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Home , Milky Way, Palomar 5, Sagittarius Stream

Captivating shimmer: The is part of the system that is home to our and an estimated 200 billion other . On a dark , it shimmers like a nebulous ribbon on the firmament. PHYSICS & ASTRONOMY_Galaxy

Archaeology of the Milky Way

The has billions and billions of , but only one that we can explore by star in all its dimensions: our Milky Way. It can be thought of as a “model organism” for the formation and evolution of galaxies and is thus a key research topic in , and the research focus of the team working with Hans-Walter Rix, Director at the Max Institute for in Heidelberg. The researchers recently found indications that quite a number of earlier ideas about our have to be revised.

TEXT THOMAS BÜHRKE

visitor approaching the en- that world-class research can be carried astronomers has now even analyzed trance to the institute on out even with relatively small instru- data from almost a billion stars. This, Königstuhl hill near Heidel- ments. The decisive factor here is that too, has led the researchers to enter the berg first notices hexagons the astronomers have been using them business of big data mining. But what is chalked on the ground and to chart the entire continuously the point of all this? Ajoined together to form a huge honey- over many years. “If you want to investigate the evo- comb. These were left over from the lution of galaxies such as the Milky last open house day and symbolize DATA MINING WITH Way, there are two possibilities,” says segments of the 39-meter-diameter 800 MILLION STARS Rix. “One is to observe galaxies that are main mirror that is to collect the light increasingly far away from us.” Because of distant stars and galaxies in the Eu- This treasure trove of sky survey data the is finite, looking into ropean Extremely Large Telescope in contains more information than can the distance is always looking into the Chile from the next decade on. Astron- currently be analyzed and modeled. “In past. It is indeed now possible to look omers at the institute are involved in the very recent past, the quantity and over a billion years into the past direct- developing two cameras for what will quality of the data has doubled every ly at galaxies that formed one billion become the largest telescope on . one to two years,” says Hans-Walter Rix. years after the . But until it has been built, the ex- “Ten years ago, we had good spectra Galaxies at different distances from plorers of the cosmos must be content from around 8,000 stars; today, it’s four us are therefore at different stages of with the telescopes that are currently million.” For a study of the spatial dis- evolution. However, it is never possible available. This isn’t necessarily a disad- tribution of the dust in the Milky Way, to see one and the same galaxy at dif-

Photo: Corbis vantage, as recent years have shown an international team led by Max Planck ferent times. Moreover, these galaxies

