- Intermediate-mass black hole
An Intermediate-mass black hole (IMBH) is a
black holewhose mass is significantly more than stellar black holes (a few tens of the mass of the Sun) yet far less than supermassive black holes (a few millions of the mass of the Sun).
There is less evidence for their existence than for the other two types. Some
ultra-luminous X ray sources (ULXs) in nearby galaxies are suspected to be IMBHs, with masses of a hundred to a thousand solar masses. [cite news
title=Black Hole Boldly Goes Where No Black Hole Has Gone Before
date=January 3, 2007
accessdate=2006-05-24 ] The ULXs are observed in star forming regions (e.g., in starburst galaxy M82cite journal
author=Patruno, A.; Portegies Zwart, S.; Dewi, J.; Hopman, C.
title=The ultraluminous X-ray source in M82: an intermediate-mass black hole with a giant companion
journal=Monthly Notices of the Royal Astronomical Society: Letters
url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005ApJ...628L..33M | doi=10.1111/j.1745-3933.2006.00176.x] , see external links for beautiful pictures of this galaxy), and are seemingly associated with young star clusters which are also observed in these regions. However only a dynamical mass measurement from the analysis of the optical spectrum of the companion star can unveil the presence of an IMBH as the compact accretor of the ULX.
Additional evidence for the existence of IMBHs can be obtained from observation of
gravitational radiation, emitted by the compact remnant that orbits the IMBH. [cite journal| id=arxiv|astro-ph|0506181| title=Gravitational waves from remnants of ultraluminous X-ray sources| first= Clovis| last= Hopman| coauthors= Simon Portegies Zwart | journal=Mon.Not.Roy.Astron.Soc.Lett.| volume= 363| year=2005| pages= L56–L60| doi=10.1111/j.1745-3933.2005.00083.x] Also, the M-sigma relationpredicts the existence of black holes with masses of 104 to 106 solar masses in low-luminosity galaxies.
However it is not clear how such a black hole would form. On the one hand, they are too massive to be formed by the collapse of a single star, which is how the
stellar black holes are thought to form. On the other hand, their environments lack the extreme conditions—i.e., high density and velocities observed at the centers of galaxies—which seemingly lead to the formation of supermassive black holes. There are two popular formation scenarios for IMBHs. The first, is the merging of stellar mass black holes and other compact objects by means of gravitational radiation. The second one is the runaway collision of massive stars in dense stellar clusters and the collapse of the collision product into an IMBH.
In November 2004 a team of astronomers reported the discovery of
GCIRS 13E, the first intermediate-mass black hole in our galaxy, orbiting three light-years from Sagittarius A*. [ [http://www.solstation.com/x-objects/s2.htm S2 and Central Black Hole] ] This medium black hole of 1,300 solar masses is within a cluster of seven stars, possibly the remnant of a massive star cluster that has been stripped down by the Galactic Centre. This observation may add support to the idea that supermassive black holes grow by absorbing nearby smaller black holes and stars. However, recently, a German research group claimed that the presence of an IMBH near the galactic center is doubtful. [cite journal| id=arxiv|astro-ph|0504474 | title= A Black Hole in the Galactic Center Complex IRS 13E?| first= R.| last= Schoedel| coauthors= A. Eckart, C. Iserlohe, R. Genzel, T. Ott| journal= Astrophys. J.| volume= 625| year=2005| pages= L111–L114 | doi= 10.1086/431307] This conclusion is based on a dynamical study of a small star cluster in which should reside the suspected intermediate mass black hole. The debate on the real existence of intermediate mass black holes is still open.
More recently, in January 2006 a team led by Prof. Philip Kaaret of the
University of Iowa, Iowa City announced the discovery of a quasiperiodic oscillation from an intermediate-mass black hole candidate located using NASA's Rossi X-ray Timing Explorer. The candidate, M82 X-1, is orbited by a red giantstar that is shedding its atmosphere into the black hole. [ [http://www.scienceblog.com/cms/dying_star_reveals_more_evidence_for_new_kind_of_black_hole_9685 Dying Star Reveals More Evidence for New Kind of Black Hole | Science Blog ] ] Neither the existence of the oscillation nor its interpretation as the orbital period of the system are fully accepted by the rest of the scientific community. While the interpretation is quite reasonable, the periodicity is claimed based on only about 4 cycles, meaning that it is quite possible for this to be random variation. If the period is real, it could be either the orbital period, as suggested, or a super-orbital period in the accretion disk, as is seen in many other systems.
* [http://chandra.harvard.edu/photo/2000/0094/index.html Chandra images of starburst galaxy M82]
* [http://hubblesite.org/newscenter/newsdesk/archive/releases/2002/18/text/ NASA press release for discovery of IMBHs by]
Hubble Space Telescope
* A New Breed of Black Holes, by Davide Castelvecchi
Sky & TelescopeApril 2006
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