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Do black holes zap galaxies into existence?

Dec. 1, 2009
Courtesy European Southern Observatory
and World Science staff

Which come first: the su­pe­r­mas­sive black holes that franti­c­ally de­vour mat­ter, or the huge ga­lax­ies where they re­side? 

A new sce­nar­i­o has emerged to an­swer this con­ten­tious “‘chicken and egg’ ques­tion,” said Da­vid El­baz of the Cen­ter for Nu­clear Stud­ies of Saclay in Gif-sur-Yvette, France, one of the re­search­ers who de­vel­oped the mod­el.

Colour com­pos­ite im­age of qua­sar HE0450-2958, the bright­est ob­ject in the im­age. The im­age was ob­tained with the VISIR in­stru­ment on ES­O’s Very Large Tel­e­scope, the Hub­ble Space Tel­e­scope and the Ad­vanced Cam­era for Sur­veys. The qua­sar is be­lieved to be zap­ping the ob­ject to its low­er left, a gal­axy, with an en­er­get­ic beam of par­t­i­cles.


The is­sue is “one of most de­bat­ed sub­jects in as­t­ro­physics,” he added. “Our study sug­gests that su­pe­r­mas­sive black holes can trig­ger the forma­t­ion of stars, thus ‘build­ing’ their own host ga­lax­ies.” 

A black hole is a supe­r-compact as­t­ro­phys­i­cal ob­ject whose gra­vity is so pow­er­ful that it drags in­to it­self any ob­ject pass­ing too near­by, in­clud­ing light. That ac­counts for the “black” mon­i­ker, but ac­tu­ally many black holes are thought to be easily vis­i­ble thanks to vi­o­lent ac­ti­vity go­ing on around them.

El­baz and col­leagues stud­ied a pe­cu­liar ob­ject some five bil­lion light years away, be­lieved to be a black hole with­out a home gal­axy and dubbed qua­sar HE0450-2958. A light year is the dis­tance light trav­els in a year.

It had been spec­u­lat­ed that the qua­sar’s host gal­axy was hid­den be­hind dust. The as­tro­no­mers thus used an in­stru­ment on the Eu­ro­pean South­ern Ob­ser­va­tory’s Very Large Tel­e­scope de­signed to de­tect so-called mid-infrared light, which would make dust clouds brightly vis­i­ble.

Yet no dust ap­peared, in­di­cat­ing there was no home gal­axy, said Knud Jahnke of the Max Planck In­sti­tute for As­tron­o­my in Hei­del­berg, Germany, who led the ob­serva­t­ions. “In­stead we dis­cov­ered that an ap­par­ently un­re­lat­ed gal­axy in the qua­sar’s im­me­di­ate neigh­bour­hood is pro­duc­ing stars at a frantic rate,” he said, the equiv­a­lent of about 350 Suns yearly. 

Ear­li­er ob­serva­t­ions had shown that the com­pan­ion gal­axy is, in fact, un­der fire: the qua­sar is spew­ing a je­t of en­er­get­ic par­t­i­cles to­wards its com­pan­ion, ac­com­pa­nied by a stream of fast-mov­ing gas. The in­jec­tion in­di­cates that the qua­sar it­self might be in­duc­ing the forma­t­ion of stars and there­by cre­at­ing its own host gal­axy, ac­cord­ing to El­baz and col­leagues. In such a sce­nar­i­o, ga­lax­ies would have evolved from clouds of gas hit by the en­er­get­ic je­ts emerg­ing from qua­sars, or giant black holes. 

“The two ob­jects are bound to merge in the fu­ture: the qua­sar is mov­ing at a speed of only a few tens of thou­sands of kilo­me­ters [or miles] per hour with re­spect to the com­pan­ion gal­axy and their separa­t­ion is only about 22,000 light-years,” said El­baz. “Although the qua­sar is still ‘naked’, it will even­tu­ally be ‘dressed’ when it merges with its star-rich com­pan­ion. It will then fi­nally re­side in­side a host gal­axy like all oth­er qua­sars.”

The find­ings may al­so rep­re­sent the long-sought mis­sing link to un­der­stand­ing why the mass of black holes is larg­er in ga­lax­ies that con­tain more stars, the re­search­ers added. “A nat­u­ral ex­ten­sion of our work is to search for si­m­i­lar ob­jects in oth­er sys­tems,” said Jahnke.

The findings are being pre­sented in new pa­pers pub­lished in the jour­nals Astro­nomy & Astro­physics and Astro­phys­ical Jour­nal.

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Which come first: the supermassive black holes that frantically devour matter, or the huge galaxies where they reside? A new scenario has emerged to answer this contentious “‘chicken and egg’ question,” said David Elbaz of the Center for Nuclear Studies of Saclay in Gif-sur-Yvette, France, one of the researchers who developed the model. The issue is “one of most debated subjects in astrophysics,” he added. “Our study suggests that supermassive black holes can trigger the formation of stars, thus ‘building’ their own host galaxies. This link could also explain why galaxies hosting larger black holes have more stars.” A black hole is a super-compact astrophysical object whose gravity is so powerful that it drags into itself any object passing too nearby, including light. That accounts for the “black” moniker, but actually many black holes are thought to be easily visible due of violent activity going on around them. Elbaz and colleagues studied a peculiar object some five billion light years away, believed to be a black hole without a home galaxy and dubbed quasar HE0450-2958. A light year is the distance light travels in a year. It had been speculated that the quasar’s host galaxy was hidden behind large amounts of dust. The astronomers thus used an instrument on ESO’s Very Large Telescope designed to detect so-called mid-infrared light, which would make dust clouds brightly visible. Yet no dust appeared, indicating there was no home galaxy, said Knud Jahnke of the Max Planck Institute for Astronomy in Heidelberg, Germany, who led the observations. “Instead we discovered that an apparently unrelated galaxy in the quasar’s immediate neighbourhood is producing stars at a frantic rate.” It’s forming the equivalent of about 350 Suns per year, he added. Earlier observations had shown that the companion galaxy is, in fact, under fire: the quasar is spewing a jet of energetic particles towards its companion, accompanied by a stream of fast-moving gas. The injection indicates that the quasar itself might be inducing the formation of stars and thereby creating its own host galaxy, according to Elbaz and colleagues. In such a scenario, galaxies would have evolved from clouds of gas hit by the energetic jets emerging from quasars. “The two objects are bound to merge in the future: the quasar is moving at a speed of only a few tens of thousands of kilometers [or miles] per hour with respect to the companion galaxy and their separation is only about 22 000 light-years,” said Elbaz. “Although the quasar is still ‘naked’, it will eventually be ‘dressed’ when it merges with its star-rich companion. It will then finally reside inside a host galaxy like all other quasars.” The findings may also represent the long-sought missing link to understanding why the mass of black holes is larger in galaxies that contain more stars, the researchers added. “A natural extension of our work is to search for similar objects in other systems,” said Jahnke.