4 | 15 MaxPlanckResearch 55 are sofarawaythatitisgenerallypos- years ago.Heinvestigatesthegalaxy around thegalacticcenter, bearing ry. Theymoveondifferentorbits stars haveformedthroughout itshisto- see itonlyinitspresentstate? homeland bereconstructed ifwecan ofourcosmic history the evolutionary axy research,” saysRix.Buthowcan something liketheRosettaStoneofgal- andchemicalcomposition.“Itis ilar toourMilkyWay intermsofsize, verse arefoundingalaxiesthatsim- be generalized. galaxy, andwhatwelearnaboutitcan Fortunately, ourMilkyWay isatypical bers andinthreedimensions,”hesays. individual starsindetail,largenum- thepropertiesof we areabletoobserve is onlyinourowngalactichomethat that isclosesttous:ourMilkyWay. “It ble torecognizeindividualstars. system asawhole,sinceitisn’t possi- sible tomakestatementsonlyaboutthe PHYSICS & ASTRONOMY_Galaxy 56 MaxPlanckResearch 4 MaxPlanckResearch The MilkyWay isa dynamicsystem: Around halfthestarsintoday’s uni- Rix wentdownadifferentpath Sun ~ 27 ,000 light-years | 15 Bulge group overmanythousandsofyearsby stand themigrationofademographic used byscientistswantingtounder- withtheaidofcomputermodels. ry possible andtodeducetheirpasthisto- mine thepropertiesofasmanystars fingerprints. Theideanowistodeter- aschemical sitions, whichcanserve they havedistinctivechemicalcompo- oftheirbirthorbit.And some memory Sun –only80light-yearsaway from along withcloudsofdustand gas.Our mass ofabout50billionsolar , percent ofallstars,andhasatotal light-years thick,ishometo around80 flatdisk,just2,000 gions. Thevery years. Itissubdividedintothreere- a diameterofaround100,000light- The MilkyWay isaspiralgalaxywith HIDING IN THE GALAXY A MASSMONSTERIS providing completelynewinsights. it galacticarchaeology, anditisnow analyzing geneticmaterial.Insiderscall This methodissimilartotheone 27,000 isaround Sun Our arms. spiral several with galaxy the of view plan a schematic depicts right the on illustration The data. of amisinterpretation on isbased but thick disk exist probably doesn’t at all, region. indicate that the outer the into farther stretches that disk athick and plane center the to close stars of disk athin bulge: central the components can be seen in addition to Two . an from looking system Way Milky our of view edge-on isan left the on graphic The galaxy: the of Two views were alreadyinexistenceat thattime time ago,swirlingupallthe starsthat ed withanotherlargegalaxy along assumed thattheMilkyWayory collid- er, remainsamystery. Thefavoredthe- tends farintotheouterregions. – includingalltheoldstarsandex- contains around10percent ofallstars knowledge. Thethickercomponent a thickerone–hasbecometextbook of starsclosetothecentergalaxy, and two components–athinonemadeup central region. disk intheso-calledhaloaround stars movefaraboveorbelowthe masses. Theremaining5percent ofthe mass ofroughlyfourmillionsolar likelyablackholewith ble. Itisvery years. Thecentralbodyitselfisinvisi- a diameterofaround16,000light- spherical regionaroundthecenterwith found inthecentralbulge.Thisisa largest population. the centerplane–alsobelongstothis Moreover, the viewthatthediskhas How thethickdiskformed, howev- Around 15percent ofthe starsare light-years away from the center. the from away light-years

Graphic: MPG, based on original material from Gaba p/CC-BY-SA 3.0 Photo: Robert Hurt/SSC/Caltech/JPL/NASA Robert Hurt just likeinarchaeology. sible toestimatetheageof thestars, the surfaceofstarandmake itpos- elements formalifelongfingerprint on dances. Themeasurableabundances of into populationswiththesame abun- around 15,000starsandgroupthem termine thechemicalcompositionof delberg makesa25percent contribution. which theMaxPlanckInstituteinHei- inNewMexico (USA),to servatory 2.5-meter telescopeatApachePointOb- isbeingundertakenwitha sky survey (SDSS).This the SloanDigitalSkySurvey had evaluatedspectroscopicdatafrom ed datamaterial. limit- misinterpretation basedonvery ly definedthickdiskisprobablya cate thattheexistenceofsuchaclear- er, thenewmeasurementsnowindi- disk. OurSunisoneofthem.Howev- galactic cataclysmarenowinthethin thick diskcomponent. and leadingtotheformationof The spectramakeitpossibletode- Hans-Walter Rixandhiscolleagues All starsbornafterthispostulated search GroupattheMaxPlanckInsti- lar SpectroscopyandPopulations Re- cently becametheLeaderof theStel- milians-Universität Munich andre- doctoral studiesattheLudwig-Maxi- Maria Bergemann,whocompleted her provided agreatmanymoreinsights. have developed. cially notthosethattheythemselves accepted teachingsoverboard–espe- many researchers don’t likethrowing conferences fortheirnewfindings: still occasionallycomeunderfireat that wasn’t quitesothin. galaxy, starsweresimplyborninadisk ing theturbulentearlyphaseof time, itseemsmostplausiblethat,dur- bly havetobediscarded.Inthemean- have formedthethickdiskwillproba- another largegalaxythatissupposedto thought thattherewasacollisionwith disk –isevident.Theoldschoolof groups –thethindiskandthick no strictsubdivisionintothetwo isthat A crucialresultoftheSDSsurvey Years later, Rixandhis colleagues However, the treasuretroveofdata Sun abundance arefoundinallbelts ofthe lationship betweenageand chemical stars thatdon’t havetheexpectedre- researchers toreconstructthepast: fact thatmadeitmoredifficult forthe borne manyfruits. spect, galacticarchaeology hasalready “similar tocities,”addsRix.Inthisre- grow graduallytowardtheoutside– therefore startfromanoldcenterand searcher. GalaxiesliketheMilkyWay lions ofyears,”explainstheyoungre- outwards overthecourseofmanybil- star birthprogressedfromtheinside tion oftheMilkyWay thatsaysthe “This supportsascenariofortheforma- OF THE SPIRALARMS MIGRATION IN THE INTERPLAY Milky Way. farther theyarefromthecenterof stars becomeincreasinglyyoungerthe mine theagesofstarsandshowthat here. Bergemannwasabletodeter- tute forAstronomy, playsacrucialrole However, naturecameupwithone PHYSIK & ASTRONOMIE_Galaxis 4 | MaxPlanckResearch 15MaxPlanckResearch 57 PHYSICS & ASTRONOMY_Galaxy

