"Long before it's in the papers"
October 24, 2012

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Stray stars may haunt vastness between galaxies

Oct. 24, 2012
Courtesy of NASA Jet Propulsion Laboratory
and World Science staff

Stray stars float­ing far be­yond the edges of ga­lax­ies, a new study sug­gests, may be the cause for a mys­te­ri­ous glow of in­fra­red light seen across the en­tire sky.

In­fra­red is a form of light that is too low-energy to be seen by the eye. But re­search with NASA’s Spitzer Space Tel­e­scope helps shed light on an “in­fra­red back­ground glow” it de­tects across the sky, as­tro­no­mers said.

The im­age on the left shows a por­tion of our sky, called the Boötes field, in in­fra­red light, while the im­age on the right shows a mys­te­ri­ous, back­ground in­fra­red glow cap­tured by NA­SA's Spitzer Space Tel­e­scope in the same re­gion of sky. Us­ing Spitzer, re­search­ers were able to de­tect this back­ground glow, which spreads across the whole sky, by mask­ing out light from ga­lax­ies and oth­er known sources of light (the masks are the gray, blotchy marks). The sci­en­tists find that this light is com­ing from stray stars that were torn away from ga­lax­ies. When ga­lax­ies tan­gle and merge -- a nat­u­ral stage of gal­axy growth -- stars of­ten get kicked out in the pro­cess. The stars are too faint to be seen in­di­vid­u­ally, but Spitzer may be see­ing their col­lec­tive glow. (cred­it: NA­SA/JPL-Caltech/UC Ir­vine)

The or­phan stars are thought to have once be­longed to ga­lax­ies be­fore vi­o­lent gal­axy merg­ers stripped them away in­to the rel­a­tively emp­ty space out­side of their form­er homes.

“The in­fra­red back­ground glow in our sky has been a huge mys­tery,” said As­an­tha Cooray of the Uni­vers­ity of Cal­i­for­nia at Ir­vine, lead au­thor of the new re­search pub­lished in the jour­nal Na­ture. “We have new ev­i­dence this light is from the stars that lin­ger be­tween ga­lax­ies. In­di­vid­u­ally, the stars are too faint to be seen, but we think we are see­ing their col­lec­tive glow.”

The find­ings dis­a­gree with an­oth­er the­o­ry ex­plain­ing the same in­fra­red light ob­served by Spitzer. A group led by Al­ex­an­der “Sasha” Kash­lin­sky of NASA’s God­dard Space Flight Cen­ter in Green­belt, Md., pro­posed in June that this light, which ap­pears in Spitzer im­ages as a blotchy pat­tern, is com­ing from the first stars and ga­lax­ies.

Cooray and col­leagues looked at da­ta from a larg­er por­tion of the sky, called the Bo­o­tes field, cov­er­ing an arc of sky of width equiv­a­lent to 50 Earth moons. These ob­serva­t­ions were not as sen­si­tive as those from the Kash­lin­sky group’s stud­ies. But the larg­er scale al­lowed re­search­ers to an­a­lyze bet­ter the pat­tern of the back­ground in­fra­red light, mem­bers of Cooray’s team said.

“We looked at the Bo­o­tes field with Spitzer for 250 hours,” said co-au­thor Dan­iel Stern of NASA’s Je­t Pro­pul­sion Lab­o­r­a­to­ry in Pas­a­de­na, Ca­lif.

When ga­lax­ies grow, they merge and be­come grav­i­ta­tion­ally en­tan­gled in a vi­o­lent pro­cess that re­sults in streams of stars be­ing ripped away from the ga­lax­ies. Such streams, called tid­al tails, can be seen in this artist's con­cept. Sci­en­tists say that Spitzer is pick­ing up the col­lec­tive glow of stars such as these, which lin­ger in the spaces be­tween ga­lax­ies. (Cred­it: NA­SA/JPL-Caltech/UC Ir­vine)

The team con­clud­ed the light pat­tern of the in­fra­red glow is not con­sist­ent with the­o­ries and com­put­er sim­ula­t­ions of the first stars and ga­lax­ies. 

The re­search­ers say the glow is too bright to be from the first ga­lax­ies, which are thought to have been small­er and less nu­mer­ous than mod­ern ones. In­stead, the sci­en­tists pro­pose a new idea to ex­plain the blotchy light.

