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rich clusters of galaxies are the largest gravitationally bound structures in the universe and should confine a representative fraction of its mass . therefore the study of their dynamical properties and radiation content should allow , amongst other things , interesting cosmological conclusions on the relative amounts of visible and dark baryonic matter , and of nonbaryonic matter ( @xcite ) . another basic characteristic , due to energetic particle confinement , is the ratio of nonthermal to thermal energy in these objects . to a significant extent that ratio should be pre - determined during the epoch of early starburst activity and thus preserve the energetic history of cluster formation . the necessary confinement of the nonthermal particle components is intimately related to the existence of strong and chaotic magnetic fields in the intracluster medium ( icm ) , and we shall propose physical mechanisms for their early generation as well as for their present fluctuations .    in principle , detailed ab initio simulations of the dynamics of cluster formation under the dominant gravitational influence of the dark matter component ( @xcite ) should establish the overall cosmological framework for the present considerations . we rather start in a complementary way with the discussion of a simplified model of cluster formation and of chemical enrichment of the intracluster gas . it has the advantage that it directly allows a discussion of the physical processes of nonthermal particle production and confinement . the main part of the paper concerns a proposal of cluster magnetic field generation in terms of galactic winds due to early starbursts and their amplification effect on magnetic fields drawn out from the progenitors of today s cluster galaxies into intracluster space . it is argued that due to these dynamical processes there is no need for the operation of a dissipative turbulent dynamo in the icm . the ongoing cluster accretion naturally leads to a strong fluctuating part of the intracluster magnetic fields . a detailed discussion of the nonthermal radiation from galaxy clusters will be given in a separate paper ( @xcite ) . we shall be concerned here with rich clusters , i.e. conglomerates with typically more than 100 member galaxies . they have typical radii @xmath2 few mpc and baryonic masses @xmath3 . many such clusters are rather evolved and contain predominantly early type s0 and e - galaxies , at least in their inner parts . examples for bright and relatively nearby clusters of this type are the perseus and the coma clusters with distances @xmath4 . the perseus cluster is the brightest cluster in soft x - rays . the large x - ray luminosity is due to the very hot ( @xmath5k ) , massive ( @xmath6 ) , and metal - rich ( @xmath7_{\rm cl } \simeq 0.35 [ { \rm fe}]_{\odot}$ ] ) icm gas ( @xcite ) . as a consequence the gas pressures are extremely high , with @xmath8 ranging from @xmath9 . the metallicity of the icm gas , for instance in terms of the fractional icm iron mass , is correlated with the total optical luminosity in the e and s0 galaxies of rich clusters ( @xcite ) . the correlation supports the view that early starbursts due to galaxy - galaxy interactions of gas - rich progenitors have produced a large number of core collapse supernovae due to massive stars ( for simplicity referred to here as sne ) . they should have heated the originally present interstellar gas and generated violent galactic winds which removed the interstellar medium , leaving gas - poor e and s0 galaxies behind . this mass loss should have led to the observed strong chemical enrichment of the icm gas . we also conjecture that the ionizing radiation , the winds , and the large - scale shocks from these early galaxy mergers - together with the hard radiation from agns - strongly heated the remaining primordial icm gas , and thus prevented further galaxy formation . a quantitative discussion of the dynamical prerequisites for galactic winds and of the total number of sne in clusters is given by vlk et al . ( ( * ? ? ? * hereafter referred to as paper i ) ) . the total number of sne since galaxy formation in the cluster , roughly a hubble time @xmath10 ago , is then given by @xmath11}_{\odot } \times m_{\rm cl}}{\delta m_{\rm fe}}\;,\ ] ] where @xmath12 is the amount of iron produced per event . in such starbursts we dominantly expect core collapse sne from massive progenitor stars to occur , with @xmath13 on average . for the perseus cluster this implies @xmath14 . the corresponding total energy input into the interstellar medium is @xmath15 , where @xmath16 is the average hydrodynamic energy release per sn in units of @xmath17 .    assuming the early starbursts to occur at a typical redshift of @xmath18 due to the merging of gas - rich progenitors in an overdense protocluster environment ( @xcite ) , with a duration of @xmath19 , we obtain @xmath20 where @xmath21 is taken as 1/(30yr ) , and @xmath22 denotes the number of galaxies in the perseus cluster . as an example we can compare to the archetypical contemporary starburst galaxy @xmath23 . it has a current sn rate @xmath24 , a wind velocity @xmath25 , and a mass - loss rate of @xmath26 ( @xcite ) . the starburst nucleus of m82 is characterized by the following values for the interstellar gas temperature @xmath27 , gas density @xmath28 , and thermal gas pressure @xmath29 at the base of the wind : @xmath30 , @xmath31 , and @xmath32 ( @xcite ) . since the thermal icm gas pressure in the perseus cluster is @xmath33 , it is clear that an object like m82 could readily drive a wind even against the _ present - day _ icm pressure . at the galaxy formation epoch the icm pressure should have been much smaller than this value .    