question is why there's such a discrepancy."
According to Comerford, there are two possibilities. The first is that, during the merging of the two galaxies, tidal and gravitational forces ripped through the black hole's neighborhood, scattering its surrounding stars. But there's another and rather curious explanation that could fill the gap in our knowledge about how black holes grow.
In black hole astrophysics, we know of "stellar mass" black holes - basically black holes of 5 to 100 solar masses that remain behind after a massive star goes supernova - and we are familiar with "supermassive" black holes at the cores of galaxies that have masses of between a couple of hundred thousand to millions (or even billions) of solar masses. You may have noticed that there's a huge mass gap between these two types of black holes. If black holes start small and grow more massive over time, what kind of black hole forms the bridge between the stellar mass black holes and the supermassive monsters?
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Astronomers have been on the lookout for "intermediate mass" black holes (IMBHs) to fill this gap, and knowing the preponderance of stellar mass and supermassive black holes, there should be a lot of IMBHs out there - basically medium-sized black holes on their way to becoming supermassive. But there are only a handful of candidates, which is just weird. If our theories of black hole evolution are correct, we shouldn't be having such a hard time tracking down intermediate black holes, which would fall in the mass range of 100 to a million solar masses. Are their emissions simply very hard for us to detect? Or is the lack of observational evidence a clue to their rarity?
Intermediate mass black holes are too massive to have been formed by exploding massive stars, so they would likely be objects that have slowly formed through black hole mergers and mass accretion processes over billions of years. Some observations of low-luminosity active galactic nuclei hint of their existence and detections of ultra-luminous X-ray sources (ULXs) in nearby galaxies have also provided clues. In addition, it is thought dwarf galaxies contain intermediate mass black holes in their cores. Dwarf galaxies have a lower density of stars than more massive galaxies.
Could it be that the "naked" black hole in SDSS J1126+2944 is in fact an intermediate mass black hole that originated inside a cannibalized dwarf galaxy?
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"Theory predicts that intermediate black holes should exist, but they are difficult to pinpoint because we don't know exactly where to look," said co-author Scott Barrows, also from the University of Colorado, Boulder. "This unusual galaxy may provide a rare glimpse of one of these intermediate mass black holes."
Should this IMBH candidate have originated from a merged dwarf galaxy, more work is needed to confirm it as such, but it is certainly a tantalizing clue as to the origins of this mysterious class of "medium-sized" black hole.