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Finding may help explain giant black holes

July 1, 2009
Courtesy University of Leicester
and World Science staff

As­tro­no­mers are re­port­ing that they have dis­cov­ered a new class of mid-sized black hole, whose ex­ist­ence might help ex­plain how the big­gest black holes orig­i­nat­ed.

The find­ing of a black hole more than 500 times the weight of our Sun in a gal­axy about 290 mil­lion light years away is re­ported July 1 in the re­search jour­nal Na­ture. A light-year is the dis­tance light trav­els in a year.

An artist's de­pic­tion of the new light source HLX-1, shown as the light blue ob­ject to the top left of the ga­lac­tic bulge in the spir­al gal­axy ESO 243-49. (Cred­it: Heidi Sa­ge­rud)


Un­til now, iden­ti­fied black holes have been ei­ther “super-mas­sive” — sev­er­al mil­lion to sev­er­al bil­lion times the weight of the Sun —or about three to 20 times the weight of the Sun. 

The new find­ing is the first sol­id ev­i­dence of me­di­um-sized black holes, ac­cord­ing to as­t­ro­phys­i­cists at the Cen­tre d’E­tude Spa­tiale des Ray­on­nements in France, who de­tected the ob­ject with the Eu­ro­pe­an Space Agen­cy’s XMM-Newton X-ray space tel­e­scope. 

“While it is widely ac­cept­ed that stel­lar mass black holes are cre­at­ed dur­ing the death throes of mas­sive stars, it is still un­known how super-mas­sive black holes are formed,” said the pa­per’s lead au­thor, Sean Far­rell, now at the Un­ivers­ity of Leices­ter, U.K.

“One the­o­ry is that super-mas­sive black holes may be formed by the merg­er of a num­ber of in­ter­me­diate-mass black holes. To rat­i­fy such a the­o­ry, how­ev­er, you must first prove the ex­ist­ence of in­ter­me­diate black holes. 

“This is the best de­tec­tion to date” of these, he added. “The iden­ti­fica­t­ion of HLX-1 is there­fore an im­por­tant step to­wards a bet­ter un­der­stand­ing of the forma­t­ion of the super-mas­sive black holes that ex­ist at the cen­tre of the Milky Way and oth­er ga­lax­ies.”

A black hole is an ob­ject, nor­mally a rem­nant of a spent star, with such a pow­er­ful gravita­t­ional field that it per­ma­nently traps an­y­thing that passes too close, even light. This ac­counts for the “black” mon­i­ker, but in fact the re­gion sur­round­ing a black hole can be ex­tremely bright. This is be­cause the black hole’s high-rate gob­bling up of near­by stel­lar ma­te­ri­al can be­come a vi­o­lent pro­cess, which heats up the ma­te­ri­al so that it shines.

The new ob­ject, dubbed HLX-1 (Hyper-Luminous X-ray source 1), lies to­wards the edge of the gal­axy ESO 243-49. It is ultra-luminous in X-rays, about 260 mil­lion times more so than the Sun, ac­cord­ing to re­search­ers.

The X-ray sig­na­ture of HLX-1 and the lack of a coun­ter­part in op­ti­cal im­ages con­firm that it is nei­ther a fore­ground star nor a back­ground gal­axy, and its po­si­tion in­di­cates that it is not the cen­tral en­gine of the host gal­axy, as su­per­mas­sive black holes usu­ally are, sci­en­tists said.

Us­ing XMM-Newton ob­serva­t­ions car­ried out on the in 2004 and 2008, the team found that HLX-1 dis­played a varia­t­ion in its X-ray sig­na­ture. This in­di­cat­ed to in­vest­i­ga­tors that it must be a sin­gle ob­ject and not a group of many faint­er sources. The huge ra­di­ance ob­served can only be ex­plained if HLX-1 con­tains a black hole more than 500 times the mass of the Sun, Far­rell and col­leagues said.


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Astronomers are reporting that they have discovered a new class of mid-sized black hole, whose existence might help explain how the biggest black holes originated. The finding of a black hole more than 500 times the weight of our Sun in a galaxy about 290 million light years away is reported July 1 in the research journal Nature. A light-year is the distance light travels in a year. Until now, identified black holes have been either “super-massive” — several million to several billion times the weight of the Sun —or about three to 20 times the weight of the Sun. The new finding is the first solid evidence of medium-sized black holes, according to astrophysicists at the Centre d’Etude Spatiale des Rayonnements in France, who detected the object with the European Space Agency’s XMM-Newton X-ray space telescope. “While it is widely accepted that stellar mass black holes are created during the death throes of massive stars, it is still unknown how super-massive black holes are formed,” said the paper’s lead author, Sean Farrell, now at the University of Leicester, U.K. “One theory is that super-massive black holes may be formed by the merger of a number of intermediate mass black holes. To ratify such a theory, however, you must first prove the existence of intermediate black holes. “This is the best detection to date” of these, he added. “The identification of HLX-1 is therefore an important step towards a better understanding of the formation of the super-massive black holes that exist at the centre of the Milky Way and other galaxies.” A black hole is an object, normally a remnant of a spent star, with such a powerful gravitational field that it permanently traps anything that passes too close, even light. This accounts for the “black” moniker, but in fact the region surrounding a black hole can be extremely bright. This is because the black hole’s high-rate gobbling up of nearby stellar material can be a rather violent process, which heats up the material so that it shines. Astrophysicists had long believed this intermediate class of black holes, with masses, or weights, between a hundred and several hundred thousand times that of the Sun, might exist. The new object, dubbed HLX-1 (Hyper-Luminous X-ray source 1), lies towards the edge of the galaxy ESO 243-49. It is ultra-luminous in X-rays, about 260 million times more so than the Sun, according to researchers. The X-ray signature of HLX-1 and the lack of a counterpart in optical images confirm that it is neither a foreground star nor a background galaxy, and its position indicates that it is not the central engine of the host galaxy, as supermassive black holes usually are, scientists said. Using XMM-Newton observations carried out on the in 2004 and 2008, the team found that HLX-1 displayed a variation in its X-ray signature. This indicated that it must be a single object and not a group of many fainter sources. The huge radiance observed can only be explained if HLX-1 contains a black hole more than 500 times the mass of the Sun, said Farrell and colleagues.