AstroTalk:BehindthenewsheadlinesofApril–June2018RicharddeGrijs(锐何思)(MacquarieUniversity,Sydney,Australia)Gaia’sseconddatarelease:agenuinetreasuretroveforstarhuntersInAprilofthisyear,themuch-awaitedsecondslewofdatafromtheEuropeanSpaceAgency’sGaiamissionwasreleased,providinginformationonaphenomenal1.7billionstars—thericheststarcataloguetodate.Toputthatvastnumberintocontext,ifyouweretocount‘only’toonebillionatarateofonecountpersecond,itwouldtakeyoumorethan30years.Thenewdatawillsurelykeepastronomersbusyforevenlonger.Amultitudeofdiscoveriesareonthehorizonafterthismuchawaitedrelease,whichisbasedon22monthsofchartingthesky.Thenewdataincludepositions,distanceindicatorsandmotionsofmorethanonebillionstars,alongwithhigh-precisionmeasurementsofasteroidswithinoursolarsystemandofstarsbeyondourownMilkyWayGalaxy.Thedatasethasalreadyrevealedpreviouslyunseendetailsaboutthemake-upoftheMilkyWay’sstellarpopulationandabouthowstarsmove,essentialinformationforinvestigatingtheformationandevolutionofourGalaxy.

“TheobservationscollectedbyGaiaareredefiningthefoundationsofastronomy,”saysGüntherHasinger,ESADirectorofScience.“Gaiaisanambitiousmissionthatreliesonahugehumancollaborationtomakesenseofalargevolumeofhighlycomplexdata.Itdemonstratestheneedforlong-termprojectstoguaranteeprogressinspacescienceandtechnologyandtoimplementevenmoredaringscientificmissionsofthecomingdecades.”

Thenewdatareleasepinsdownthepositionsofnearly1.7billionstars,andwithamuchgreaterprecisionthanbefore.Forsomeofthebrighteststarsinthesurvey,thelevelofprecisionequatestoEarth-boundobserversbeingabletospotacoinlyingonthesurfaceoftheMoon.Withtheseaccuratemeasurementsitispossibletoseparatetheparallaxofstars—anapparentshiftontheskycausedbyEarth’sannualorbitaroundourSun—fromtheirtruemovementsthroughtheGalaxy.Thenewcatalogueliststheparallaxesandvelocitiesacrossthesky,or‘propermotions,’formorethan1.3billionstars.Fromthemostaccurateparallaxmeasurements,abouttenpercentofthetotal,astronomerscandirectlyestimatedistancestoindividualstars.

“ThesecondGaiadatareleaserepresentsahugeleapforwardwithrespecttoESA’sHipparcossatellite,Gaia’spredecessorandthefirstspacemissionforastrometry,whichsurveyedsome118,000starsalmostthirtyyearsago,”saysAnthonyBrownofLeidenUniversityintheNetherlands.

“Thesheernumberofstarsalone,withtheirpositionsandmotions,wouldmakeGaia'snewcataloguealreadyquiteastonishing,”addsBrown.“Butthereismore:thisuniquescientificcatalogueincludesmanyotherdatatypes,withinformationaboutthepropertiesofthestarsandothercelestialobjects,makingthisreleasetrulyexceptional.”

Aswellaspositions,thedataincludebrightnessinformationofallsurveyedstarsandcolourmeasurementsofnearlyall,plusinformationonhowthebrightnessandcolourofhalfamillionvariablestarschangeovertime.Italsocontainsthevelocitiesalongthelineofsightofasubsetofsevenmillionstars,thesurfacetemperaturesofaboutahundredmillionandtheeffectsofinterstellarduston87million.Gaiaalsoobservedobjectsinoursolarsystem:theseconddatareleasecomprisesthepositionsofmorethan14,000knownasteroids,whichallowsprecisedeterminationoftheirorbits.AmuchlargerasteroidsamplewillbecompiledinGaia’sfuturereleases.Furtherafield,Gaiaclosedinonthepositionsofhalfamilliondistantquasars,brightgalaxiespoweredbytheactivityofthesupermassiveblackholesintheircores.ThesesourcesareusedtodefineareferenceframeforthecelestialcoordinatesofallobjectsintheGaiacatalogue,somethingthatisroutinelydoneinradiowavesbutnowforthefirsttimeisalsopossibleatopticalwavelengths.MajordiscoveriesareexpectedtocomeoncescientistsstartexploringGaia’snewrelease.

