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Unifying principle said to govern all galaxies

March 6, 2007
Special to World Science  

As­tro­no­mers have found a math­e­mat­i­cal prin­ci­ple that they say sur­pris­ing­ly fits all gal­ax­ies, from the state­ly spi­ral-shaped ones to the messy “train wrecks.” This could re­veal some­thing deep about the evo­lu­tion of the un­i­verse, they claim.

In a new study us­ing the W.M. Keck Ob­serv­a­to­ry in Ha­waii, the sci­en­tists found that this com­mon­al­i­ty has linked all galax­ies for eight bil­lion years, near­ly half the age of the Uni­verse.

An im­age of col­lid­ing galax­ies cap­tured in 1999 us­ing the Hub­ble Space Tel­e­scope. The ob­jects are part of a large gal­axy clus­ter des­ig­nat­ed MS1054-03. It is about eight bil­lion light-years away, hence shows it­self to us as it looked that num­ber of years ago. (Cour­te­sy Pie­ter van Dokkum, Mar­ijn Franx [U­ni­ver­si­ty of Gronin­gen/Lei­den], ESA and NASA)


All galax­ies, they said, fol­low a con­sist­ent re­la­tion­ship be­t­ween their mass, or weight, and the ve­loc­i­ties of the stars and gas clouds that com­pose them. “We were tru­ly sur­prised at how well” the pat­tern fits a diz­zy­ing ar­ray of ga­l­axy types, said San­d­ra Fa­ber of the Uni­ver­si­ty of Ca­l­i­for­nia, San­ta Cruz, co-au­thor of the stu­dy. 

The re­port is to ap­pear in a forth­com­ing spe­cial is­sue of the re­search pub­li­ca­tion As­t­ro­phys­i­cal Jour­nal Let­ters.

Galax­ies fall in­to three bas­ic types: spi­r­al or disk-like ones such as our own Milky Way; those shaped like round­ish clouds, known as el­lip­ti­cal ga­l­ax­ies; and messy, bashed-up or odd­ball ga­l­ax­ies. These are usu­al­ly thought to be rem­nants of gal­axy col­li­sions, and some­times dubbed “train wrecks.”

Astronomers noted years ago that for spi­r­als and el­lip­ti­cals, there were spe­ci­fic re­la­tion­ships be­t­ween their masses and the ve­loc­i­ties of their con­s­tit­u­ent stars. These rules are called the Tul­ly-Fish­er and the Fa­ber-Jack­son re­la­tions, re­spec­tive­ly.

What was­n’t known, Fa­ber and col­leagues said, was that these two re­la­tion­ships—which seem­ing­ly work dif­ferent­ly—are them­selves re­la­ted, as as­pects of one over­ar­ch­ing rule. And this prin­ci­ple al­so ap­plies to the “train wrecks,” not pre­vi­ously known to re­spect any such law.

Ac­cord­ing to Faber’s group, in all galax­ies, there is some or­der­ly, reg­u­lar ro­ta­tion: the con­stit­u­ent stars and gas clouds re­volve to­geth­er about a com­mon cen­ter. For train-wreck galax­ies, this neat ro­ta­tion is over­laid with a cer­tain amount of mixed-up ve­loc­i­ties.

The re­search­ers de­vised a new meas­ure of the com­po­nents’ to­tal ve­loc­i­ty, which they called a “speed in­di­ca­tor.” It com­bines both the or­der­ly ro­ta­tion ve­loc­i­ty and the ran­dom or dis­or­dered mo­tion. This property turns out to be strict­ly re­lat­ed to the mass of gal­ac­tic com­p­o­nents, said Su­san Kassin, a post­doc­tor­al re­searcher at the uni­ver­si­ty and the stu­dy’s lead au­thor. “Sur­pris­ingly, if you use this new speed in­di­ca­tor to meas­ure the mo­tions of stars and gas in a gal­axy, you can pre­dict the mass in stars the gal­axy has with pret­ty high ac­cu­ra­cy.”

Galax­ies like our Milky Way con­sist of bil­lions of stars formed in­to a spir­al disk along with some gas. Our gal­axy al­so spins like a pin­wheel at a few hun­dred kilo­me­ters (miles) per sec­ond. 

Such el­e­gant galax­ies were scarce once, the sci­en­tists said. As­tro­no­mers can ob­serve the an­cient uni­verse by look­ing ex­treme­ly far away, be­cause this means the light we see left those ar­eas bil­lions of years ago. Half of the age of the uni­verse ago, many galax­ies look more di­shev­eled, as they were be­ing as­sem­bled through gal­axy col­li­sions and the pil­ing on of new gas and stars, Kass­in’s team said.

