arXiv:0901.0111v1 [astro-ph.GA] 31 Dec 2008
Pro eedingsIAUSymposiumNo.259,2009
K.G.Strassmeier,A.G.Kosovi hev&J.E.Be kman,eds.
2009InternationalAstronomi alUnion DOI:00.0000/X000000000000000X
Cosmic-ray driven dynamo
in galactic disks
M. Hanasz
1, K. Otmianowska-Mazur
2, H. Lesch
3, G. Kowal
2;4,
M. Soida
2, D. W´
olta´
nski
1K. Kowalik
1, R.K. Paw laszek
1B. Kulesza- ˙
Zydzik
2 1Centrefor Astronomy,Ni holasCoperni usUniversity,PL-87148Piwni e/Torun,Poland, mhanaszastri.uni.torun.pl
2
Astronomi alObservatory,JagiellonianUniversity,ul.Orla171, 30-244Krakow,
3
Astronomi alObservatory,Muni hUniversity,S heinerstr.1,D-81679, Germany,
4
DepartmentofPhysi sandAstronomy,M MasterUniversity,1280MainSt.W., Hamilton,ONL8S4M1,Canada.
Abstract.
We present newdevelopmentsonthe Cosmi {Ray driven,gala ti dynamo, mod-eledbymeansofdire t,resistiveCR{MHD simulations,performed withZEUSandPIERNIK odes.Thedynamo a tion,leading tothe ampli ationoflarge{s ale gala ti magneti elds on gala ti rotation times ales,appearsas a result of gala ti di erential rotation, buoyan y ofthe osmi ray omponentandresistivedissipationofsmall{s aleturbulentmagneti elds. Ournewresultsin ludedemonstrationoftheglobal{gala ti dynamoa tiondrivenbyCosmi Rayssuppliedinsupernovaremnants.Anessentialout omeofthenewseriesofglobalgala ti dynamomodelsistheequipartitionofthegasturbulentenergywithmagneti eldenergyand osmi rayenergy,insaturatedstatesofthedynamoonlargegala ti s ales.Keywords.
Galaxies:ISM{magneti elds{ISM: osmi rays{magneti elds{MHD1. Introduction
Numerous pro esses have been proposed to explain initial magneti elds in early galaxies. However all these pro esses, like phase transitions in the early universe or Biermann battery in protogala ti obje ts provide only very week magneti elds of the order of B 10
20
G at the beginning of gala ti evolution. On the other hand, Rees(1987)proposedthatinitialmagneti eldsingalaxiesmighthavebeengenerated in rst stars and then s attered in the interstellar medium (ISM) by SN explosions, and subsequentlyampli ed in plerioni (Crab{type) supernova remnants(SNRs). The meanmagneti eldonthegala ti s alehasbeenestimatedtobeoftheorderof10
9 G. Radioobservations(seeBe kBe k2009,thisvolume)indi atethattypi al ontemporary magneti eldsinspiralgalaxiesareoftheorderoffewuptofewtensofG,whi hmeans that magneti eldshaveundergoneampli ationbyatleastfourordersofmagnitudes withinthegala ti lifetime.Therefore,amodelforeÆ ientmagneti eldampli ation duringgala ti evolutionisne essary.Thestandardmodelofmagneti eldampli ation indiskgalaxiesisbasedonthetheoryofturbulentmean elddynamo(seeWidrow2002 forare entreview).
Themean{ elddynamotheoryhasbeensu essfulinexplainingmagneti eldsin var-iousastrophysi alobje ts,however,the lassi al,kinemati dynamo seemsto berather slowingalaxies.Magneti eldampli ation times alet
dynamo
(0:51)10 9
too longto explain 1G magneti elds ingalaxies whi h are only few 10 9
yrs old (z 13) (Wolfe,Lanzetta &Oren1992, Kronbergetal.2008). These ir umstan es seem to indi ate a need of an alternativeapproa h.We suggest that dire tnumeri al MHDmodelingofISMdynami s,through onsiderationsofdetailedphysi alpro esses, involvingmajor ISM omponents:gas, magneti eldsand osmi rays,is ne essaryto followmagneti historyofgalaxies.
2. Cosmic Rays in the interstellar medium
The dynami alrole of CRswas re ognized forthe rst time by Parker (1966), who noti edthataverti allystrati edISM, onsistingofthermalgasmagneti eldandCRs is unstable due to buoyan y of the weightless omponents: magneti elds and CRs. The CR omponentappears to be an important ingredient of this pro ess. A ording to thedi usivesho ka elerationmodelsCRsare ontinuouslysuppliedto ISMbySN remnants.Therefore,strongbuoyan ye e ts duetoCRsareunavoidable.
