Black Holes - Astronomy News

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Date: Apr 30 15:36 2009

Black Holes	Permalink

Title: Star Clusters Around Recoiled Black Holes in the Milky Way Halo Authors: Ryan M. O'Leary, Abraham Loeb (Harvard University) (Version v2) Gravitational wave emission by coalescing black holes (BHs) kicks the remnant BH with a typical velocity of hundreds of km/s. This velocity is sufficiently large to remove the remnant BH from a low-mass galaxy but is below the escape velocity from the Milky Way (MW) galaxy. If central BHs were common in the galactic building blocks that merged to make the MW, then numerous BHs that were kicked out of low-mass galaxies should be freely floating in the MW halo today. We use a large statistical sample of possible merger tree histories for the MW to estimate the expected number of recoiled BH remnants present in the MW halo today. We find that hundreds of BHs should remain bound to the MW halo after leaving their parent low-mass galaxies. Each BH carries a compact cluster of old stars that populated the core of its original host galaxy. Using the time-dependent Fokker-Planck equation, we find that present-day clusters are ~< 1 pc in size, and their central bright regions should be unresolved in most existing sky surveys. These compact systems are distinguishable from globular clusters by their internal (Keplerian) velocity dispersion greater than one hundred km/s and their high mass-to-light ratio owing to the central BH. An observational discovery of this relic population of star clusters in the MW halo, would constrain the formation history of the MW and the dynamics of BH mergers in the early Universe. A similar population should exist around other galaxies, and may potentially be detectable in M31 and M33. Read more (45kb, PDF) __________________

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Date: Apr 29 13:51 2009

Rogue Black Holes	Permalink

It sounds like the plot of a sci-fi movie: rogue black holes roaming our galaxy, threatening to swallow anything that gets too close. In fact, new calculations by Ryan O'Leary and Avi Loeb (Harvard-Smithsonian Centre for Astrophysics) suggest that hundreds of massive black holes, left over from the galaxy-building days of the early universe, may wander the Milky Way. Good news, however: Earth is safe. The closest rogue black hole should reside thousands of light-years away. Astronomers are eager to locate them, though, for the clues they will provide to the formation of the Milky Way. "These black holes are relics of the Milky Way's past. You could say that we are archaeologists studying those relics to learn about our galaxy's history and the formation history of black holes in the early universe" - Avi Loeb Read more __________________

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Date: Apr 27 16:32 2009

RE: Black Holes	Permalink

Dare to fall into a black hole and you would get vaporised in what is probably the most violent place in the universe. But the journey would yield some amazing sights, though you might need three eyes for the best view of what's going on, new research suggests. Read more __________________

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Date: Apr 20 13:43 2009

Black-hole masses	Permalink

Title: Black-hole masses of distant quasars Authors: M. Vestergaard (Steward Observatory, University of Arizona, current address: Tufts University) A brief overview of the methods commonly used to determine or estimate the black hole mass in quiescent or active galaxies is presented and it is argued that the use of mass-scaling relations is both a reliable and the preferred method to apply to large samples of distant quasars. The method uses spectroscopic measurements of a broad emission-line width and continuum luminosity and currently has a statistical 1 sigma uncertainty in the absolute mass values of about a factor of 4. Potentially, this accuracy can be improved in the future. When applied to large samples of distant quasars it is evident that the black hole masses are very large, of order 1 to 10 billion solar masses, even at the highest redshifts of 4 to 6. The black holes must build up their mass very fast in the early universe. Yet they do not grow much larger than that: a maximum mass of about 10 billion solar masses is also observed. Preliminary mass functions of active black holes are presented for several quasar samples, including the Sloan Digital Sky Survey. Finally, common concerns related to the application of the mass scaling relations, especially for high redshift quasars, are briefly discussed. Read more (286kb, PDF) __________________

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Date: Feb 25 17:06 2009

Carter constant	Permalink

A physicist at Washington University in St. Louis has found a new twist on a 40-year-old discovery - "the Carter constant" - about the motion of particles in the external field of a rotating black hole. Clifford M. Will, Ph.D., the James S. McDonnell Professor of Physics in Arts & Sciences, has shown that even in Newton's gravity, arrangements of masses exist whose gravitational field also admits a Carter-like constant of motion. The finding has implications for gravitational-wave astronomy, he says. Read more __________________