The new findings in essence show that the mixed-up and or­der­ly ve­loc­i­ties are some­how re­lated, said Ben Weiner of the Uni­ver­si­ty of Ar­i­zo­na in Tuc­son, Ariz., an­oth­er co-au­thor of the stu­dy. “The mixed-up ve­loc­i­ties may set­tle down to or­der­ly ro­ta­tion over time as the uni­verse ages,” he pro­posed.

The find­ings prob­a­bly re­flect an even deepe­r prope­rty of the cos­mos, said Faber, one of the name­sakes of the Faber-Jackson re­la­tion, which she helped de­vel­op in 1976. “Both of these re­la­tions were im­printed by the na­ture of fluc­tu­a­tions [in the uni­verse] that made galax­ies in the first place,” she said. 

“This re­la­tion holds for all the galax­ies, no mat­ter what they look like,” Kassin re­marked. “It ties to­geth­er the Faber-Jackson re­la­tion with the Tully-Fisher re­la­tion and works for all kinds of odd­ball galax­ies that are more com­mon in the ear­ly uni­verse.” The study in­volved 544 far-off galax­ies of var­i­ous types. Kassin said that makes it the larg­est study to date of the speed and move­ment of dis­tant galax­ies’ stars and oth­er mat­ter.


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Astronomers have found a mathematical principle that they say surprisingly fits all galaxies, from the stately spirals to the messy “train wrecks.” In a new study using the W.M. Keck Observatory in Hawaii, the scientists found that this commonality has linked all galaxies for eight billion years, nearly half of the age of the Universe. All galaxies, they found, follow a consistent relationship between their mass, or weight, and the velocities of the stars and gas clouds that make them up. “We were truly surprised at how well” the pattern fits a dizzying array of galaxy types, said Sandra Faber of the University of California, Santa Cruz, co-author of the study. The report is to appear in a forthcoming special issue of the research publication Astro physical Journal Letters. Galaxies fall into three basic types: spiral or disk-like galaxies like our own Milky Way; those shaped like smooth clouds, known as elliptical galaxies; and messy, bashed-up or oddball galaxies, usually thought to be remnants of galaxy collisions and sometimes dubbed “train wrecks.” It was already known that for spiral and elliptical galaxies, there were relationships between their mass and the velocities of their constituent stars. These rules are known as the Tully-Fisher and the Faber-Jackson relations, respectively. What wasn’t known, Faber and colleagues said, was that there these two rules are themselves aspects of a single, overarching principle—which also turns out to cover messy galaxies not previously known to respect any such law. According to Faber’s group, in all galaxies, there is a certain amount of orderly, regular rotation; the constituent stars and gas clouds revolve together about a common center. For train wreck galaxies, though, a certain amount of mixed-up velocities are overlaid over the orderly rotation. The researchers devised a new measure of the components’ total velocity, which they called a “speed indicator.” It takes into account both the orderly rotation velocity, and the random or disordered motion. This speed indicator turns out to be strictly related to the mass of the stars in the galaxy, said Susan Kassin, a postdoctoral researcher at the university and the study’s lead author. “Surprisingly, if you use this new speed indicator to measure the motions of stars and gas in a galaxy, you can predict the mass in stars the galaxy has with pretty high accuracy.” Galaxies like our Milky Way consist of billions of stars formed into a spiral disk along with some gas. Like a spinning pinwheel, our galaxy also spins, but at a speed of a few hundred kilometers (miles) per second. Such elegant galaxies were scarce once, the scientists said. Astronomers can observe the ancient universe by looking extremely far away. This means the light we see from those areas, left them billions of years ago. Half of the age of the universe ago, many galaxies look more disheveled, as they were assembled through galaxy collisions and the piling on of new gas and stars, Kassin’s team said. The fact that mixed-up and orderly velocities follow the same rule suggests the phenomena are related, said Ben Weiner of the University of Arizona in Tucson, Ariz., another co-author of the study. “The mixed-up velocities may settle down to orderly rotation over time as the universe ages,” he said. The findings probably reflect an even deeper property of the cosmos, said Faber, one of the namesakes of the Faber-Jackson relation, which she helped develop in 1976. “Both of these relations were imprinted by the nature of fluctuations [in the universe] that made galaxies in the first place,” she said. “This relation holds for all the galaxies, no matter what they look like,” Kassin remarked. “It ties together the Faber-Jackson relation with the Tully-Fisher relation and works for all kinds of odd-ball galaxies that are more common in the early universe.” The study involved 544 distant galaxies of various types, which according to Kassin makes this the largest study to date of the speed and movement of distant galaxies’ stars and other matter.