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Black holes came first, astronomers conclude

Jan. 6, 2009
Courtesy National Radio Astronomy Observatory 
and World Science staff

As­tro­no­mers may have solved a cos­mic chicken-and-egg prob­lem—the ques­tion of which formed first, ga­lax­ies or the gi­ant black holes found at their cores.

“It looks like the black holes came first. The ev­i­dence is pil­ing up,” said Chris Car­illi of the Na­tional Ra­dio As­tron­o­my Ob­serv­a­to­ry in Char­lottes­ville, Va. 

An im­age of the gal­axy M81 tak­en with the Hub­ble Space Tele­scope. The cen­ters of most ga­lax­ies are dom­i­nat­ed by gi­ant black holes. De­spite their names, the black holes ap­pear as bright areas, be­cause of the fiery ac­tiv­i­ty gen­er­at­ed when they vi­o­lently dra­g near­by stars and gas to­ward them­selves. Many gal­ax­ies also have a "bulge" of stars and gas dom­in­at­ing the central re­gion. (Cre­dit: NA­SA, ESA, Hub­ble Her­i­tage Team (STScI/AURA)) 


Black holes are ob­jects so com­pact that their gravita­t­ional pull grows strong enough to suck in eve­ry­thing near­by, in­clud­ing light. Most ga­lax­ies have at their cen­ters enor­mous, or “su­per­mas­sive,” black holes, whose gra­vity holds the stars in a gal­axy to­geth­er.

Car­illi pre­sented the new find­ings, from re­search done by a group stu­dy­ing con­di­tions in the first bil­lion years of the Uni­verse’s his­to­ry, at the Amer­i­can As­tro­nom­i­cal So­ci­ety’s meet­ing in Long Beach, Cal­if. on Jan. 6.

Ear­li­er stud­ies of ga­lax­ies and their cen­tral black holes in the near­by Uni­verse re­vealed an in­tri­guing link­age be­tween the mass­es of the black holes and of the cen­tral “bulges” of stars and gas in the ga­lax­ies. The black hole con­sist­ently weighs about one-thou­s­andth of what the sur­round­ing ga­lac­tic bulge weighs.

The con­stan­cy of this rela­t­ion­ship “indi­cates that the black hole and the bulge af­fect each oth­ers’ growth” as a gal­axy forms, said Do­minik Riech­ers of the Cal­i­for­nia In­sti­tute of Tech­nol­o­gy, a mem­ber of the re­search team. “The big ques­tion has been wheth­er one grows be­fore the oth­er or if they grow to­geth­er.”

Sci­en­tists have been us­ing the Na­tional Sci­ence Founda­t­ion’s Very Large Ar­ray ra­dio tel­e­scope and the Plat­eau de Bu­re In­ter­fer­om­eter in France to peer far back in the 13.7 bil­lion-year his­to­ry of the Uni­verse, to the dawn of the first ga­lax­ies. An in­ter­fer­om­eter is a device that makes mea­sure­ments of light waves based on pat­terns that arise when the waves merge, or in­ter­fere.

“We fi­nally have been able to meas­ure black-hole and bulge mass­es in sev­er­al ga­lax­ies seen as they were in the first bil­lion years af­ter the Big Bang,” a sort of ex­plo­sion be­lieved to have giv­en birth to our uni­verse, said Fa­bi­an Wal­ter of the Max-Planck In­sti­tute for Ra­dio Astro­nomy in Bonn. Ob­serva­t­ions of the dis­tant uni­verse re­veal its state at some time in the past, since the light takes time to get he­re.

The ev­i­dence sug­gests that the thousand-to-one ra­tio seen near­by may “not hold in the early Uni­verse. The black holes in these young ga­lax­ies are much more mas­sive com­pared to the bulges than those seen in the near­by Uni­verse,” Wal­ter went on.

“The im­plica­t­ion is that the black holes started grow­ing first.”

The next chal­lenge is to fig­ure out how the black hole and the bulge af­fect each oth­ers’ growth. “We don’t know what mech­an­ism is at work he­re, and why, at some point in the pro­cess, the ‘s­tan­dard’ ra­tio be­tween the mass­es is es­tab­lished,” Riech­ers said.

“To un­der­stand how the Uni­verse got to be the way it is to­day, we must un­der­stand how the first stars and ga­lax­ies were formed when the Uni­verse was young. With the new ob­ser­va­to­ries we’ll have in the next few years, we’ll have the op­por­tun­ity to learn im­por­tant de­tails from the era when the Uni­verse was only a tod­dler com­pared to to­day’s adult,” Car­illi said.


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Astronomers may have solved a cosmic chicken-and-egg problem—the question of which formed first in the early Universe, galaxies or the giant black holes found at their cores. “It looks like the black holes came first. The evidence is piling up,” said Chris Carilli of the National Radio Astronomy Observatory in Charlottesville, Va. Black holes are objects so compact that their gravitational pull grows strong enough to suck in everything nearby, including light. Most galaxies have at their centers enormous, or “supermassive,” black holes, whose gravity holds the stars in a galaxy together. Carilli presented the new findings, from research done by a group studying conditions in the first billion years of the Universe’s history, at the American Astronomical Society’s meeting in Long Beach, California on Jan. 6. Earlier studies of galaxies and their central black holes in the nearby Universe revealed an intriguing linkage between the masses of the black holes and of the central “bulges” of stars and gas in the galaxies. The black hole consistently weighs about one-thousandth of what the surrounding galactic bulge weighs. The constancy of this relationship “indicates that the black hole and the bulge affect each others’ growth” as a galaxy forms, said Dominik Riechers of the California Institute of Technology, a member of the research team. “The big question has been whether one grows before the other or if they grow together.” Scientists have been using the National Science Foundation’s Very Large Array radio telescope and the Plateau de Bure Interferometer in France to peer far back in the 13.7 billion-year history of the Universe, to the dawn of the first galaxies. An interferometer is an instrument that makes measurements of light waves based on patterns that arise when the waves merge, or interfere. “We finally have been able to measure black-hole and bulge masses in several galaxies seen as they were in the first billion years after the Big Bang,” a sort of explosion believed to have given birth to our universe, said Fabian Walter of the Max-Planck Institute for Radioastronomy in Bonn. Observations of the distant universe reveal its state at some time in the past, since the light takes time to get here. The evidence suggests that the thousand-to-one ratio seen nearby may “not hold in the early Universe. The black holes in these young galaxies are much more massive compared to the bulges than those seen in the nearby Universe,” Walter went on. “The implication is that the black holes started growing first.” The next challenge is to figure out how the black hole and the bulge affect each others’ growth. “We don’t know what mechanism is at work here, and why, at some point in the process, the ‘standard’ ratio between the masses is established,” Riechers said. “To understand how the Universe got to be the way it is today, we must understand how the first stars and galaxies were formed when the Universe was young. With the new observatories we’ll have in the next few years, we’ll have the opportunity to learn important details from the era when the Universe was only a toddler compared to today’s adult,” Carilli said.