The­o­ries pre­dict a dif­fuse smat­ter­ing of stars be­yond the ha­los, or out­er reaches, of ga­lax­ies, and in the spaces be­tween clus­ters of ga­lax­ies. The pres­ence of these stars can be at­trib­ut­ed to two pro­cesses. Early in the histo­ry of our uni­verse as ga­lax­ies grew in size, they col­lid­ed with oth­er ga­lax­ies and merged. As the col­lid­ing ga­lax­ies be­came tan­gled through the at­trac­tion of each oth­er’s gra­vity, strips of stars were shred­ded and tossed in­to space. Ga­lax­ies al­so grow by swal­low­ing smaller dwarf ga­lax­ies, a messy pro­cess that al­so re­sults in stray stars.

“A light bulb went off when read­ing some re­search pa­pers pre­dicting the ex­ist­ence of dif­fuse stars,” Cooray said. “They could ex­plain what we are see­ing with Spitzer.”

More re­search is needed to con­firm this sprin­kling of stars makes up a sig­nif­i­cant frac­tion of the back­ground in­fra­red light, he added. For in­stance, it would be nec­es­sary to find a si­m­i­lar pat­tern in fol­low-up ob­serva­t­ions in vis­i­ble light. NASA’s upcom­ing James Webb Space Tel­e­scope might help set­tle the mat­ter, the as­tro­no­mers said.

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Stray stars floating far beyond the edges of galaxies, a new study suggests, may be the cause for a mysterious glow of infrared light seen across the entire sky. Infrared is a form of light that is too low-energy to be seen by the eye. But research with NASA’s Spitzer Space Telescope helps shed light on an “infrared background glow” detected across the sky, astronomers said. The orphan stars are thought to have once belonged to the galaxies before violent galaxy mergers stripped them away into the relatively empty space outside of their former homes. “The infrared background glow in our sky has been a huge mystery,” said Asantha Cooray of the University of California at Irvine, lead author of the new research published in the journal Nature. “We have new evidence this light is from the stars that linger between galaxies. Individually, the stars are too faint to be seen, but we think we are seeing their collective glow.” The findings disagree with another theory explaining the same infrared light observed by Spitzer. A group led by Alexander “Sasha” Kashlinsky of NASA’s Goddard Space Flight Center in Greenbelt, Md., proposed in June this light, which appears in Spitzer images as a blotchy pattern, is coming from the first stars and galaxies. Cooray and colleagues looked at data from a larger portion of the sky, called the Bootes field, covering an with equivalent to 50 full Earth moons. These observations were not as sensitive as those from the Kashlinsky group’s studies. But the larger scale allowed researchers to analyze better the pattern of the background infrared light, members of Cooray’s team said. “We looked at the Bootes field with Spitzer for 250 hours,” said co-author Daniel Stern of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Studying the faint infrared background was one of the core goals of our survey, and we carefully designed the observations in order to directly address the important, challenging question of what causes the background glow.” The team concluded the light pattern of the infrared glow is not consistent with theories and computer simulations of the first stars and galaxies. Researchers say the glow is too bright to be from the first galaxies, which are thought not to have been as large or as numerous as the galaxies we see around us today. Instead, the scientists propose a new theory to explain the blotchy light, based on theories of “intracluster” or “intrahalo” starlight. Theories predict a diffuse smattering of stars beyond the halos, or outer reaches, of galaxies, and in the spaces between clusters of galaxies. The presence of these stars can be attributed to two processes. Early in the history of our universe as galaxies grew in size, they collided with other galaxies and merged. As the colliding galaxies became tangled through the attraction of each other’s gravity, strips of stars were shredded and tossed into space. Galaxies also grow by swallowing smaller dwarf galaxies, a messy process that also results in stray stars. “A light bulb went off when reading some research papers predicting the existence of diffuse stars,” Cooray said. “They could explain what we are seeing with Spitzer.” More research is needed to confirm this sprinkling of stars makes up a significant fraction of the background infrared light, he added. For instance, it would be necessary to find a similar pattern in follow-up observations in visible light. NASA’s upcoming James Webb Space Telescope might help settle the matter, the astronomers said.