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Black holes spinning faster and faster, researchers say

May 24, 2011
Courtesy of the Royal Astronomical Society (U.K.)
and World Science staff

The gi­ant black holes in the cen­ters of ga­lax­ies are spin­ning faster than ev­er be­fore on av­er­age, two U.K. as­tro­no­mers have con­clud­ed based on tel­e­scope ob­serva­t­ions.

Alejo Martinez-Sansigre of the Uni­vers­ity of Ports­mouth and Steve Raw­lings of the Uni­vers­ity of Ox­ford are re­port­ing the find­ings in the jour­nal Monthly No­tices of the Roy­al As­tro­nom­i­cal So­ci­e­ty.

A merger of im­ages from tele­scopes cap­tur­ing light at diff­er­ent wave­lengths shows jets and ra­dio wave-emitt­ing lobes em­an­at­ing from the cen­tral black hole of ga­laxy Cen­taur­us A. (Cred­it: ESO/WFI (visible); MPIfR/ESO/APEX/A.Weiss et al. (microwave); NASA/CXC/CfA/R.Kraft et al. (X-ray))


Sci­en­tists be­lieve most ga­lax­ies har­bor at their cen­ters huge, “su­per­mas­sive” black holes that weigh the equiv­a­lent of a mil­lion to a bil­lion times our sun. Black holes are ob­jects so dense and heavy that their gra­vity sucks in an­y­thing that strays too close, even light rays. While black holes can’t be seen di­rect­ly, it’s pos­si­ble to see the ma­te­ri­al that’s grad­u­ally fall­ing in­to a black hole. Usu­ally spi­ral­ing in­ward and form­ing a disk shape around the cen­tral mass, the ma­te­ri­al can be­come very hot and emit radia­t­ion in­clud­ing X-rays de­tect­a­ble by space-based tel­e­scopes. Ra­di­o waves are al­so emitted, de­tect­a­ble from ground tel­e­scopes.

Twin jets of par­t­i­cles of­ten spray out and away from black holes and their “ac­cre­tion disks.” How these jets arise is un­clear, but a ma­jor fac­tor is thought to be that black holes are spin­ning. Phys­i­cists have thus been try­ing to learn more in­forma­t­ion about black hole spins, in­clud­ing how these might be chang­ing.

The spin of black holes can al­so “tell you a lot about how they formed,” Mar­tinez-San­sigre said. “Our re­sults sug­gest that in re­cent times a large frac­tion of the most mas­sive black holes have some­how spun up. A likely ex­plana­t­ion is that they have merged with oth­er black holes of si­m­i­lar mass, which is a truly spec­tac­u­lar event, and the end prod­uct of this merg­er is a faster-spin­ning black hole.”

Mar­tinez-San­sigre and Raw­lings com­pared the­o­ret­i­cal mod­els of spin­ning black holes with tel­e­scope ob­serva­t­ions us­ing ra­di­o, X-ray and visible-light da­ta. They con­clud­ed that ex­ist­ing the­o­ries can ex­plain the popula­t­ion of su­per­mas­sive black holes with jets.

Us­ing ra­di­o ob­serva­t­ions, the as­tro­no­mers sam­pled the popula­t­ion of black holes, de­duc­ing the spread of the pow­er of the jets. By es­ti­mat­ing how fast they draw in ma­te­ri­al, the re­search­ers could then in­fer how quickly these ob­jects are spin­ning. The spins could then be com­pared at dif­fer­ent times in the un­iverse, be­cause as­tro­no­mers can get a look at ear­li­er pe­ri­ods simply by look­ing fur­ther away. This works be­cause the light from more dis­tant ob­jects takes long­er to reach us, re­veal­ing them as they ap­peared in a more dis­tant past.

“Later this dec­ade we hope to test our idea that these su­per­mas­sive black holes have been set spin­ning rel­a­tively re­cently,” Raw­lings said. “Black hole merg­ers cause pre­dict­a­ble dis­tor­tions in space and time—so-called gravita­t­ional waves. With so many col­li­sions, we ex­pect there to be a cos­mic back­ground of gravita­t­ional waves.” This in turn should “change the tim­ing of the pulses of ra­di­o waves that we de­tect from the rem­nants of mas­sive stars known as pul­sars,” he added. “If we are right, this tim­ing change should be pick­ed up by the Square Kil­o­me­tre Ar­ray, the gi­ant ra­di­o ob­servatory due to start op­er­at­ing in 2019.”

The new find­ings al­so sug­gest that su­per­mas­sive black holes that grow by swal­low­ing mat­ter will spin slow­ly, while those that merge with oth­er black holes will end up spin­ning rap­id­ly, the re­search­ers said.


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The giant black holes in the centers of galaxies are on average spinning faster than ever before, two U.K. astronomers have concluded based on telescope observations. Alejo Martinez-Sansigre of the University of Portsmouth and Steve Rawlings of the University of Oxford are reporting the findings in the journal Monthly Notices of the Royal Astronomical Society. Scientists believe most galaxies harbor at their centers huge, “supermassive” black holes that weigh the equivalent of a million to a billion times our sun. Black holes are objects so dense and heavy that their gravity sucks in anything that strays too close, even light rays. While black holes can’t be seen directly, it’s possible to see the material that’s gradually falling into a black hole. Usually spiraling inward and forming a disk shape around the central mass, the material can become very hot and emit radiation including X-rays detectable by space-based telescopes. Radio waves are also emitted, detectable from ground telescopes. Twin jets of particles often spray out and away from black holes and their accretion disks. How these jets arise is unclear, but a major factor is thought to be that black holes are spinning. Physicists have thus been trying to learn more information about black hole spins, including how these might be changing. The spin of black holes can also “tell you a lot about how they formed,” Martinez-Sansigre said. “Our results suggest that in recent times a large fraction of the most massive black holes have somehow spun up. A likely explanation is that they have merged with other black holes of similar mass, which is a truly spectacular event, and the end product of this merger is a faster-spinning black hole.” Martinez-Sansigre and Rawlings compared theoretical models of spinning black holes with telescope observations using radio, X-ray and visible-light data. They concluded that existing theories can explain the population of supermassive black holes with jets. Using radio observations, the astronomers sampled the population of black holes, deducing the spread of the power of the jets. By estimating how fast they draw in material, the researchers could then infer how quickly these objects are spinning. The spins could then be compared at different times in the universe, because astronomers can get a look at earlier periods in the universe simply by looking further away. This works because the light from more distant objects takes longer to reach us, revealing them as they appeared in a more distant past. “Later this decade we hope to test our idea that these supermassive black holes have been set spinning relatively recently,” Rawlings said. “Black hole mergers cause predictable distortions in space and time—so-called gravitational waves. With so many collisions, we expect there to be a cosmic background of gravitational waves, something that will change the timing of the pulses of radio waves that we detect from the remnants of massive stars known as pulsars. “If we are right, this timing change should be picked up by the Square Kilometre Array, the giant radio observatory due to start operating in 2019,” he added. The findings also suggest that supermassive black holes that grow by swallowing matter will spin slowly, while those that merge with other black holes will end up spinning rapidly, the researchers said.