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"Long before it's in the papers"
July 08, 2015

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Monster black holes said to come out of hiding

July 8, 2015
Courtesy of the Royal Astronomical Society
and World Science staff

Much as X-rays can re­veal the con­tents of a suit­case, en­er­get­ic X-rays have now helped un­veil a pos­sibly large popula­t­ion of dis­tant, gi­ant and pre­vi­ously hid­den black holes, as­tro­no­mers say.

“For the first time we have been able to clearly see these hid­den mon­sters that are pre­dicted to be there, but have pre­vi­ously been elu­sive,” said George Lans­bury of Dur­ham Uni­vers­ity in the U.K., lead au­thor of a re­port on the find­ings.

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Shrouds of gas and dust are block­ing the ob­jects from clear view, the sci­en­tists ex­plain.

Us­ing a NASA sat­el­lite called the Nu­clear Spec­tro­scop­ic Tel­e­scope Ar­ray, Lans­bury and col­leagues said they de­tected high-en­er­gy X-rays from five of the “su­per­mas­sive” black holes. The as­tro­no­mers be­lieve there could be mil­lions more. 

A black hole is an ob­ject so com­pact that it takes on over­whelm­ing gravita­t­ional strength, and thus swal­lows any oth­er ob­ject that strays too close. A black hole gets its name from its abil­ity to eat up even light. But black holes don’t nec­es­sarily ap­pear black, as their some­times vi­o­lent gob­bling ac­ti­vi­ties can gen­er­ate plen­ty of heat, and thus light.

Most ga­lax­ies are thought to har­bor gi­ant, or “su­per­mas­sive” black holes at their cores.

The as­tro­no­mers point­ed the tel­e­scope at nine sus­pected, hid­den su­per­mas­sive black holes that were thought to be feast­ing on ma­te­ri­al at the cen­tre of ga­lax­ies, ac­ti­vity partially ob­scured by gas and dust. In five cases, re­leases of high-en­er­gy X-rays con­firmed the pres­ence of black holes, the re­search­ers said.

Such ob­serva­t­ions were impos­sible be­fore the sat­el­lite, they added. It launched in 2012 and de­tects much high­er en­er­gy X-rays than pre­vi­ous types.

“For a long time we have known about su­per­mas­sive black holes that are not ob­scured... but we sus­pected that many more were hid­den,” said Lans­bury. While five is not much, he added, “when we ex­trap­o­late our re­sults across the whole uni­verse then the pre­dicted num­bers are huge and in agree­ment with what we would ex­pect to see.”

De­tect­ing very en­er­get­ic X-rays was crit­i­cal be­cause “high-en­er­gy X-rays are more pen­e­trat­ing than low-en­er­gy X-rays,” said Dan­iel Stern, proj­ect sci­ent­ist for the sat­el­lite. Thus, “we can see deeper in­to the gas.” 

The researchers pre­sented their find­ings July 6 at the Roy­al As­tro­nom­i­cal So­ci­ety’s Na­t­ional As­tron­o­my Meet­ing in Llan­dud­no, Wales. The re­port has also been ac­cept­ed for pub­lica­t­ion in The As­t­ro­phys­i­cal Jour­nal.


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Much as X-rays can reveal the contents of a suitcase, energetic X-rays have now helped unveil a possibly large population of distant, giant and previously hidden black holes, astronomers say. “For the first time we have been able to clearly see these hidden monsters that are predicted to be there, but have previously been elusive,” said George Lansbury of Durham University in the U.K., lead author of a report on the findings. Shrouds of gas and dust are blocking the objects from clear view, the scientists explain. Using a NASA satellite called the Nuclear Spectroscopic Telescope Array, Lansbury and colleagues said they detected high-energy X-rays from five of the “supermassive” black holes. The astronomers believe there could be millions more. They presented their findings July 6 at the Royal Astronomical Society’s National Astronomy Meeting in Llandudno, Wales. A black hole is an object so compact that it takes on overwhelming gravitational strength, and thus swallows any other object that strays too close. A black hole gets its name from its ability to eat up even light. But black holes don’t necessarily appear black, as their sometimes violent gobbling activities can generate plenty of heat, and thus light. Most galaxies are thought to harbor giant, or “supermassive” black holes at their cores. The astronomers pointed the telescope at nine suspected, hidden supermassive black holes that were thought to be feasting on material at the centre of galaxies, but where shrouds of gas and dust might be obscuring some of the activity. In five cases, releases of high-energy x-rays confirmed the presence of black holes, the researchers said. Such observations were impossible before the satellite, they added. It launched in 2012 and detects much higher energy X-rays than previous types. “For a long time we have known about supermassive black holes that are not obscured... but we suspected that many more were hidden,” said Lansbury. While five is a small number, he added, “when we extrapolate our results across the whole Universe then the predicted numbers are huge and in agreement with what we would expect to see.” Detecting very energetic X-rays was critical because “high-energy X-rays are more penetrating than low-energy X-rays,” said Daniel Stern, project scientist for the satellite. Thus, “we can see deeper into the gas.” The report has been accepted for publication in The Astrophysical Journal.