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Black holes are sometimes huge – supermassive as they are called, billions of times the mass of our sun. Other times they are petite with just a few times the sun’s mass. But do black holes also come in size medium? A new study suggests that, most likely, the answer is no. Astronomers have long suspected that the best place to find a medium-mass black hole would be at the core of a miniature galaxy-like object called a globular cluster. Yet nobody has been able to find one conclusively. And now, a team of astronomers has thoroughly examined a globular cluster called RZ2109 and determined that it cannot possess a medium black hole, leading researchers to believe that black holes don’t come in medium, or at most are very rare.
“Some theories say that small black holes in globular clusters should sink down to the center and form a medium-sized one, but our discovery suggests this isn’t true,” said Daniel Stern of NASA’s Jet Propulsion Laboratory. Stern is second author of a study detailing the findings in the Aug. 20 issue of Astrophysical Journal. The lead author is Stephen Zepf of Michigan State University, East Lansing.
Black holes are incredibly dense points of matter, whose gravity prevents even light from escaping. The least massive black holes known are about 10 times the mass of the sun and form when massive stars blow up in supernova explosions. The heftiest black holes are up to billions of times the mass of the sun and lie deep in the bellies of almost all galaxies.
That leaves black holes of intermediate mass, which were thought to be buried at the cores of globular clusters. Globular clusters are dense collections of millions of stars, which reside within galaxies containing hundreds of billions of stars. Theorists argue that a globular cluster should have a scaled down version of a galactic black hole. Such objects would be about 1,000 to 10,000 times the mass of the sun, or medium in size on the universal scale of black holes.
The research team used the Keck Observatory on Mauna Kea in Hawaii to look at the spectrum of the cluster, which revealed that the black hole is petite, with roughly 10 times the mass of our sun.
According to theory, a cluster with a small black hole cannot have a medium one, too. Medium black holes would be quite hefty with a lot of gravity, so if one did exist in a globular cluster, scientists argue that it would quickly drag any small black holes into its grasp.
“If a medium black hole existed in a cluster, it would either swallow little black holes or kick them out of the cluster,” said Stern. In other words, the small black hole in RZ2109 rules out the possibility of a medium one.
The researchers believe other globular clusters would have a similar makeup and the likelihood for finding a medium black hole is not good. Zepf said it is possible such objects are hiding in the outskirts and of galaxies like our Milky Way, either in surrounding so-called dwarf galaxies or in the remnants of dwarf galaxies being swallowed by a bigger galaxy. If so, the black holes would be faint and difficult to find.
News Source: JPL
Am I to understand then that there definitely is a black hole at the center of glubular cluster RZ2109? I wasn’t aware that having ANY kind of black hole at the center of such clusters had been proven.
Ah, a quick reading of the related stories shows my lack of awareness to be due to bad memory. Alas.
As far as I recall, the Omega Centauri cluster has been proven to contain a black hole.
The article states:
“Black holes are incredibly dense points of matter”
This is not the case. A black hole is no point, it has a diameter, the radius of which rises with the mass the black hole contains. This radius is called the schwarzschild radius, as Carl Schwarzschild set up the formula to calculate that radius a centuri ago.
We cannot get any information from the interior of a black hole. The assuption, that all the mass is concentrated in a singularity, is only an assumption.
If you put the mass of our universe into the formula to calculate the schwarzschild radius of a black hole of the mass of the universe, the resulting radius is some 13 billion light years. From that point of view, we are all living inside a black hole of the mass of the universe, and if I want to see how the inside of such a huge black hole looks like, I just have to look out of the window.
Regards,
Günther
I seem to remember reading somewhere that if you take the Schwartzschild radius as a measure of the size of a supermassive black hole, and calculate its density on that basis, the density comes out to be rather less than that of air.
Can anyone confirm or refute this?
This lack of intermediate sized black holes, together with the flat rotation curves observed in spiral galaxies, makes me wonder about alternatives to last century’s BH assumptions.
What we measure is not so much a “black hole”, but a gravitational center accompanied by an event horizon. “Black hole” serves well as a catchy description, but less well as a model.
Regarding supermassive “black holes” at the center of spiral galaxies, what we measure are characteristic emissions, and stellar orbits on geodesics initially interpreted as being imposed by a large central mass.
However, let a spiral galaxy be associated with a vortex in a belt of dark matter, centered on some topological anomaly in our 3-brane, e.g. a puncturing intrusion, or perturbation from the bulk. The circling motion of such a belt of pre-galactic “pre-photonic” matter about its center could help explain those flat rotation curves, and become another model to explore.
As for stellar mass “black holes”, once we replace last century’s point-like lepto-quarks with rings and strings of Planck scale oscillators, such a repository of near-ultimately compacted matter seems a neat precursor for that which goes bang for GLAST.
Nigel
For GBendt – A recent study of Omega Cen by Gemini-S & HST (arXiv:0801.2782) claims to have found a 40,000 solar mass BH at its’ center. Previous studies of this oversized globular cluster( or possibly stripped core of a dwarf galaxy) have produced equivocal results. Just as this story was posted, a paper on NGC 2808 (arXiv:0808.2661) using HST, Chandra & XMM-Newton found no BH at its’ core & put an upper limit on the mass of any central BH in NGC 2808 at 140 solar masses. Recent stdies of G1 (another super-sized GCl in M 31) & M 32 found no evidence for an IMBH in either object. Despite many years of searching, astronomers are still having a hard time pinning down IMBHs in any stellar system. Even so, the search for these objects is certainly well warranted. In either case(IMBH exist or not), the result will have profound implications for black hole researchers.
Can’t speak for anyone else, but I’d consider my black hole to be medium size. Not too big, and not too small. Goldilocks would agree.
I don’t think “Black Hole” has even been more than a “catchy description” – at least outside of the public’s mind.
Isn’t “a gravitational center accompanied by an event horizon” pretty much the formal description of a Black Hole?
Does anyone know what happens if Dark Matter falls into one? Does it even happen? I mean, since it’s dark, it cannot really cool through emitting radiation, so perhaps it can only skirt by a BH.
It seems to me that the SMBH at the center of spiral galaxies are the engines that drive the spinning of the galaxies to begin with. Therefor, how could there be a BH of any apreciable size at the center of GC. The clusters themselves are too small and “globular”, as opposed to being a flat disk, such as an accretion disk of a black hole, to have such an object within them in the first place. Thank you.
For those interested in the SMBH in our galaxy, a recent paper entitled ‘Is there a Super-Massive Black Hole at the center of the Milky Way?’ was posted at the arXiv site( arXiv:0808.2624 ). This paper gives a great overview of how astronomers came to believe a SMBH is at the center of our galaxy. With very few equations and written for a general audience (& many great pictures), interested readers may find out how the hunt for a SMBH in our galaxy came about.