in an expanding galactic wind flow the sn - heated gas will cool adiabatically to quite small temperatures . however it will be reheated in the termination shock , where the ram pressure of the wind adjusts to the icm pressure . much beyond this point the ejected galactic gas is expected to slowly exchange energy and some metal - rich material with the unprocessed icm gas . cluster formation also implies the production of a strong nonthermal component of relativistic particles . they will be accelerated during the early phase - and presumably also in later accretion events that have shock waves associated with them . in the supernova remnants ( snrs ) the main acceleration should occur at the outer shock , with a very high efficiency of @xmath34 ( @xcite ; @xcite ) , the rest of @xmath35 going into the thermal gas ( @xmath36 ) ( @xcite ) , and radiation ( @xmath37 percent ) . however , since the particles are ultimately removed from the galaxies in a strong galactic wind ( see paper i and section 3 . below ) , they will cool adiabatically like the thermal gas and transfer their energy to the kinetic energy of the wind flow . as a consequence the original snr - accelerated particles should constitute a negligible fraction of the present - day nonthermal particle content of the cluster . but this is not the end of the story . at distances @xmath38 from the galaxies , fresh particle acceleration will occur at the strong galactic wind termination shocks ( fig . [ fig1 ] ) . we estimate the overall acceleration efficiency in these shocks to be again of the order of 10 percent . over the early phase of galaxy formation , using our estimate for the total number of sne , this should result in a total gas internal energy in the postshock region @xmath39 erg and a nonthermal energy @xmath40 erg for a system like the perseus cluster , ultimately driven by star formation and the subsequent sn explosions . since the confinement time in the cluster ( see next subsection ) exceeds the cluster lifetime , the energy spectrum of these particles in the icm is basically the same as the source spectrum at the termination shocks . in cosmic ray ( cr ) parlance these are _ cosmological _ crs . since the galaxies are distributed across the cluster quasi - uniformly , this should originally also be true for the nonthermal particle population . the ensuing gravitational contraction / accretion of the cluster will subsequently energize crs and thermal gas at least adiabatically or , more likely , shock accelerate / irreversibly heat both components so that finally the total energy @xmath41 of energetic particles should reach at least the adiabatic value @xmath42 in the cluster ; @xmath43 now denotes the total present internal energy of the icm gas ( paper i ) . it is worthwhile to compare the expected nonthermal energy with the thermal energy content of the cluster galaxies . assuming the stars internally to be in virial equilibrium and , for purposes of estimate , all of them to have a solar mass and radius , then @xmath44 . for a total mass of about @xmath45 contained in the _ galaxies _ of the perseus cluster this gives a total thermal energy in stars @xmath46 , and thus @xmath47 . this means that the nonthermal icm energy should be at least as large as the total thermal energy content of all the stars in all the galaxies together . it has been argued more recently that , apart from star formation and overall gravitational contraction , also individual giant radio galaxies should have injected large and in fact comparable amounts of nonthermal particles during the life time of a cluster ( @xcite ) . this is no doubt an important additional possibility . a weakness of this argument consists in the fact that per se it is predicated on statistical knowledge about the luminosity function for active galaxies in clusters in general , and not on direct observations of the individual cluster to which it is applied . energetic particle confinement has been discussed in a number of papers in recent years ( paper i , berezinsky et al . 1997 , colafrancesco & blasi 1998 ) . we review here this important point , also in the light of the magnetic field structure to be discussed in the next section .    the large - scale magnetic field in the icm gas may be quite chaotic and not well connected over distances exceeding typical intergalactic distances . thus energetic particles may not readily escape from the cluster due to such topological characteristics . however , already a consideration of pure pitch angle diffusion along straight magnetic field lines with superposed turbulent fluctuations gives important insights into the confinement properties of galaxy clusters . standard quasilinear theory yields a spatial diffusion coefficient @xmath48 along the large scale field @xmath49 due to a power spectrum @xmath50 of magnetic field fluctuations with wavelength @xmath51 in the following form : @xmath52 where @xmath53 is the gyro radius of a particle with momentum @xmath29 , @xmath54 , and @xmath55 denotes the wavenumber of the field fluctuations . let us assume a relative fluctuation field strength of order unity at the inter - galaxy distance @xmath56 , i.e. a totally turbulent field @xmath57 on this scale , and a power law form of @xmath58 . then the diffusion time across the cluster @xmath59 for @xmath60 , for @xmath61 and @xmath62 , respectively ( paper i ) . also @xmath63 for nuclear collisions in the icm gas . therefore - except at subrelativistic energies with their prevailing