Theories ofdi usivesho ka elerationpredi t thatabout10% ofthe 10 51
ergof theSN II explosionenergyis onvertedto theCRenergy(see e.g.Joneset al1998and referen estherein).Observationaldataindi atethatgas,magneti eldsandCRsappear inapproximateenergeti equipartition,whi h meansthatallthe three omponentsare dynami ally oupled.Moreover,numeri alexperimentsbyGia alone&Jokipii (1999) indi ate that CRs di use anisotropi ally in the ISM, along the mean magneti eld dire tion.
3. Cosmic Ray transport
Toin orporateCRpropagationinMHD onsiderationsweusethedi usion-adve tion equation(e.g.S hli keiser&Ler he1985)
e r t +r(e r v)= p r rv+r( ^ Kre r ) (3.1) +CR sour es(SNremnants)
where the di usion term in written in the tensorial form to a ount for anisotropi di usivity of CRs. The sour es of CRs on the rhs. of Eqn. 3.1 orrespond to the CR produ tioninSupernovaremnants.
Thedynami sofISMin ludingtheenergeti sofCRs anbenowdes ribedbytheCR transportEqn. (3.1) andthe system of MHD equations.Wenote that inthe presen e of osmi rays additionalsour e term: rP
CR
shouldbe in ludedinthe gasequation ofmotion (seee.g.Berezinskiet al.1990),inordertoin orporatethee e tsof CRson gasdynami s.The osmi raydi usion{adve tionEqn.(3.1)hasbeensupplementedto theMHDalgorithmofZeus{3DMHD odebyHanasz&Les h(2003a)withtheaimof studyingISMphysi alpro esses,e.g.Parkerinstability,inwhi hCRsplayadynami ally importantrole.
4. CR-driven dynamo
CR{drivendynamohasbeenraisedbyParker(1992).Our rstlo al(shearing{box)CR{ drivendynamonumeri alexperiments(Hanaszetal.2004,2006;Otmianowska-Mazuret al.2007)relyonthefollowingingredients:(1.)the osmi ray omponentdes ribedbythe di usion{adve tiontransportequation (3.1),in ludinglo alizedsour esof osmi rays | supernovaeremnants,explodingrandomlyinthediskvolume,whileSNsho ksand thermale e tsarenegle ted(seeGresseletal.2008a,2008b)forare entdynamomodel relyingonthethermalenergyoutputfromSNe);(2.)resistivityoftheISM(seeHanasz, Otmianowska{MazurandLes h2002),leadingtotopologi alevolutionofmagneti elds; (3) shearing boundary onditions (Hawley, Gammie and Balbus 1995) together with Coriolisand tidalfor es, aimed at modelingof di erentiallyrotatingdisksinthe lo al approximation;(4)realisti verti aldiskgravityandrotationfollowingthemodelofISM intheMilkyWaybyFerriere(1998).
In the most re ent paper (Hanaszet al. 2009) we presenta parameterstudy of the CR{drivendynamomodelbyexaminingthedependen eofmagneti eldampli ation on magneti di usivity, supernovae rate determining the CR inje tion rate, temporal modulation of SN a tivity, grid resolution, and CR di usion oeÆ ients. We nd the dominatingin uen eofmagneti di usivity(treatedasa freeparameter),amongother parameters,(Fig.1)ontheeÆ ien yof magneti eldampli ation.
Figure 1.
Timeevolutionofazimuthalmagneti uxandtotalmagneti energyfordi erent valuesofmagneti di usivityinsimulationseriesA.The urvesrepresent respe tively asesof =0(A1),=1(A2),=10(A3),=100(A4)and=1000(A5)inunits p2 Myr
1 .
Thefastest magneti eldampli ation,observedintheexperiments, oin ideswith the magneti di usivity omparable to '
1 3 turb (obs), where turb (obs) ' 1 3 100p 10kms 1 ' 10 26 ms 1
. The question of whi h physi alpro ess an be responsible forthat largemagneti di usivity oeÆ ientspointsouttowardsthe urrentstudieson magneti re onne tion(Lazarianetal.2004,Cassaket al.2006)
Toinvestigateobservationalpropertiesof urrentdynamomodelsweperformaseries of numeri al simulations, pla ing the lo al shearing{boxes at di erent gala to entri radii. We repli ate the ontents of lo al ubes into rings, and then ombine rings to build upa disk(Otmianowska{Mazuret al.2009).A syntheti polarizationradio{map of syn hrotronemission onstru ted, on thebase of simulatedmagneti elds andCR
Figure 2.
