"Long before it's in the papers"
April 27, 2015


“Runaway galaxies” found

April 27, 2015
Courtesy of Harvard-Smithsonian 
Center for Astrophysics
and World Science staff

We know of about two doz­en run­a­way stars, and have even found one run­a­way star clus­ter es­cap­ing its gal­axy for­ev­er. 

Now, as­tro­no­mers have spot­ted what they call en­tire run­a­way ga­lax­ies. These is­lands of stars are leav­ing the gal­axy clus­ters where they orig­i­nally formed, hav­ing gained enough speed to over­come their gravita­t­ional clutches. The 11 new­found run­a­way ga­lax­ies are ex­pected to end up wan­der­ing the void of in­ter­ga­lac­tic space.

This schematic illustrates how a runaway galaxy forms in theory. In the first panel, an "intruder" spiral galaxy approaches the center of a galaxy cluster, where a compact elliptical galaxy (cE) already revolves around a massive central elliptical galaxy. In the second panel, a close encounter occurs and the compact elliptical receives a gravitational kick from the intruder. In the third panel, the compact elliptical escapes the galaxy cluster while the intruder is devoured by the giant elliptical galaxy in the cluster center.

They’re “fac­ing a lonely fu­ture,” quipped as­tron­o­mer Igor Chilin­gar­ian of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics in Cam­bridge, Mass., and of Mos­cow State Uni­vers­ity. Chilin­gar­ian is the lead au­thor of the stu­dy, which ap­pears in the re­search jour­nal Sci­ence.

An ob­ject is des­ig­nat­ed a run­a­way if it’s mov­ing faster than “escape ve­lo­city,” which means it will de­part a home base nev­er to re­turn. In the case of a run­a­way star, the re­quired speed is more than a mil­lion miles per hour (500 km/s). A run­a­way gal­axy has to race even faster, trav­el­ing at up to 6 mil­lion miles per hour (3,000 km/s).

Chilin­gar­ian and co-au­thor Ivan Zolo­tuk­hin in­i­tially set out to find new mem­bers of a class of ga­lax­ies called com­pact el­lip­ti­cals. These ti­ny blobs of stars are big­ger than star clus­ters but smaller than a typ­i­cal gal­axy, span­ning only a few hun­dred light-years. In com­par­i­son, the Milky Way is 100,000 light-years across. Com­pact el­lip­ti­cals al­so weigh 1,000 times less than a gal­axy like our Milky Way.

Be­fore the stu­dy, only about 30 com­pact el­lip­ti­cal ga­lax­ies were known, all in gal­axy clus­ters. 

To find new ones, Chilin­gar­ian and Zo­lo­tu­khin sorted through pub­lic ar­chives of da­ta from the Sloan Dig­it­al Sky Sur­vey—a proj­ect to map the uni­verse in­volv­ing many in­sti­tu­tions—and from a NASA sat­el­lite known as GALEX. 

They iden­ti­fied al­most 200 pre­vi­ously un­known com­pact el­lip­ti­cals, of which 11 were iso­lat­ed and far from any large gal­axy or gal­axy clus­ter.

“The first com­pact el­lip­ti­cals were all found in clus­ters be­cause that’s where peo­ple were look­ing. We broad­ened our search, and found the un­ex­pected,” said Zo­lo­tuk­hin, of the In­sti­tute for Re­search in As­t­ro­phys­ics and Plan­e­tol­o­gy in Tou­louse, France, and Mos­cow State Uni­vers­ity.

The iso­lat­ed com­pact ga­lax­ies were un­ex­pected be­cause the­o­rists thought they orig­i­nat­ed from larg­er ga­lax­ies that had lost most of their stars be­cause of in­ter­ac­tions with an even big­ger gal­axy, the sci­en­tists added. So, the com­pact ga­lax­ies should all be found near big ga­lax­ies. But not only were the new­found com­pact el­lip­ti­cals iso­lat­ed, they were al­so found to be mov­ing faster than their breth­ren in clus­ters.