“ThenewGaiadataaresopowerfulthatexcitingresultsarejustjumpingatus,”saysAntonellaVallenarifromtheIstitutoNazionalediAstrofisica(INAF)andtheAstronomicalObservatoryofPadova,Italy.“Forexample,wehaveconstructedthemostdetailedHertzsprung–Russelldiagramofstarsevermadeonthefullskyandwecanalreadyspotsomeinterestingtrends.ItfeelslikeweareinauguratinganeweraofGalacticarchaeology.”

Namedafterthetwoastronomerswhodeviseditintheearlytwentiethcentury,theHertzsprung–Russelldiagramcomparestheintrinsicbrightnessofstarswiththeircolourandisafundamentaltooltostudypopulationsofstarsandtheirevolution.Anewversionofthisdiagram,basedonfourmillionstarswithinfivethousandlight-yearsfromtheSunselectedfromtheGaiacatalogue,revealsmanyfinedetailsforthefirsttime.Thisincludesthesignatureofdifferenttypesofwhitedwarfs—thedeadremnantsofstarslikeourSun—suchthatadifferentiationcanbemadebetweenthosewithhydrogen-richcoresandthosedominatedbyhelium.CombinedwithGaiameasurementsofstarvelocities,thediagramenablesastronomerstodistinguishbetweenvariouspopulationsofstarsofdifferentagesthatarelocatedindifferentregionsoftheMilkyWay,suchasthediskandthe‘halo’—thesphericalcloudofstarssurroundingthemaindiskandbulgeof

ourGalaxy—andthatformedindifferentways.Furtherscrutinysuggeststhatthefast-movingstarsthoughttobelongtothehaloencompasstwostellarpopulationsthatoriginatedviatwodifferentformationscenarios,callingformoredetailedinvestigations.

“GaiawillgreatlyadvanceourunderstandingoftheUniverseonallcosmicscales,”saysTimoPrusti,GaiaprojectscientistatESA.“EvenintheneighbourhoodoftheSun,whichistheregionwethoughtweunderstoodbest,Gaiaisrevealingnewandexcitingfeatures.”

Forasubsetofstarswithinafewthousandlight-yearsoftheSun,Gaiahasmeasuredthevelocityinallthreedimensions,revealingpatternsinthemotionsofstarsthatareorbitingtheGalaxyatsimilarspeeds.Infact,theMilkyWayhaslikelyformedinpartfromthemergingofmanysmallersystems.Howexactlythathappened,isstillapuzzle,however.TolearnmoreaboutthehistoryofformationoftheMilkyWay,astronomersfromtheUniversityofGroningenintheNetherlandsandtheUniversityofCaliforniaatRiversideintheUSAinspectedthemotionsofstarsintheGalactichalo.StarsinthehaloaremorepristinethanthoseinthecentralregionsandtheGalacticdisk,andtheyspendmostoftheirtimeoutsideofthedisk-likestructurethatgivestheMilkyWayitsname.ItisthoughtthatthesehalostarsarethestarsthatjoinedtheMilkyWayonboardofsmallgalaxies.Forthisstudy,ateamledbymycolleagueAminaHelmi,professorofdynamics,structureandformationoftheMilkyWayattheUniversityofGroningen,combinedthevastGaiadatasetwithdatafromtheRAVEsurvey.Theresearchersdiscoveredthatalargefractionofthehalostarstravelingroups.Helmi:

“ThisindicatesthatthestarsindeedoriginatefromsmallgalaxiesthatwerecannibalisedbytheMilkyWayaverylongtimeago.”

TheastronomersdescribethesegroupsaslargeflowsofstarslikeflocksofbirdstravelingtogetherthroughtheMilkyWay.

“Webelievetheremightbetensorevenhundredssuchflocks.Atthemoment,weonlyseesmallgroupswithjustafewstars,butthatisprobablybecausewedonotyethaveallthenecessarydata.”

