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“Ghost galaxies” seen as cosmic relics

July 10, 2012
Courtesy of the Space 
Telescope Science Institute
and World Science staff

The ex­treme faint­ness of some ga­lax­ies that are al­most to­tally lack­ing stars may help ex­plain why thou­sands of small ga­lax­ies seem to be mis­sing, as­tro­no­mers say.

Sci­en­tists have puz­zled over why some pu­ny, ex­tremely faint dwarf ga­lax­ies spot­ted in our Milky Way ga­laxy’s back yard con­tain so few stars. The ghost-like ga­lax­ies are thought to be some of the ti­ni­est, old­est, and most pris­tine ga­lax­ies, and have been disco­vered over the past dec­ade by as­tro­no­mers us­ing au­to­mat­ed com­put­er tech­niques to search through im­ages com­piled by a proj­ect known asof the Sloan Dig­it­al Sky Sur­vey. 

As­tro­no­mers used the Hub­ble Space Tel­e­scope to un­mask the dim, star-starved dwarf gal­axy Le­o IV. These Hub­ble im­ages dem­on­strate why as­tro­no­mers had a tough time spot­ting this small-fry gal­axy. The im­age at left shows part of the gal­axy, out­lined by the white rec­tan­gu­lar box. The box meas­ures 83 light-years wide by 163 light-years long. The few stars in Le­o IV are lost amid neigh­bor­ing stars and dis­tant ga­lax­ies.A close-up view of the back­ground ga­lax­ies with­in the box is shown in the mid­dle im­age. The im­age at right shows on­ly the stars in Le­o IV. The gal­axy, which con­tains sev­er­al thou­sand stars, is com­posed of Sun-like stars, faint­er, red dwarf stars, and some red gi­ant stars brighter than the Sun. As­tro­no­mers dis­cov­ered Le­o IV in Sloan Dig­it­al Sky Sur­vey im­ages by spot­ting a re­gion where a clump of stars was hud­dled clos­er to­geth­er than stars in ar­eas around it. Re­sid­ing 500,000 light-years from Earth, Le­o IV is one of more than a doz­en ultra-faint dwarf ga­lax­ies found lurk­ing around our Milky Way gal­axy. These ga­lax­ies are dom­i­nat­ed by dark mat­ter, an in­vis­i­ble sub­stance that makes up the bulk of the uni­verse's mass. As­tro­no­mers used Hub­ble to meas­ure the ages of the stars in Le­o IV and two oth­er ultra-faint dwarf ga­lax­ies. The meas­urements re­vealed that the stars in all three ga­lax­ies are more than 13 bil­lion years old, al­most as old as the 13.7-bil­lion-year-old uni­verse. Be­cause the stars in these ga­lax­ies are so an­cient and share the same age, as­tro­no­mers sug­gest that a glob­al event, such as reion­iza­tion, shut down star for­ma­tion in them. Reion­iza­tion is a tran­si­tion­al phase in the ear­ly uni­verse when the first stars burned off a fog of cold hy­dro­gen. The Hub­ble im­age is a com­pos­ite of ex­po­sures tak­en in Jan­u­ary 2012 by the Ad­vanced Cam­era for Sur­veys. (Cred­it: NA­SA, ESA, and T. Brown (STScI))

Now, views of three of these ga­lax­ies cap­tured with NASA’s Hub­ble Space Tel­e­scope re­veal their stars share the same birth time, as­tro­no­mers say: the ga­lax­ies all started form­ing stars more than 13 bil­lion years ago, then ab­ruptly stopped, all in the first bil­lion years af­ter the uni­verse was born. 

The rel­ic ga­lax­ies are now con­sid­ered ev­i­dence for a tran­si­tion­al phase in the early uni­verse that shut down star-making fac­to­ries in ti­ny ga­lax­ies. Dur­ing this time, the first stars burned off a fog of cold hy­dro­gen mo­le­cules in a pro­cess called reion­iz­a­tion.

“Some­thing came down like a guil­lo­tine and turned off the star forma­t­ion at the same time in these ga­lax­ies,” said Tom Brown of the Space Tel­e­scope Sci­ence In­sti­tute in Bal­ti­more, Md., the stu­dy’s lead­er. “The most likely ex­plana­t­ion is reion­iz­a­tion.” This pro­cess be­gan when radia­t­ion from the very first stars knocked elec­trons off pri­me­val hy­dro­gen atoms, “ion­iz­ing,” or charg­ing, the cool hy­dro­gen gas. Reion­iz­a­tion al­lowed the hy­dro­gen gas to be­come trans­par­ent to ul­tra­vi­o­let light.

But the same radia­t­ion seems to have squelched star-making ac­ti­vi­ties in dwarf ga­lax­ies, such as those in Brown’s stu­dy, ac­cord­ing to the re­search­ers. These formed an es­ti­mat­ed 100 mil­lion years be­fore reion­iz­a­tion be­gan and had just started to churn out stars. Roughly 2,000 light-years wide (a light-year is the dis­tance light trav­els in a year), the ga­lax­ies are the smaller cousins of the brighter star-making dwarf ga­lax­ies near our Milky Way. The ga­lax­ies were too small to shield them­selves from the harsh ul­tra­vi­o­let light, sci­en­tists claim; what lit­tle gas they had was stripped away as the flood of ul­tra­vi­o­let light rushed through them, and most star forma­t­ion was choked off.