center of the Milky Way. “This finding dust cloud and subsequently drifted the hot gas from being compressed. can be explained by the fact that some away from each other through influ- The dark-matter particles, in contrast, stars don’t remain on their original or- ences such as the of spiral arms. don’t exert any electric forces and bit around the center, but are able to It looks as though the astronomers clustered together to form huge clouds migrate inwards and outwards,” says have to revise not only their ideas and long filaments. Rix. New computer simulations sup- about the evolution of the Milky Way, Their gravitational force attracted port this scenario for the process of but those about its formation, as well. normal gas particles. These collected in star migration. This requires them to move mentally to a well like marbles, and the gas com- The Milky Way is a in the beginning of the universe. pacted to form the first stars and galax- which the spirals represent aggrega- ies. Without the midwifery of the dark tions of gas and stars. If a star comes DARK-MATTER PARTICLES EXERT matter, there would probably be no gal- close to a spiral arm, it is attracted by NO ELECTRIC FORCES axies or stars. the stronger gravity and accelerated – According to the conventional like a surfer on a big wave. If that star After the Big Bang 13.8 billion years school of thought, the large galaxies like rides in front of this spiral wave, the ago, the primal gas comprised mainly our Milky Way didn’t form to full size. star moves farther away from the cen- of and , on the one Rather, there were initially smaller sub- ter of the Milky Way onto a larger or- hand, and on the other, units that collided, merged and slowly bit. If, after a few hundred million formed a relatively uniformly distrib- grew. In this so-called hierarchical for- years, this star happens to come under uted haze. Although researchers still mation scenario, large galaxies emerge the influence of another spiral arm, but don’t know precisely what dark mat- only in a later phase of the universe. approaches it from behind, it becomes ter is, everything currently points to it In recent years, however, astrono- decelerated and moves inwards. Over being an unknown form of elementa- mers have found more and more large billions of years, the result is that the ry particle that can presently be de- galaxies in the infant universe. “The star doesn’t move on one simple circu- tected only via gravitational effects. idea that small galaxies have been the lar orbit around the center, but mi- Particularly important is the fact that most important building blocks of the grates from one orbit to another – and the particles don’t exert any mutually large galaxies is a myth,” says Rix. thus conceals its place of origin. repulsive electric forces. This was cru- Two-thirds of all stars in the Milky “We want to try to reconstruct these cial at the beginning of the universe Way only formed during the past sev- orbital disturbances by looking for stars because the gravitational force tried to en to eight billion years, for example, that have identical chemical composi- concentrate this matter into large and thus can’t originate from early tions at different locations in the Milky lumps. But the hydrogen and helium mergers. The likely scenario is that Way,” explains Rix. It can then be as- nuclei were electrically charged and only around 10 percent of the stars in

sumed that they originated in the same repelled each other. This prevented the Milky Way originate from what Photo: ESA//CC BY-SA 3 IGO

