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April 01, 2015

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Galaxies may have formed in groups as “fireworks”

April 1, 2015
Courtesy of the University of Arizona
and World Science staff

Vast clus­ters of ga­lax­ies may have formed bil­lions of years ago amid “fire­works,” form­ing new stars at a break­neck pa­ce com­pared to the more staid ga­lax­ies of to­day, sci­en­tists say.

Cos­mol­o­gists in a new study identified what they said could be ba­by gal­axy clus­ters, by com­bin­ing ob­serva­t­ions of the dis­tant uni­verse made with the Eu­ro­pe­an Spa­ce Agen­cy’s Her­schel and Planck spa­ce ob­servatories.

This Hubble Space Telescope image focuses on two galaxies named "10" and "15." Neither looks as big and beau­tiful as the Milky Way but each is still a galaxy. Each appears twice on the sky. In fact, there only is one real galaxy 10 and one real galaxy 15, but a natural telescope in space has doubled their images. (Courtesy Brenda Frye)


Ga­lax­ies such as our Milky Way, with its 100 bil­lion stars, are sel­dom found alone. Many are in dense clus­ters of tens or even hun­dreds of ga­lax­ies. These clus­ters haven’t al­ways ex­isted, and how they came to be is con­sid­ered a key ques­tion in ex­plain­ing their ev­o­lu­tion.

In the re­search, as­tro­no­mers found ob­jects seen at a time when the universe was an es­ti­mat­ed 3 bil­lion years old, only about a fifth of its cur­rent age. Be­cause light takes time to trav­el, look­ing far enough away re­veals ob­jects as they looked when they gave off the light that we de­tect from them.

“In con­trast to pre­vi­ous ob­serva­t­ions, for which the odd one or two ba­by clus­ters was found which one would put in a zoo, we now have found a real sam­ple of 200 ba­by clus­ters,” said Bren­da L. Frye of the Uni­vers­ity of Ari­zon­a’s Stew­ard Ob­serv­a­to­ry, who was in­volved in the re­search.

Planck was de­signed mainly to pro­vide the most pre­cise map of radia­t­ion left over from the Big Bang. But it al­so scanned the sky at wave­lengths, or “col­ors,” not di­rectly tied to that proj­ect. It was in some of this da­ta—the “short-wavelength” da­ta—that sci­en­tists were able to iden­ti­fy 234 bright sources of light with char­ac­ter­is­tics sug­gest­ing they were in the dis­tant, early uni­verse.

Da­ta from an­oth­er spa­ce ob­servatory, Her­schel, re­vealed that the vast ma­jor­ity of these sources are con­sist­ent with dense con­centra­t­ions of ga­lax­ies in the early uni­verse, vig­or­ously form­ing new stars, the in­vest­i­ga­tors said.

Each of these young ga­lax­ies was seen to be con­vert­ing gas and dust in­to stars at a rate equivalent to a few hun­dred to 1,500 suns per year, ac­cord­ing to as­tro­no­mers. By com­par­i­son, our own Milky Way gal­axy to­day is pro­duc­ing an average of just one.

“It was not known wheth­er young ga­lax­ies form stars grad­u­al­ly, like mar­a­thon run­ners pac­ing them­selves, or in bursts,” Frye said. “It turns out these young ga­lax­ies were not form­ing slow­ly, but in a dra­mat­ic way. Light­ing up with star forma­t­ion, they ap­pear like fire­works go­ing off in the sky. It’s like sprint­ing the first mile of a 26-mile mar­a­thon, and then walk­ing the rest of the way.”

Be­cause of their ex­treme dis­tance, as­tro­no­mers would not be able to see these “fire­works” in sin­gle ga­lax­ies, but be­cause most oc­cur in clus­ters, they look bright. How­ev­er, here and the­re, they de­tected sin­gle ga­lax­ies that ap­peared much brighter than they should.

“In a small num­ber of cases, we are find­ing only one ob­ject in­stead of a clus­ter, which we should­n’t be able to see, so there is some oth­er mys­tery the­re,” Frye said.

Frye was called to join the re­search team to study those rare cases of “mys­tery fire­works ga­lax­ies” in part be­cause of her expe­rtise in “gravita­t­ional lens­ing,” an ef­fect in which gra­vity from heavy ob­jects bends the light from oth­er ob­jects.

“The an­swer is that these ob­jects are bright­ened up by what you could call a nat­u­ral tel­e­scope in spa­ce,” she ex­plained. “In a few cases, we can see in­di­vid­ual far-a­way ga­lax­ies light­ing up with star forma­t­ion be­cause they are sit­u­at­ed along just at the right line of sight where their light passes through a mas­sive gal­axy clus­ters close to Earth.”

As pre­dicted by Ein­stein’s The­o­ry of Gen­er­al Rel­a­ti­vity, the fore­ground clus­ter bends the light from the back­ground gal­axy due to its gra­vity, leading to a brighter im­age of the far-a­way gal­axy.