The find­ing, they add, could help ex­plain the so-called “mis­sing sat­el­lite prob­lem,” in which only a few doz­en dwarf ga­lax­ies have been seen around the Milky Way while the sim­pler com­put­er sim­ula­t­ions pre­dict thou­sands should ex­ist. A pos­si­ble ex­plana­t­ion is that these ga­lax­ies are very dark, lack­ing stars. The Sloan sur­vey re­cently un­co­vered more than a doz­en of these star-starved ga­lax­ies in our Milky Way’s neigh­bor­hood while scan­ning just a quar­ter of the sky. As­tro­no­mers think the rest of the sky should con­tain doz­ens more of these ob­jects, dubbed ul­tra-faint dwarf ga­lax­ies.

“By meas­ur­ing the star forma­t­ion his­to­ries of the ob­served dwarfs, Hub­ble has con­firmed ear­li­er the­o­ret­i­cal pre­dictions that star forma­t­ion in the small­est clumps would be shut down by reion­iz­a­tion,” said Ja­son Tum­lin­son of the Space Tel­e­scope Sci­ence In­sti­tute in Balti­more, a mem­ber of the re­search team. The find­ings ap­peared in the July 1 is­sue of the jour­nal As­t­ro­phys­i­cal Jour­nal Let­ters.

“These are the fos­sils of the ear­li­est ga­lax­ies in the uni­verse,” Brown said. “They haven’t changed in bil­lions of years. These ga­lax­ies are un­like most near­by ga­lax­ies, which have long star-forma­t­ion his­to­ries.”

These fos­sil ga­lax­ies have a few hun­dred to a few thou­sand stars the size of our Sun or smaller, as­tro­no­mers es­ti­mate. The ga­lax­ies may be star-deprived, but are thought to have plen­ty of dark mat­ter, the un­der­ly­ing scaf­fold­ing up­on which ga­lax­ies are built. Nor­mal dwarf ga­lax­ies near the Milky Way con­tain an es­ti­mat­ed 10 times more dark mat­ter than the or­di­nary mat­ter that makes up gas and stars. In ul­tra-faint dwarf ga­lax­ies, dark mat­ter is be­lieved to out­weigh or­di­nary mat­ter by at least a fac­tor of 100. “The small ga­lax­ies in our study are made up mostly of dark mat­ter be­cause their hy­dro­gen gas was ion­ized and the stars got turned of­f,” Brown said.

* * *

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The extreme faintness of some galaxies that are almost totally lacking stars may help explain why thousands of small galaxies seem to be missing, astronomers say. Scientists have puzzled over why some puny, extremely faint dwarf galaxies spotted in our Milky Way galaxy’s back yard contain so few stars. The ghost-like galaxies are thought to be some of the tiniest, oldest, and most pristine galaxies, and have been discovered over the past decade by astronomers using automated computer techniques to search through images compiled by a project known asof the Sloan Digital Sky Survey. Now, views of three of these galaxies captured with NASA’s Hubble Space Telescope reveal their stars share the same birth time, astronomers say: the galaxies all started forming stars more than 13 billion years ago, then abruptly stopped, all in the first billion years after the universe was born. The relic galaxies are now considered evidence for a transitional phase in the early universe that shut down star-making factories in tiny galaxies. During this time, the first stars burned off a fog of cold hydrogen molecules in a process called reionization. “Something came down like a guillotine and turned off the star formation at the same time in these galaxies,” said Tom Brown of the Space Telescope Science Institute in Baltimore, Md., the study’s leader. “The most likely explanation is reionization.” This process began when radiation from the very first stars knocked electrons off primeval hydrogen atoms, “ionizing,” or charging, the cool hydrogen gas. Reionization allowed the hydrogen gas to become transparent to ultraviolet light. But the same radiation seems to have squelched star-making activities in dwarf galaxies, such as those in Brown’s study, according to the researchers. These formed an estimated 100 million years before reionization began and had just started to churn out stars. Roughly 2,000 light-years wide (a light-year is the distance light travels in a year), the galaxies are the smaller cousins of the brighter star-making dwarf galaxies near our Milky Way. The galaxies were too small to shield themselves from the harsh ultraviolet light, scientists claim; what little gas they had was stripped away as the flood of ultraviolet light rushed through them, and most star formation was choked off. The finding, they add, could help explain the so-called “missing satellite problem,” in which only a few dozen dwarf galaxies have been seen around the Milky Way while the simpler computer simulations predict thousands should exist. A possible explanation is that these galaxies are very dark, lacking stars. The Sloan survey recently uncovered more than a dozen of these star-starved galaxies in our Milky Way’s neighborhood while scanning just a quarter of the sky. Astronomers think the rest of the sky should contain dozens more of these objects, dubbed ultra-faint dwarf galaxies. “By measuring the star formation histories of the observed dwarfs, Hubble has confirmed earlier theoretical predictions that star formation in the smallest clumps would be shut down by reionization,” said Jason Tumlinson of the Space Telescope Science Institute, a member of the research team. The findings appeared in the July 1 issue of The Astrophysical Journal Letters. “These are the fossils of the earliest galaxies in the universe,” Brown said. “They haven’t changed in billions of years. These galaxies are unlike most nearby galaxies, which have long star-formation histories.” These fossil galaxies have a few hundred to a few thousand stars the size of our Sun or smaller, astronomers estimate. The galaxies may be star-deprived, but are thought to have plenty of dark matter, the underlying scaffolding upon which galaxies are built. Normal dwarf galaxies near the Milky Way contain an estimated 10 times more dark matter than the ordinary matter that makes up gas and stars. In ultra-faint dwarf galaxies, dark matter is believed to outweigh ordinary matter by at least a factor of 100. “The small galaxies in our study are made up mostly of dark matter because their hydrogen gas was ionized and the stars got turned off,” Brown said.