TheteamwerebewilderedofthebehaviourofhalostarsthatspendmostofthetimeintheoutskirtsoftheMilkyWay.Surprisingly,morethan70%ofthosestarsappeartobemovingintheoppositesensethanthevastmajorityofstarsintheMilkyWay.Suchahighfractionisunexpectedincurrentmodels.Helmi:

“Onemaycomparestarsfromtheouterhalowithcommutersthatdrivethewrongway.Wedonotyetquiteunderstandwhy.”

Thesediscoveriesweremadeusinghalostarsthat,intheirjourneythroughtheMilkyWay,arebychancecurrentlyclosetotheSun.GaiawillprovideuswithdatafromstarsfromallovertheMilkyWay.Helmi:

“WithsuchdatawewillgetmanynewinsightsonhowtheMilkyWayformedandbeabletoreconstructitsgenealogytree.”

TheteamdiscoveredrelicsofmergereventsintheMilkyWayhalo.Fivesmallgroupsofstarsappeartorepresentmergerswithsmallergalaxies,whileabig‘blob’comprisinghundredsofstarsappearstobetheremnantofalargemergerevent.

“OuraimistostudyhowtheMilkyWayhasevolved,”saysHelmerKoppelman,aPh.D.studentinHelmi’sresearchgroup.Theideaisthatsmallergalaxiesmergetoformlargerones.“Oneofthequestionsiswhetheralotofsmallgalaxiesmerge,orafewlargeones.”

AsmoststarsintheMilkyWay’shaloarethoughttoberemnantsofmergerevents,KoppelmanandhiscolleaguesfocusedonhalostarsintheGaiadata.

“Wecollectedinformationfromstarswithin3000lightyearsoftheSun,astheaccuracyofthepositionandmovementishighestforstarsthatarenearus,”Koppelmanexplains.ThefirststepwastofilteroutthestarsfromtheMilkyWaydisk.“Thesestarsmovearoundthecentreofthedisk,soareeasilyidentified.”

Whatremainedwereabout6000halostars.Bycalculatingtheirtrajectory,Koppelmanwasabletoidentifystarswithasharedorigin.

“Wediscoveredfivesmallclusterswhichwebelieveareremnantsoffivemergerevents.”However,manyoftheremainingstarsalsoappearedtohaveasharedhistory.“Thesestarsformahuge‘blob’witharetrogrademovementcomparedtothedisk.Thissuggeststheyaretheresultofamergerwithalargegalaxy.Infact,webelievethatthismergereventmusthaveremodelledthediskinourMilkyWay.”

Amoredetailedstudyofthenatureofthismergerisnowunderway.

“Atthispointintime,wecansaythatourMilkyWaywasshapedbyamassivemergereventandsomesmallermergers.”

Koppelmanalsolookedforstarsbelongingtothe‘Helmistream,’whichisnamedafterhisPh.D.supervisorwhoidentifieditbackin1999astheremnantofamergerevent.

“Uptonow,fewerthan20starsbelongingtotheHelmistreamhadbeenidentified.TheGaiadatahasaddedover100newstars.”Furtheranalysisshouldclarifythenatureofthegalaxythatproducedthisstream.“Wewillalsobelookingatstarsbeyond3000light-yearstodiscovermoremembers

ofthedifferentstreamsweidentified.Togetherwithsimulationsofgalaxyevolution,thisshouldgiveusexcitingnewinsightsintotheevolutionoftheMilkyWay.”

AtGaia’sprecision,itisalsopossibletoseethemotionsofstarswithinsomeglobularclusters—ancientsystemsofstarsboundtogetherbygravityandfoundinthehalooftheMilkyWay—andwithinourneighbouringgalaxies,theSmallandLargeMagellanicClouds.Gaiadatawereusedtoderivetheorbitsof75globularclustersand12dwarfgalaxiesthatrevolvearoundtheMilkyWay,providingall-importantinformationtostudythepastevolutionofourGalaxyanditsenvironment,thegravitationalforcesthatareatplay,andthedistributionoftheelusivedarkmatterthatpermeatesgalaxies.MeasuringthepropermotionofseveralmillionstarsintheLargeMagellanicCloud,astronomerswereabletoseeanimprintofthestarsrotatingclockwisearoundthecentreofthegalaxy.

