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January 18, 2011

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Astronomers puzzled by galaxies that formed “too early”

Nov. 25, 2010
Courtesy of Tufts University
and World Science staff

Some of the big­gest ga­lax­ies may have formed bil­lions of years ear­li­er than pre­vail­ing sci­en­tif­ic mod­els pre­dict, sug­gest­ing some­thing is wrong with the mod­els, as­tro­no­mers are re­port­ing.

“We have found a rel­a­tively large num­ber of very mas­sive, highly lu­mi­nous ga­lax­ies that ex­isted al­most 12 bil­lion years ago when the uni­verse was still very young, about 1.5 bil­lion years old. These re­sults ap­pear to dis­a­gree with the lat­est pre­dictions from mod­els of gal­axy forma­t­ion and evo­lu­tion,” said as­t­ro­phys­i­cist Da­nil­o March­esini of Tufts Uni­vers­ity in Mas­sa­chu­setts, lead au­thor of a re­port on the find­ings pub­lished on­line Nov. 24 in the As­t­ro­phys­i­cal Jour­nal.

The ga­lax­ies, es­ti­mat­ed to be five to ten times more mas­sive than our own, were among a sam­ple of ga­lax­ies stud­ied by his March­esini’s group and said to be 1.5 bil­lion to 2 bil­lion years old. The ages were gauged based on the ga­lax­ies’ dis­tance and the time their light would take to reach us.

These es­ti­mates might be some­what off, but it’s not clear whe­ther any such error could be large en­ough to ex­plain the find­ings, the re­search­ers said. Ei­ther way, they added, the discovery of such mas­sive, old ga­lax­ies would it­self be no­ta­ble, since such a gal­axy popula­t­ion has nev­er been clearly seen. 

The scient­ists said they got a full­er view of gal­axies at this early stage than was pre­vi­ously avail­a­ble by com­bin­ing ex­ist­ing da­ta with deep im­ages ob­tained through a new sys­tem of cus­tomized light fil­ters. The galaxies glow in in­fra­red light, which is in­visible to the un­aided eye, in an effect thought to be caused by their vast dis­tance com­bined with an on­going ex­pan­sion of the uni­verse.

More than four in five of these mas­sive ga­lax­ies were al­so very bright in in­fra­red light, show­ing that they were either un­der­go­ing vi­o­lent ac­ti­vity, rap­idly grow­ing or both, the in­vest­i­ga­tors added. The vi­o­lent ac­ti­vity would probably be due to black holes at the cen­ters of the ga­lax­ies eat­ing vast amounts of ma­te­ri­al, which is drawn to their in­tense gra­vity.

“It is clear that our un­der­stand­ing of how mas­sive ga­lax­ies form is still far from sat­is­fac­to­ry,” said March­esini. He did­n’t ven­ture an ex­plana­t­ion for the find­ings, but added that “the ex­ist­ence of these ga­lax­ies so early in the his­to­ry of the uni­verse, as well as their prop­er­ties, can pro­vide very im­por­tant clues on how ga­lax­ies formed and evolved shortly af­ter the Big Bang,” the mo­ment when phys­i­cists be­lieve the uni­verse was born.

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Some of the biggest galaxies may have formed billions of years earlier than prevailing scientific models predict, suggesting something is wrong with the models, astronomers are reporting. “We have found a relatively large number of very massive, highly luminous galaxies that existed almost 12 billion years ago when the universe was still very young, about 1.5 billion years old. These results appear to disagree with the latest predictions from models of galaxy formation and evolution,” said astrophysicist Danilo Marchesini of Tufts University in Massachusetts, lead author of a report on the findings published online Nov. 24 in the Astrophysical Journal. The galaxies, estimated to be five to ten times more massive than our own Milky Way, were among a sample of galaxies studied by his Marchesini’s group and said to be 1.5 billion to 2 billion years old. The ages were gauged based on the galaxies’ distance and the time their light would take to reach us. The astronomers said that the age and distance estimates might be somewhat off, but it’s not clear whether correcting any such error would make enough difference to reconcile the findings with prevailing models. Either way, they added, the discovery of the existence of such massive, old and apparently very dusty galaxies would itself be notable, since such a galaxy population has never been seen. By combining existing data with deep images obtained through a new system of customized light filters, the researchers said they got a fuller view of this early galaxy population than was previously available. More than four in five of these massive galaxies were also very bright, showing that they are undergoing violent activity, rapidly growing or both, the astronomers added. The violent activity would probably be due to black holes at the centers of the galaxies eating vast amounts of material, which is drawn to their intense gravity. “Either way, it is clear that our understanding of how massive galaxies form is still far from satisfactory,” said Marchesini. He didn’t venture an explanation for the findings, but added that “the existence of these galaxies so early in the history of the universe, as well as their properties, can provide very important clues on how galaxies formed and evolved shortly after the Big Bang,” the moment when physicists believe the universe was born.