coulomb losses - up to these energies cr hadrons should accumulate in the cluster since the galaxy formation epoch , and that is what we called _ cosmological _ crs before . the situation is different for relativistic electrons , which suffer radiative losses . electrons , and the nonthermal radiation from galaxy clusters are discussed extensively in the paper by atoyan & vlk ( 2000 ) . the accumulated cosmological cr protons and nuclei will produce high energy @xmath64 rays from inelastic pp - collisions on the intracluster gas which , in particular , lead to @xmath65-production and subsequent decay . [ fig2 ] shows the @xmath65-decay energy fluxes expected from the hadronic crs of the coma cluster with a differential energy spectrum @xmath66 , with @xmath67 and an upper cutoff energy @xmath68 . the solid and the dashed curve assume @xmath69 and @xmath70 erg , respectively , in an icm with a gas density of @xmath71 . also the egret upper limit by sreekumar et al . ( @xcite ) is shown . the observed size of the radio emission produced by high - energy electrons in coma is about half a degree . the brightness of the _ hadronic _ tev emission should be significantly higher in the central region of the cluster , since the icm gas density strongly increases towards the center . detection of an extended and weak ( @xmath720.1 crab ) tev flux by current instruments is problematic , but such fluxes are quite accessible for future imaging atmospheric cherenkov telescope arrays like h.e.s.s . , veritas , and cangaroo iii ( see fig.2 ) . the magnetic field strengths in the icm of rich clusters , which may be as large as @xmath73 as inferred by faraday rotation measurements ( @xcite ) , are not easily explained by a contemporary dissipative mechanism because present - day turbulent dynamo effects in such a large - scale system should be extremely slow . this problem is compounded by the extraordinary smallness of the expected intergalactic seed fields . therefore we suggest a field configuration that is due to the early formation history of galaxy clusters as discussed above ; it should be preserved in its essential features to this day ( @xcite ) . this field should even be still in a state of development at the present epoch . the model derives from the violent early galactic winds which accompany the starbursts responsible for the predominance of the early type galaxies in rich clusters .    in a general form the ejection of galactic fields has been discussed by kronberg ( @xcite . for the generation of the general intergalactic magnetic field , related arguments have been advanced independently by kronberg et al . ( @xcite ) , assuming very early formation of dwarf galaxies with wind outflows at redshifts @xmath74 . we assume first of all that the gas - rich progenitors whose mergers supposedly constitute the building blocks for the e and s0 galaxies , can be specifically pictured as protospirals that had already generated galactic magnetic fields of @xmath75 g strength . this should indeed be possible within a time of @xmath76 yr or less , i.e. a time scale of the order of a rotation period of our galaxy or even shorter . the explanation derives from fast turbulent dynamo action that invokes boyancy effects due to crs that inflate the magnetic flux tubes together with magnetic reconnection over spatial scales of order @xmath77 ( @xcite ) . in starburst galaxies also systematic dynamo effects might play an important role ( section 3.1 ) .    in a second stage , i.e. during the galaxy mergers , the resulting supersonic galactic winds will extend these fields from the interacting galaxies to almost intergalactic distances . in the final and by far longest stage that lasts until now , the fields should be recompressed by the contraction of the cluster to its present size . in addition , the continuing accretion of subclusters and individual galaxies constantly perturbs this field , keeping the fluctuations around this large scale field at a high level . the icm fields do not reconnect on the intergalactic scale in a hubble time . consequently there is no need for a continuous regeneration of these fields since their formation . however , this also implies that a topologically connected overall icm field will on average not be formed either and that the icm field is chaotic on a scale smaller or equal to the present intergalactic distance . in detail we draw on arguments which we have used in the past for the field configuration in a galactic wind from our own galaxy ( @xcite ) , see also fig . they are based on estimates of the relative amount of field line reconnection vs. the extension of galactic field lines by a wind to `` infinity '' ( @xcite ) . the basic result is that the rates of reconnection - and thus of the formation of `` parker bubbles '' leaving the galaxy by their boyancy and allowing the generation of the disk magnetic field - and the rates of extension of this field into the galactic halo by the pressure forces of the wind are roughly equal . thus both effects occur with about equal probability . for the cluster galaxies this means that magnetic energy can be generated on the large scale of the wind at the expense of the thermal and nonthermal enthalpies produced in the starburst . the geometry of the field should roughly correspond to straight field lines out to meridional distances @xmath78 of the order of the starburst ( sb ) radius , @xmath79 kpc in the protogalactic disk , and spherically diverging field lines beyond that . the slow rotation of the system should then lead to an azimuthal field component , decreasing with the wind distance @xmath80 , which dominates at large distances over the radial component . however , in contrast to the familiar situation in the solar wind equatorial plane , the axis of rotation is rather parallel than perpendicular to the flow at the base of the wind , and thus the dominance of the azimuthal field component is by no means as drastic as in the case of a stellar wind ( fig . [ fig3 ] ) . the wind becomes supersonic at about the same critical distance , @xmath81 . far beyond this critical point the mass velocity @xmath82 becomes constant and the density falls off @xmath83 .      