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Biggest black holes formed early, study finds

Aug. 26, 2010
Courtesy of the University of Zurich
and World Science staff

The first “su­per­mas­sive” black holes were formed shortly af­ter the birth of the uni­verse. That’s the con­clu­sion of phys­i­cists led by Lu­cio May­er of the Uni­vers­ity of Zu­rich, Switz­er­land, re­port­ing their find­ings in the cur­rent is­sue of the re­search jour­nal Na­ture.

Black holes are ce­les­tial ob­jects that are so com­pact or dense that their gra­vity be­comes over­pow­ering, trap­ping even light rays that pass too close. Par­tic­u­larly large, or su­per­mas­sive, black holes lie at the cen­ters of ga­lax­ies.

Gas near the cen­ter of a ga­la­xy be­gins to con­dense to form a black hole, in a si­mu­la­tion by a re­search group led by Lu­cio May­er of the Uni­vers­ity of Zu­rich. (Cour­tesy U. of Zu­rich)
These most pow­er­ful of black holes were formed through the col­li­sion of ga­lax­ies 13 bil­lion years ago, ac­cord­ing to May­er’s group, adding that the find­ings are im­por­tant to help ex­plain the or­i­gin of gravita­t­ion and cos­mo­lo­g­i­cal struc­tures. 

In com­put­er sim­ula­t­ions, they mod­elled the forma­t­ion of ga­lax­ies and black holes dur­ing the first bil­lion years af­ter the “Big Bang,” a sort of ex­plo­sion be­lieved to have formed our uni­verse. The re­sults sug­gested the first su­per­mas­sive black holes formed when those early ga­lax­ies col­lid­ed with each oth­er and merged.

For more than two dec­ades, May­er’s group not­ed, sci­ence has as­sumed that ga­lax­ies grow hi­er­ar­chic­ally: small mass­es are pulled to­geth­er by gravita­t­ion, and from them, larg­er struc­tures form step by step. May­er’s group claims to have turned that as­sump­tion up­side down. “Large struc­tures such as ga­lax­ies and mas­sive black holes formed quickly in the his­to­ry of the uni­verse,” he ex­plained. 

“At first glance, this seems to con­tra­dict the stand­ard the­o­ry with cold dark ma­te­ri­al which de­scribes the hi­er­ar­chical build­ing of ga­lax­ies,” he added. In this mod­el, so-called dark mat­ter, a sub­stance de­tect­a­ble only through its gravita­t­ional ef­fects and per­va­sive uni­verse-wide, plays a lead­ing role in the build­up of cos­mic struc­tures.

But actual­ly, May­er said, “nor­mal mat­ter, from which the vis­i­ble parts of the ga­lax­ies and su­per­mas­sive black holes are formed, col­lapses more strongly than dark ma­te­ri­al, form­ing quickly the most mas­sive ga­lax­ies in the dens­est re­gions of the Uni­verse.” This leads to the rela­tive­ly quick form­a­t­ion of ga­lax­ies and black holes, as ma­te­ri­al com­pacts it­self in­to smaller and smaller re­gions be­cause of gra­vity, he added.

Thus, May­er went on, huge ga­lax­ies and su­per­mas­sive black holes form early. Small ga­lax­ies, on the oth­er hand, such as our own, the Milky Way and its rel­a­tively small black hole in the cen­ter, would have formed more slow­ly. Our gal­ax­y’s black hole is es­ti­mat­ed to weigh only a mil­lion suns in­stead of the bil­lion suns of the black holes sim­ulated by May­er and col­leagues.

The ga­lax­ies in the sim­ula­t­ion would count among the big­gest known to­day, May­er said, about 100 times larg­er than the Milky Way. A gal­axy that could have aris­en from a col­li­sion in that way is our neigh­bour­ing gal­axy M87 in the Vir­go clus­ter, lo­cat­ed at 54 mil­lion light years from us.

The new find­ings have con­se­quenc­es for cos­mol­o­gy, May­er and col­leagues said. A wide­spread as­sump­tion that the char­ac­ter­is­tics of ga­lax­ies and the mass of the black hole are re­lat­ed to each oth­er be­cause they grow in par­al­lel might have to be re­vised: in May­er’s mod­el, the black hole grows much more quickly than the gal­axy, so the black hole’s char­ac­ter­is­tics may in­flu­ence the gal­axy growth more than than the oth­er way around.

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The first “supermassive” black holes were formed shortly after the birth of the universe. That’s the conclusion of physicists led by Lucio Mayer of the University of Zurich, Switzerland, reporting their findings in the current issue of the research journal Nature. Black holes are celestial objects that are so compact or dense that their gravity becomes overpowering, trapping even light rays that pass too close. Particularly large, or supermassive, black holes lie at the centers of galaxies. These most powerful of black holes were formed through the collision of galaxies 13 billion years ago, according to Mayer’s group, adding that the findings are important to help explain the origin of gravitation and cosmological structures. In computer simulations, they modelled the formation of galaxies and black holes during the first billion years after the “Big Bang,” a sort of explosion believed to have formed our universe. The results suggested the first supermassive black holes formed when those early galaxies collided with each other and merged. For more than two decades, Mayer’s group noted, science has assumed that galaxies grow hierarchically: small masses are pulled together by gravitation, and from them, larger structures form step by step. Mayer’s group claims to have turned that assumption upside down. “Large structures such as galaxies and massive black holes formed quickly in the history of the universe,” he explained. “At first glance, this seems to contradict the standard theory with cold dark material which describes the hierarchical building of galaxies,” he added. In this model, so-called dark matter, a substance detectable only through its gravitational effect, plays a leading role in the buildup of cosmic structures. But actually, Mayer said, “normal matter from which the visible parts of the galaxies and supermassive black holes are formed collapse more strongly than dark material, forming quickly the most massive galaxies in the densest regions of the Universe, where gravity begins to form structures earlier than elsewhere.” This leads to the formation of galaxies and black holes, as material compacts itself into smaller and smaller regions because of gravity, he added. Thus, Mayer went on, huge galaxies and supermassive black holes form quickly. Small galaxies, on the other hand, such as our own, the Milky Way and its relatively small black hole in the center, would have formed more slowly. Our galaxy’s black hole is estimated to weigh only a million suns instead of the billion suns of the black holes simulated by Mayer and colleagues. The galaxies in the simulation would count among the biggest known today in reality, Mayer said, about 100 times larger than the Milky Way. A galaxy that could have arisen from a collision in that way is our neighbouring galaxy M87 in the Virgo cluster, located at 54 million light years from us. The new findings have consequences for cosmology, Mayer and colleagues said. A widespread assumption that the characteristics of galaxies and the mass of the black hole are related to each other because they grow in parallel might have to be revised: in Mayer’s model, the black hole grows much more quickly than the galaxy, so the black hole’s characteristics may influence the galaxy growth more than than the other way around.