“We asked our­selves, what else could ex­plain them? The an­swer was a clas­sic ‘three-body in­ter­ac­tion,’” said Chi­lin­gar­ian. What does that mean? A star can be kicked up to a tre­men­dous speed if, along with a part­ner star, it wan­ders close to a gi­ant black hole, a cen­ter of over­whelm­ing gra­vity. In the re­sult­ing gravita­t­ional in­ter­ac­tions, the hole will swal­low up one star and throw out the oth­er.

Sim­i­lar­ly, a com­pact el­lip­ti­cal could pair up with the big gal­axy that stripped it of its stars, Chilin­gar­ian said. Then a third gal­axy blun­ders in­to the dance and flings the com­pact el­lip­ti­cal away. As pu­nishment, the in­trud­er ends up stuck as part of the re­main­ing big gal­axy.

The dis­cov­ery rep­re­sents a suc­cess of the “Vir­tual Ob­ser­va­to­ry,” he added—a proj­ect to make da­ta from large as­tro­nom­i­cal sur­veys easily avail­a­ble to re­search­ers. So-called da­ta min­ing can re­sult in finds nev­er ex­pected when the orig­i­nal da­ta was col­lect­ed. “We rec­og­nized we could use the pow­er of the ar­chives to po­ten­tially un­earth some­thing in­ter­est­ing, and we did,” said Chilin­gar­ian.

* * *

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We know of about two dozen runaway stars, and have even found one runaway star cluster escaping its galaxy forever. Now, astronomers have spotted what they call entire runaway galaxies. These islands of stars are leaving the galaxy clusters where they originally formed, having gained enough speed to overcome their gravitational clutches. The 11 newfound runaway galaxies are expected to end up wandering the void of intergalactic space. They’re “facing a lonely future,” quipped astronomer Igor Chilingarian of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and of Moscow State University. Chilingarian is the lead author of the study, which appears in the research journal Science. An object is designated a runaway if it’s moving faster than “escape velocity,” which means it will depart a home base never to return. In the case of a runaway star, the required speed is more than a million miles per hour (500 km/s). A runaway galaxy has to race even faster, traveling at up to 6 million miles per hour (3,000 km/s). Chilingarian and co-author, Ivan Zolotukhin initially set out to find new members of a class of galaxies called compact ellipticals. These tiny blobs of stars are bigger than star clusters but smaller than a typical galaxy, spanning only a few hundred light-years. In comparison, the Milky Way is 100,000 light-years across. Compact ellipticals also weigh 1000 times less than a galaxy like our Milky Way. Before the study, only about 30 compact elliptical galaxies were known, all in galaxy clusters. To find new ones Chilingarian and Zolotukhin sorted through public archives of data from the Sloan Digital Sky Survey—a project to map the universe involving many institutions—and a NASA satellite known as GALEX. They identified almost 200 previously unknown compact ellipticals, of which 11 were completely isolated and far from any large galaxy or galaxy cluster. “The first compact ellipticals were all found in clusters because that’s where people were looking. We broadened our search, and found the unexpected,” said Zolotukhin, of the Institute for Research in Astrophysics and Planetology in Toulouse, France, and Moscow State University. The isolated compact galaxies were unexpected because theorists thought they originated from larger galaxies that had lost most of their stars because of interactions with an even bigger galaxy, the scientists added. So, the compact galaxies should all be found near big galaxies. But not only were the newfound compact ellipticals isolated, they were also found to be moving faster than their brethren in clusters. “We asked ourselves, what else could explain them? The answer was a classic ‘three-body interaction,’“ said Chilingarian. What does that mean? A star can be kicked up to a tremendous speed if two partner stars wander close to a giant black hole, a center of overwhelming gravity. In the resulting gravitational interactions, the hole will swallow up one star and throw away the other. Similarly, a compact elliptical could be paired with the big galaxy that stripped it of its stars, Chilingarian said. Then a third galaxy blunders into the dance and flings the compact elliptical away. As punishment, the intruder ends up stuck as part of the remaining big galaxy. This discovery represents a success of the “Virtual Observatory,” he added—a project to make data from large astronomical surveys easily available to researchers. So-called data mining can result in finds never expected when the original data was collected. “We recognized we could use the power of the archives to potentially unearth something interesting, and we did,” said Chilingarian.