“Why we find these en­ig­mat­ic ex­am­ples of sin­gle ones when they should be in clus­ters, is what we’re stu­dying he­re” at the uni­vers­ity, Frye said. “These might be faint­er ex­am­ples in gen­er­al, or they, too, may have friends, which haven’t turned on their fire­works just yet.”

While the as­tro­no­mers haven’t proved the ages and lu­mi­nos­i­ties of many of the new­found gal­axy con­centra­t­ions, they say these are the best can­di­dates yet found for “proto-clus­ters” — pre­cur­sors of mod­ern large, ma­ture gal­axy clus­ters.

“Hints of these kinds of ob­jects had been found ear­li­er in da­ta from Her­schel and oth­er tel­e­scopes, but the all-sky ca­pa­bil­ity of Planck re­vealed many more can­di­dates for us to stu­dy,” said Hervé Dole of the In­sti­tut d’As­tro phy­sique Spa­tiale in Or­say, France. Dole is the lead sci­ent­ist of the anal­y­sis, pub­lished in the jour­nal As­tron­o­my & As­t­ro­phys­ics. “We still have a lot to learn about this new popula­t­ion, re­quir­ing fur­ther fol­low-up stud­ies with oth­er ob­servatories. But we be­lieve that they are a mis­sing piece of cos­mo­lo­g­i­cal struc­ture forma­t­ion.”


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Vast clusters of galaxies may have formed billions of years ago amid “fireworks,” forming new stars at a breakneck pace compared to the more staid galaxies of today, scientists say. Cosmologists in a new study discovered what could be “baby” galaxy clusters by combining observations of the distant universe made with the European Space Agency’s Herschel and Planck space observatories. Galaxies such as our Milky Way, with its 100 billion stars, are seldom found alone. Many are in dense clusters of tens or even hundreds of galaxies. These clusters haven’t always existed, and how they came to be is considered a key question in explaining their evolution. In the research, astronomers found objects seen at a time when it was an estimated 3 billion years old, only about a fifth of its current age. Because light takes time to travel, looking far enough away reveals objects as they looked when they gave off the light that we detect from them. “In contrast to previous observations, for which the odd one or two baby clusters was found which one would put in a zoo, we now have found a real sample of 200 baby clusters,” said Brenda L. Frye of the University of Arizona’s Steward Observatory, who was involved in the research. Planck was designed mainly to provide the most precise map of radiation left over from the Big Bang. But it also scanned the sky at wavelengths, or “colors,” not directly tied to that project. It was in some of this data—the “short-wavelength” data—that scientists were able to identify 234 bright sources of light with characteristics suggesting they were in the distant, early universe. Data from another space observatory, Herschel, revealed that the vast majority of these sources are consistent with dense concentrations of galaxies in the early universe, vigorously forming new stars. Each of these young galaxies is seen to be converting gas and dust into stars at a rate of a few hundred to 1,500 times the mass of our sun per year, according to astronomers. By comparison, our own Milky Way galaxy today is producing stars at an average rate of just one solar mass per year. “It was not known whether young galaxies form stars gradually, like marathon runners pacing themselves, or in bursts,” Frye said. “It turns out these young galaxies were not forming slowly, but in a dramatic way. Lighting up with star formation, they appear like fireworks going off in the sky. It’s like sprinting the first mile of a 26-mile marathon, and then walking the rest of the way.” Because of their extreme distance, astronomers would not be able to see these “fireworks” in single galaxies, but because most occur in clusters, they appear bright. However, here and there, they detected single galaxies that appeared much brighter than they should. “In a small number of cases, we are finding only one object instead of a cluster, which we shouldn’t be able to see, so there is some other mystery there,” Frye said. Frye was called to join the research team to study those rare cases of “mystery fireworks galaxies” in part because of her expertise in “gravitational lensing,” an effect in which gravity from heavy objects bends the light from other objects. “The answer is that these objects are brightened up by what you could call a natural telescope in space,” she explained. “In a few cases, we can see individual far-away galaxies lighting up with star formation because they are situated along just at the right line of sight where their light passes through a massive galaxy clusters close to Earth.” As predicted by Einstein’s Theory of General Relativity, the foreground cluster bends the light from the background galaxy due to its gravity, resulting in an image of the far-away galaxy that is boosted in brightness. “Why we find these enigmatic examples of single ones when they should be in clusters, is what we’re studying here” at the university, Frye said. “These might be fainter examples in general, or they, too, may have friends, which haven’t turned on their fireworks just yet.” While the astronomers haven’t proved the ages and luminosities of many of the newfound galaxy concentrations, they say these are the best candidates yet found for “proto-clusters” — precursors of modern large, mature galaxy clusters. “Hints of these kinds of objects had been found earlier in data from Herschel and other telescopes, but the all-sky capability of Planck revealed many more candidates for us to study,” said Hervé Dole of the Institut d’Astrophysique Spatiale in Orsay, France, lead scientist of the analysis published in the journal Astronomy & Astrophysics. “We still have a lot to learn about this new population, requiring further follow-up studies with other observatories. But we believe that they are a missing piece of cosmological structure formation.”