A Possible Origin of Magnetic Fields in Galaxies and Clusters Strong Magnetic fields at z~1

We propose that strong magnetic fields should be generated at shock waves associated with formation of galaxies or clusters of galaxies by the Weibel instability, an instability in collisionless plasmas. The strength of the magnetic fields generated throug

Mon.Not.R.Astron.Soc.000,(0000)Printed5February2008

A(MNLTEXstyle lev2.2)

APossibleOriginofMagneticFieldsinGalaxiesand

Clusters:StrongMagnetic eldsatz～10?

arXiv:astro-ph/0508589v1 26 Aug 2005

1,2 1

YutakaFujitaandTsunehikoN.Kato1

2Department

NationalAstronomicalObservatory,Osawa2-21-1,Mitaka,Tokyo181-8588,Japan

ofAstronomicalScience,TheGraduateUniversityforAdvancedStudies,Osawa2-21-1,Mitaka,Tokyo181-8588,Japan

Accepted0000December00.Received0000December00;inoriginalform0000October00

ABSTRACT

Weproposethatstrongmagnetic eldsshouldbegeneratedatshockwavesassociatedwithformationofgalaxiesorclustersofgalaxiesbytheWeibelinstability,aninstabilityincollisionlessplasmas.Thestrengthofthemagnetic eldsgeneratedthroughthismechanismisclosetotheorderofthoseobservedingalaxiesorclustersofgalaxiesatpresent.Ifthegenerated eldsdonotdecayrapidly,thisindicatesthatstrongampli cationofmagnetic eldsafterformationofgalaxiesorclustersofgalaxiesisnotrequired.Thismechanismcouldhaveworkedevenataredshiftof～10,andthereforethegeneratedmagnetic eldsmayhavea ectedtheformationofstarsinprotogalaxies.Thismodelwillpartiallybecon rmedbyfutureobservationsofnearbyclustersofgalaxies.Mechanismsthatpreservethemagnetic eldsforalongtimewithoutconsiderabledecayarediscussed.

Keywords:instabilities—magnetic elds—galaxies:general—galaxies:clusters:general

1INTRODUCTION

Thequestionoftheoriginofgalacticmagnetic eldsisoneofthemostchallengingproblemsinmodernastrophysics.Itisgenerallyassumedthatmagnetic eldsinspiralgalaxiesareampli edandmaintainedbyadynamothroughrotationofthegalaxiesThedynamorequiresseed eldstobeampli ed.However,observationsofmicrogauss eldsingalaxiesatmoderateredshiftsstronglyconstrainthelowerboundaryoftheseed elds(Athreyaetal.Moreover,magnetic eldsarealsoobservedinellipticalgalaxiesandgalaxyclusters,inwhichrotationcannotplayacentralroleasthedynamomechanism(Clarke,Kronberg,&B¨ohringer2001;Widrow2002;Vall´ee2004).

TheWeibelinstabilityisanothermechanismtogener-atestrongmagnetic elds(WeibelFriedThisinstabilityisdriveninacollisionlessplasma,oratenuousionisedgas,bytheanisotropyoftheparticlevelocitydis-tributionfunction(PDF)oftheplasma.WhenthePDFisanisotropic,currentsandthenmagnetic eldsaregen-eratedintheplasmasothattheplasmaparticlesarede- ectedandthePDFbecomesisotropicThroughtheinstability,thefreeenergyattributedtothePDFanisotropyistransferredtomagnetic elden-ergy.Thisinstabilitydoesnotneedseedmagnetic elds.Itcanbesaturatedonlybynonlineare ects,andthus

themagnetic eldscanbeampli edtoveryhighval-ues.Thisinstabilityhasbeenobserveddirectlyinre-centlaserexperimentsInastrophysicalplasmas,theinstabilityisexpectedtodevelopatshocksoratsteeptemperaturegradients,wherethePDFisanisotropic.Examplesofthesitesarepulsarwinds,shocksproducedbygamma-raybursts,jetsfromactivegalacticnuclei(AGNs),cosmologicalshocks,andcoldfronts(con-tactdiscontinuitiesbetweencoldandhotgas)inclus-tersofgalaxies(Medvedev&LoebKazimuraetal.1998;Nishikawaetal.2003;Schlickeiser&2003;Okabe&Hattori2003).Althoughtheinstabilitywasfoundin1959,itsnonlinearnaturehadpreventedusfromunder-standingitslong-termevolution.Recently,however,ascom-puterpowerincreases,detailedparticlesimulationsofplas-mashavebeeninitiatedandtheyhaverevealedtheevolu-tionofmagnetic eldsevenaftersaturationoftheinstabilityetal.Basedonthesere-sults,weconsiderthegenerationofmagnetic eldsatgalaxyandcluster-scaleshocksthroughtheWeibelinstabilityattheformationofgalaxies(bothellipticalsandspirals)andclusters.Weusethecosmologicalparametersof 0=0.3,λ=0.7,theHubbleconstantofH0=70kms 1Mpc 1,andσ8=0.9.

E-mail:yfujita@th.nao.ac.jp(YF);tkato@th.nao.ac.jp(TNK)

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