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Galaxy growth examined like tree rings

Nov. 4, 2013
Courtesy of the Jet Propulsion Laboratory
and World Science staff

Watch­ing a tree grow might be more frus­trat­ing than wait­ing for a pot to boil, but luckily there are tree rings. Be­gin­ning at a tree trunk’s com­pact co­re and mov­ing out to the soft bark, con­cen­tric rings mark the pas­sage of time, re­veal­ing chap­ters of the tree’s his­to­ry.

In this im­age of a gal­axy called NGC 3377, in­fra­red light from WISE is col­ored red, and ultra­violet light from GALEX is green and blue. The cen­ter of the gal­axy ap­pears white, where all three "colors" are there and add up. The out­side of the gal­axy is most­ly ultra­violet, and thus con­tains more blue and green. The dots are stars in the fore­ground. (Cour­te­sy JPL)


Ga­lax­ies out­live trees by bil­lions of years. But like bi­ol­o­gists, as­tro­no­mers can read the rings in the disk of a gal­ax­y to un­rav­el its past, a study sug­gests. 

Us­ing da­ta from two NASA tele­scopes, sci­en­tists have gained more ev­i­dence for an “inside-out” the­o­ry of gal­axy growth, show­ing, they say, that bursts of star forma­t­ion in cen­tral re­gions were fol­lowed one to two bil­lion years lat­er by star birth in the out­er fringes.

“Ini­tially, a rap­id star-forming pe­ri­od formed the mass [ma­te­ri­al] at the cen­ter of these ga­lax­ies, fol­lowed lat­er by a star-forming phase in the out­er re­gions. Even­tu­al­ly, the ga­lax­ies stop mak­ing stars,” said Sara Pet­ty of Vir­gin­ia Tech in Blacks­burg, Va. She is the lead au­thor of a pa­per on the work in the Oc­to­ber is­sue of The As­tro­nom­i­cal Jour­nal, al­so posted here

“This lat­er star-forming phase could have been caused by mi­nor merg­ers with gas-rich neigh­bors, which pro­vide the fu­el for new stars.”

The dis­cov­ery may al­so solve a mys­tery of eld­erly ga­lax­ies, she said. The ga­lax­ies in the stu­dy, known as “red and dead” for their red col­or and lack of new star births, have a sur­pris­ing amount of ul­tra­vi­o­let light em­a­nat­ing from the out­er re­gions. Of­ten, that light comes from hot, young stars, but these ga­lax­ies were con­sid­ered too old to host many of those, Pet­ty ex­plained.

The so­lu­tion to the puz­zle is likely hot, old stars, ac­cord­ing to the re­search­ers. They used a new ap­proach an­a­lyz­ing light at mul­ti­ple wave­lengths, or “col­ors,” to show that the un­ex­plained ul­tra­vi­o­let light seems to be com­ing from a late phase in the lives of old­er stars, when they b­low off their out­er lay­ers and heat up.

The as­tro­no­mers used two NASA tele­scopes, the Wide-field In­fra­red Sur­vey Ex­plor­er and Gal­axy Ev­o­lu­tion Ex­plor­er. The first sees the in­fra­red light com­ing from old­er stars, whe­reas the other is sen­si­tive to ul­tra­vi­o­let. Both are forms of light in en­er­gy ranges, or “col­ors,” in­vis­i­ble to the un­aided eye. Both tele­scopes have large fields of view, al­low­ing them to easily cap­ture im­ages of en­tire ga­lax­ies.

“The syn­er­gy be­tween GALEX and WISE pro­duces a very sen­si­tive meas­ure­ment of where the hot, old­er stars re­side in these red-and-dead ga­lax­ies,” said Don Neill, co-au­thor of the pa­per from the Cal­i­for­nia In­sti­tute of Tech­nol­o­gy, Pas­a­de­na. “This al­lows us to map the prog­ress of star forma­t­ion with­in each gal­axy.”


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Watching a tree grow might be more frustrating than waiting for a pot to boil, but luckily there are tree rings. Beginning at a tree trunk’s compact core and moving out to the soft bark, concentric rings mark the passage of time, revealing chapters of the tree’s history. Galaxies outlive trees by billions of years. But like biologists, astronomers can read the rings in a galaxy’s disk to unravel its past, a study suggests. Using data from two NASA telescopes, scientists have gained more evidence for an “inside-out” theory of galaxy growth, showing, they say, that bursts of star formation in central regions were followed one to two billion years later by star birth in the outer fringes. “Initially, a rapid star-forming period formed the mass [material] at the center of these galaxies, followed later by a star-forming phase in the outer regions. Eventually, the galaxies stop making stars,” said Sara Petty of Virginia Tech in Blacksburg, Va., lead author of a paper in the October issue of the Astronomical Journal, and also online here. “This later star-forming phase could have been caused by minor mergers with gas-rich neighbors, which provide the fuel for new stars.” The discovery may also solve a mystery of elderly galaxies, she said. The galaxies in the study, known as “red and dead” for their red color and lack of new star births, have a surprising amount of ultraviolet light emanating from the outer regions. Often, that light comes from hot, young stars, but these galaxies were considered too old to host many of those, Petty explained. The solution to the puzzle is likely hot, old stars, according to the researchers. They used a new approach analyzing light at multiple wavelengths, or “colors,” to show that the unexplained ultraviolet light seems to be coming from a late phase in the lives of older stars, when they blow off their outer layers and heat up. The astronomers used two NASA telescopes, the Wide-field Infrared Survey Explorer and Galaxy Evolution Explorer. WISE sees the infrared light coming from older stars, whereas GALEX was sensitive to ultraviolet light. Both are forms of light in energy ranges, or “colors,” invisible to the unaided eye. Both telescopes have large fields of view, allowing them to easily capture images of entire galaxies. “The synergy between GALEX and WISE produces a very sensitive measurement of where the hot, older stars reside in these red-and-dead galaxies,” said Don Neill, co-author of the paper from the California Institute of Technology, Pasadena. “This allows us to map the progress of star formation within each galaxy.”