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Date: Feb 6 01:41 2009

Black Holes	Permalink

Title: Small, dark, and heavy: But is it a black hole? Authors: Matt Visser (Victoria University of Wellington), Carlos Barcelo (Astrophysics Institute of Andalusia), Stefano Liberati (SISSA and INFN, Trieste), Sebastiano Sonego (University of Udine) Astronomers have certainly observed things that are small, dark, and heavy. But are these objects really black holes in the sense of general relativity? The consensus opinion is simply "yes", and there is very little "wriggle room". We discuss one of the specific alternatives. Read more (528kb, PDF) __________________

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Date: Jan 21 17:28 2009

Naked Singularities	Permalink

Do Naked Singularities Break the Rules of Physics? * Conventional wisdom has it that a large star eventually collapses to a black hole, but some theoretical models suggest it might instead become a so-called naked singularity. Sorting out what happens is one of the most important unresolved problems in astrophysics. * The discovery of naked singularities would transform the search for a unified theory of physics, not least by providing direct observational tests of such a theory. Source __________________

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Date: Jan 15 16:54 2009

RE: Black Holes	Permalink

How did black holes lurking at the centre of galaxies like ours get so big so quickly? That's the puzzle posed by the discovery of fully grown, supermassive black holes surrounded by fledgling galaxies early in the history of the universe. Chris Carilli of the National Radio Astronomy Observatory in Socorro, New Mexico, and colleagues studied four galaxies from less than 2 billion years after the big bang. They found the black holes at the centres of these galaxies were as heavy as anything seen in the modern universe, with one estimated to have the mass of 20 billion suns. Meanwhile, it looked as if the host galaxies had only begun to grow, with only modest masses compared with galaxies harbouring similarly sized black holes today. "This suggests the black holes came first" - Chris Carilli, who presented the results at a meeting of the American Astronomical Society in Long Beach, California. Read more __________________

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Date: Jan 7 00:39 2009

Supermassive Black Holes	Permalink

Astronomers may have solved a cosmic chicken-and-egg problem -- the question of which formed first in the early Universe -- galaxies or the supermassive black holes seen at their cores. "It looks like the black holes came first. The evidence is piling up" - Chris Carilli, of the National Radio Astronomy Observatory (NRAO). Carilli outlined the conclusions from recent research done by an international team studying conditions in the first billion years of the Universe's history in a lecture presented to the American Astronomical Society's meeting in Long Beach, California. Earlier studies of galaxies and their central black holes in the nearby Universe revealed an intriguing linkage between the masses of the black holes and of the central "bulges" of stars and gas in the galaxies. The ratio of the black hole and the bulge mass is nearly the same for a wide range of galactic sizes and ages. For central black holes from a few million to many billions of times the mass of our Sun, the black hole's mass is about one one-thousandth of the mass of the surrounding galactic bulge. "This constant ratio indicates that the black hole and the bulge affect each others' growth in some sort of interactive relationship. The big question has been whether one grows before the other or if they grow together, maintaining their mass ratio throughout the entire process" - Dominik Riechers, of Caltech. In the past few years, scientists have used the National Science Foundation's Very Large Array radio telescope and the Plateau de Bure Interferometer in France to peer far back in the 13.7 billion-year history of the Universe, to the dawn of the first galaxies. "We finally have been able to measure black-hole and bulge masses in several galaxies seen as they were in the first billion years after the Big Bang, and the evidence suggests that the constant ratio seen nearby may not hold in the early Universe. The black holes in these young galaxies are much more massive compared to the bulges than those seen in the nearby Universe. The implication is that the black holes started growing first" - Fabian Walter of the Max-Planck Institute for Radioastronomy (MPIfR) in Germany. Source __________________

Blobrana

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Posts: 131433

Date: Dec 16 18:54 2008

RE: Black Holes	Permalink

You wouldn't want to eat one, but you might take comfort in the new knowledge that a black hole and its surrounding material take the shape of a doughnut regardless of the mass of the black hole itself. Read more __________________

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