“Gaiaisastronomyatitsfinest,”saysFredJansen,GaiamissionmanageratESA.“Scientistswillbebusywiththisdataformanyyears,andwearereadytobesurprisedbytheavalancheofdiscoveriesthatwillunlockthesecretsofourGalaxy.”

Figure1:Gaia’sall-skyviewofourMilkyWayGalaxyandneighbouringgalaxies,basedonmeasurementsofnearly1.7billionstars.ThemapshowsthetotalbrightnessandcolourofstarsobservedbytheESAsatelliteineachportionoftheskybetweenJuly2014andMay2016.Brighterregionsindicatedenserconcentrationsofespeciallybrightstars,whiledarkerregionscorrespondtopatchesoftheskywherefewerbrightstarsareobserved.ThecolourrepresentationisobtainedbycombiningthetotalamountoflightwiththeamountofblueandredlightrecordedbyGaiaineachpatchofthesky.ThebrighthorizontalstructurethatdominatestheimageistheGalacticplane,theflatteneddiskthathostsmostofthestarsinourGalaxy.Inthemiddleoftheimage,theGalacticcentreappearsvividandteemingwithstars.DarkerregionsacrosstheGalacticplanecorrespondtoforegroundcloudsofinterstellargasand

dust,whichabsorbthelightofstarslocatedfurtheraway,behindtheclouds.Manyoftheseconcealstellarnurserieswherenewgenerationsofstarsarebeingborn.Sprinkledacrosstheimagearealsomanyglobularandopenclusters—groupingsofstarsheldtogetherbytheirmutualgravity,aswellasentiregalaxiesbeyondourown.ThetwobrightobjectsinthelowerrightoftheimagearetheLargeandSmallMagellanicClouds,twodwarfgalaxiesorbitingtheMilkyWay.(Credit:GaiaDataProcessingandAnalysisConsortium(DPAC);A.Moitinho/A.F.Silva/M.Barros/C.Barata,UniversityofLisbon,Portugal;H.Savietto,ForkResearch,Portugal)

Figure2:Gaia’sall-skyviewofourMilkyWayGalaxyandneighbouringgalaxies.Themapsshowthetotalbrightnessandcolourofstars(top),thetotaldensityofstars(middle)andtheinterstellardustthatfillstheGalaxy(bottom).(Credit:GaiaDataProcessingandAnalysisConsortium(DPAC);Topandmiddle:A.Moitinho/A.F.Silva/M.Barros/C.Barata,UniversityofLisbon,Portugal;H.Savietto,ForkResearch,Portugal;Bottom:GaiaCoordinationUnit8;M.Fouesneau/C.Bailer-Jones,MaxPlanckInstituteforAstronomy,Heidelberg,Germany)

Figure3:Leftpanel:differentstarstreams(coloureddots),theMilkyWaydisk(blue)andinblacktherestofthehalostars,inwhichthehorizontalcigar-shapedblobisvisible.Rightpanel:samedata,nowseenforma90degreerotatedangle.(Credit:Koppelmanetal.)

Figure4:TheMilkyWaydiskisembeddedinaroundishhaloofstars.Thestars(inpurple)arefromacomputersimulationoftheremainsfromamergerwithasmallgalaxy.Thearrowsindicatethemotionofthesestarsthatarenowpartofthehalo.Largerarrowsindicatefastermotion.Theastronomerssuspectthattenstohundredsofsuchflowsofstarsarecriss-crossingtheMilkyWay.(Credit:AminaHelmi/JovanVeljanoski/MaartenBreddels/UniversityofGroningen)

Figure5:ThisimageoftheLargeMagellanicCloudcombinesthetotaldensityofstarsdetectedbyGaiaineachpixelwithinformationaboutthe‘propermotion’ofstars—theirvelocityacrossthesky—whichisrepresentedasthetextureoftheimage,givingitafingerprint-likeappearance.Measuringthepropermotionofseveralmillionstars,astronomerswereabletoseeanimprintofthestarsrotatingclockwisearoundthecentreofthegalaxy.Theimpressionofmotionisevokedbytheswirlingnatureofthelinetexture.(Credit:ESA/Gaia/DPAC)