58 MaxPlanckResearch 4 | 15 Graphic: Maria Bergemann/MPI for Astronomy disk. Theturbulencefrom back then matter, whereitcollectsinaturbulent into thegravitationalwells ofdark from theouterregionsinitially streams Astronomy, forexample, showthatgas putes attheMaxPlanckInstitute for that theoreticianGregStinsoncom- dark matter. one ofthesegravitationaltraps bly formedalmost“inonepiece”in block galaxies.TheMilkyWay proba- had previouslybeensmallbuilding- sadipscing elitr. Lorem ipsumdolorsitamet,consetetur sanctus estLoremipsumdolorsitamet. Stet clitakasdgubergren,noseatakimata accusam etjustoduodoloresearebum. aliquyam erat,seddiamvoluptua.Atveroet tempor inviduntutlaboreetdoloremagna sadipscing elitr,seddiamnonumyeirmod Lorem ipsumdolorsitamet,consetetur Computer simulationslikethose it orbitstheMilkyWay, passingthrough called Palomar5.Theydiscovered that ular starcluster75,000light-years away in Heidelberginvestigateda small glob- cesses areevidentevennow. Milky Way. thesepro- Onthecontrary: no roleatallintheevolutionof swallowing upofsmallgalaxiesplayed This doesn’t mean,however, thatthe finds asimpleexplanation. reflected inthethickdisk,whichthus still hasn’t completelysubsidedandis Just overtenyearsago,astronomers December in launched was which Gaia, probe space European The survey: sky The Above: accuracy. The scout covers more than 100 than more covers scout The accuracy. high very with spectrum visible inthe sky of and when it was formed. itwas when and of iscomposed star the material which out find to aspectrum such use astronomers from a specific . The sents an absorption line that originates by lines. Each of the dark stripes repre- here shown colors, detailed its into down isbroken astar of light The Rainbow: Left: Clouds Magellanic Small the and Large the galaxies, neighboring two our are image the of part lower the of side right-hand the on spots hazy two The galaxy. the of catalog inventory the shows photo Way. The Milky inour stars billion other starsintheMilkyWay. Theycan their membersdon’t differfromthe difficult tofindbecause,atfirst glance, solve completely. 100 millionyearsandprobably dis- 5 willdiveintotheMilkyWay againin Computer modelspredictthatPalomar which arealsoknownasstellarstreams. around 15,000light-yearsinlength, the clusterandnowresideintwotails On itstravels,starswerepulledoutof its diskagainandasitdoesso. These stellarstreamsareextremely PHYSIK & ASTRONOMIE_Galaxis 2013 , is to chart the complete complete the chart , isto 4 | MaxPlanckResearch 15MaxPlanckResearch

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be found only by determining the chemical composition and especially the spatial of as many stars as possible. The stripped-off remnants of what used to be the companion galaxy then become recognizable, similar to a shoal of fish in the . They can be detected only in the data of modern sky surveys. The scientists are currently aware of half a dozen or so stellar streams. The newest members are the stream, named after the Ophiuchus in which it was discov- ered, and the , the first and biggest stream known, located in the eponymous constellation. The latter forms the tidal debris tail of a that orbits the Milky Way on an orbit that is almost perpendicu- lar to the center plane. The Max Planck astronomers were recently able to record this stellar stream in unprecedented detail with their PanSTARRS sky survey. PanSTARRS has been running on Mount Haleakal in Hawaii for five years. A 1.8-meter telescope equipped with the world’s largest digital camera, with 1.4 billion pixels, records three-quarters of the ce- lestial sphere visible from Hawaii every four months. One feature of such analyses is that they allow the merger and its effects on the Milky Way to be studied in detail. There is, however, a second aspect that makes the stellar streams of interest to cosmologists: their members move on broad trajectories outside the Milky Way through its halo, where they are also subjected to the gravitational force exerted by the dark matter. With the aid of computer simula- tions, the distribution of this invisible mass can be computed from the mo-