Syntheti radio{mapofpolarizedradioemissionfor ourmodelatthein lination i=80Æ
andamapofrealgalaxyNGC5775
Apolarizedemissionmaptogetherwithsuperposedpolarizationve torsofsyn hrotron emission of an edge{on galaxy NGC 5775 is shown for omparison. As it is apparent the X{shaped stru turesof magneti eld, presentinthe realgalaxy,formalso inthe syntheti radioimage.ThepresentedresultsdemonstratethattheCR{MHDsimulations provideawayofveri ationofCR{dynamomodelsagainsttheobservationaldata.
Tosummarizetheoverallout omeofthelo al simulationsofCR{drivendynamo,we notethatthemodelprovideseÆ ientampli ationoflarge{s alemagneti elds,whi h attain,inthesaturatedstate,equipartitionwithturbulentgasmotions.However,wenote alsoadrawba kofthelo almodels,sin etheCRenergydensityremainsmorethanan order of magnitude larger than gas and magneti energy densities. We re ognize that improvementsofthe urrentmodelsarene essary,sin ethepossiblereasonoftheex ess of osmi rays may resultfrom thee e t of trapping of osmi raysby predominantly horizontalmagneti eldinthe omputationalboxwhi hisperiodi inhorizontal dire -tions. Therefore,globalgala ti disksimulationsshouldbeperformed toenablees ape ofCRsalongthepredominantlyhorizontalmagneti eldinthegala ti plane,inorder to avoidtheCRex ess.
5. Global disk simulations
Re entlywestartedanewseriesofglobalsimulationsofCR{drivendynamowiththe aid of PIERNIKMHD ode (seeHanasz et al.2009a, 2009b, 2009 , 2009d),whi h is a grid{basedMPIparallelized,resistiveMHD ode basedontheRelaxingTVD (RTVD) s hemeby Jin& Xin(1995) andPen et al.(2003). Theoriginals heme isextended to dealwiththedi usiveCR omponent(seeHanasz&Les h2003a).
In Fig. 3 we show results of one of the rst semi{globalsimulations of CR{driven dynamo.Thesimulationshavebeenperformedforaquarterofgala ti diskresembling MilkyWay,withparallelCRdi usion oeÆ ientK
k =310 28 m 2 s 1 ,ina omputa-tionaldomain20kp 20kp 20kp ,andresolutionof400400160grid ells,in 16MPIblo ks.
Figure 3.
Horizontal and verti al sli es through the omputational box for a simulation of CR{drivendynamoinasemi{globalgala ti disk.The olormapsre e t:1:gasdensity,2:CR energydensity,3:magneti eldstrength,4.theratioofmagneti toCRenergydensitiesand5. syn hrotronemissivityforasaturatedstateofthedynamo.Thegray{s alesinallpanels,ex ept 4.are hosenarbitrarily.Thedarkershades orrespond tolargervaluesofdisplayedquantities. Thegreys aleofpanel4.issu hthatbla kareasrepresentvaluesofemag=e r'1.eld: growth time of the mean magneti ux is approximately 210 Myr in the whole galaxy,whi his loseto thegala ti rotationperiodneartheorbitofSun.Asexpe ted, the globalsimulationsintrodu e a signi antimprovementwith respe t to the former shearing{boxsimulations.It is apparentinFig. 3that theratioof magneti to osmi ray energy is lose to one in the syn hrotronemitting volume( ompare panels 4.and 5.), whileover{equipartition of CRenergy with respe t to magneti energy still holds outsidethedisk.
ThelatestdevelopmentofourCR{dynamomodelisafullyglobalgala ti disk simula-tion(seethe omplementarypaperbyHanasz,etal.thisvolume),wherewedemonstrate thatdipolarmagneti eldssuppliedonsmallSN{remnants ales, anbeampli ed expo-nentiallybytheCR{drivendynamotothepresentequipartitionvalues,andtransformed simultaneouslyto largegala ti {s ales.
6. Conclusions
WehaveshownthattheCR ontributiontothedynami sofISM,studiedbymeansof CR{MHDsimulations,inbothlo alandglobals ales,leadstoaveryeÆ ientmagneti eldampli ationingala ti disks.
The osmi ray driven dynamo ampli es eÆ iently gala ti magneti eldson a ti-mes aleofgala ti rotation.Theresultinggrowthofthelarge{s alemagneti eldby4 ordersofmagnitudewithin 2Gyr(Hanaszatal.2004),isfastenoughtoexpe t1G magneti eldingalaxies atz13.
Wepointouttheadvantageofglobaldiskmodels,whi honthe ontrarytoshearing{ box models, provide CR energy equipartition with magneti eld in the syn hrotron emittingpartofthedisks.
A knowledgements
ThisworkwassupportedbyPolishMinistryofS ien eandHigherEdu ationthroughthe grants92/N{ASTROSIM/2008/0and PB0656/P03D/2004/26and byNi olaus
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