Top: Cosmic detectives: Maria Bergemann and Hans-Walter Rix from the Max Planck Institute for Astronomy in Heidelberg are trying to get to the bottom of how the gal- axies evolved. Our Milky Way serves as their “model organism.” Bottom: Birth in a computer: Theoreticians like Greg Stinson reproduce the birth of en- tire galaxies in complex simulations. The computations show that gas streams into the gravitational wells of dark matter and

collects in a turbulent disk. Photos: Thomas Hartmann (top), Stinson et al. (2013) “MaGICC” (bottom)

60 MaxPlanckResearch 4 | 15 tions of the stars. “If we do this for sev- eral stellar streams moving on different TO THE POINT trajectories, we can even determine the ● spatial distribution of the dark matter,” Our Milky Way serves the astronomers in Hans-Walter Rix’s team as a kind of Rosetta Stone that they use to study the evolution of galaxies. says Hans-Walter Rix. ● By determining the properties of as many stars as possible and entering them into Rix’s team is very well integrated computer models, the researchers discover the past history of the galaxies. into this pioneering research through its ● The textbook teaching that the stars in the galaxy are spread over a thin and a thick participation in the PanSTARRS sky sur- disk can no longer be maintained. The theory stating that our Milky Way collided vey. The astronomers additionally hope with another large galaxy at some time in the past, leading to the formation of the that the data from the Gaia space tele- thick disk we observe, will probably also have to be discarded. scope of the ● The team headed by Rix has also exposed what some believe – that large galaxies ESA, which was launched at the end of grew gradually by colliding with many small stellar clusters – to be a myth. 2013, will take them a giant leap for- Two-thirds of all stars in the Milky Way only formed during the past seven to eight billion years, for example, and thus cannot originate from early mergers. ward. Gaia is currently recording the po- sitions, , brightness and colors of one billion stars in the Milky Way. GLOSSARY Max Planck astronomers are work- ing on the classification of the stars. In European Extremely Large Telescope: This project of the European Southern Observatory (ESO) is currently being built on Cerro Armazones, a mountain that rises to 3,000 meters one to two years, they will be able to in the Chilean Andes. The E-ELT will have a main mirror constructed from 798 hexagonal analyze the first datasets of the mis- elements and measuring 39 meters in diameter, and will be the largest telescope on Earth sion. “We hope this will lead to the on its planned completion date in 2024. discovery of five to ten new stellar : The SDSS is a survey that will cover a quarter of the sky. Its streams,” says Rix. And who knows objective is to record the positions and brightness of more than 100 million celestial objects what surprises this treasure trove of at five wavelengths. It will also acquire spectra from individual objects. data still has in store.

Call for Nominations Max Planck Research Award 2016 The International Research Award of the Alexander von Humboldt Foundation and the Max Planck Society

The Alexander von Humboldt Foundation and the Max Planck Society jointly confer the Max Planck Research Award, which is funded by the German Federal Ministry for Education and Research, on exceptionally highly-qualified German and foreign scientists. The researchers are expected to have already achieved international recognition and to continue to produce outstanding academic results in international collaboration – not least with the assistance of this award. Every year, two research awards are conferred on internationally renowned scientific researchers. One of the awards should be given to a researcher working in Germany and the other to a researcher working abroad. As a rule, each Max Planck Research Award is endowed with 750,000 Euros. Nominations of qualified female scientific researchers are especially welcome. On an annually-alternating basis, the call for nomina- tions addresses areas within the natural and engineering sciences, the life sciences, the humanities and the social sciences. The Max Planck Research Award 2016 will be conferred in the area of life sciences in the subject Organismal Biology with particular focus on sensing the environment

The Rectors/Presidents of German universities or research organisations and the scientific heads of institutes of these organisations are eligible to nominate candidates. Nominations must be submitted to the Alexander von Humboldt Foundation. Applications by prospective candidates themselves are not possible. The deadline for nominations is 15 January 2016.

Further information can be obtained from the

Alexander von Humboldt-Stiftung, Bonn (Germany) www.humboldt-foundation.de/web/max-planck-award.html E-Mail: [email protected]

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