choosing a present average baryon density @xmath84 , i.e. assuming most of the baryonic matter to be in the form of intergalactic gas , the mean density at the formation stage of the early type galaxies ( at redshifts @xmath85 ) was then @xmath86 . with a present icm density @xmath87 , we have @xmath88 due to gravitational compression of the icm gas . with a dominant thermal gas pressure the corresponding adiabatic pressure increase @xmath89 amounts to @xmath90 ; for the following we shall use @xmath91 . the wind termination shock distance @xmath92 is then given by @xmath93 , where @xmath94 is the wind mass density upstream of the shock .    to estimate the wind characteristics we assume a quasi - steady state and a strong starburst for which we may disregard gravity , magnetic forces and cr pressure gradients in the overall energy balance equation ( e.g. @xcite ) . it reads in this case :    @xmath95_s \simeq \frac{\gamma}{\gamma -1}\frac { p_{\rm base}}{\rho_{\rm base}},\ ] ] where in addition @xmath96 is assumed at the base @xmath97 of the wind . the critical point @xmath98 is given by @xmath99 . approximately @xmath100 , where @xmath101 is the asymptotic wind speed . beyond the critical point at @xmath102 the wind achieves spherical symmetry so that from mass conservation    @xmath103 if the icm pressure is small compared to the base pressure then we can assume that the wind is highly supersonic at the shock distance which implies @xmath104 . this should be true for m82-like objects - although they should have a considerably larger scale @xmath105 kpc - which have @xmath106 . in this case we can assume the wind pressure to evolve adiabatically to lowest order , so that    @xmath107 as a result , taking into account that in a flow dominated by the thermal gas the adiabatic index @xmath108 @xmath109^{1/2 } \left(\frac{p_{\rm base}}{p_{\rm cl}}\right)^{1/2 } \simeq 1.27~ \left(\frac{p_{\rm base}}{p_{\rm cl}}\right)^{1/2 } \simeq 400\ ] ] for the above starburst parameters . therefore @xmath110 the wind bubble containing the shock - heated wind gas will have a radius still exceeding @xmath92 . beyond the bubble , part of the external gas will be shocked by the rapidly expanding bubble gas . this shock heated gas will exchange energy with the cold ambient gas by heat conduction and instabilities . to some extent such exchanges will also take place with the bubble gas . even though the volume of at least initially unmagnetized icm gas may be large enough so that the icm gas mass exceeds the mass associated with galaxies by a factor of a few - as observed - it may therefore be that some wind bubbles touch . thus we should consider whether the magnetic fields of the bubbles can reconnect with each other to produce field lines that pervade the entire cluster . let us assume that the fastest rate of reconnection proceeds with a speed between 1 and 10 percent of the alfv@xmath111n speed @xmath112 , to be specific , with @xmath113 ( @xcite ) . then the present - day reconnection time across a termination shock scale is @xmath114 greater than a hubble time , even for a present - day icm magnetic field as high as @xmath1 g. therefore , many bubble fields might not yet be reconnected and much of the field structure could well remain topologically disconnected until today . the field strength @xmath115 in the early wind bubbles should be of the order of @xmath116 or somewhat larger , if the field in the bubble increases in the decelerating postshock flow . the subsequent and still ongoing overall cluster contraction / accretion compresses the field to lowest order isotropically @xmath117 , with the scale factor @xmath118^{1/3}/(1+z),\ ] ] where @xmath119 and @xmath120 . for a present mean baryon number density @xmath121 , we obtain @xmath122 .    from these estimates , we obtain for the present - day icm field : @xmath123 . therefore the present - day icm magnetic field should have a mean strength of the order of @xmath0 g , from @xmath124 `` primordial '' seed fields , and should be randomly directed on an intergalactic scale . although smaller by about one order of magnitude than estimated from faraday rotation measurements , such fields need not necessarily be unrealistic , considering that observations might emphasize regions of high magnetic fields . indeed the simplest inverse compton interpretation of the euv excess and the excess x - ray flux in the coma cluster requires such low field strengths ( @xcite ) . an additional possibility is that the `` initial '' fields for such starburst galaxies might be an order of magnitude stronger than assumed . apart from a fast dynamo whose strength could be directly proportional to the star formation rate ( @xcite ) , we could invoke a systematic field amplification through the commencing galactic outflow that `` combs '' the field outward . it might occur on very short time scales of about @xmath125 yr . empirically this field amplification is suggested by the statistical time - independence of the radio synchrotron to far infrared emission ratio in starburst galaxies ( @xcite ) which can hardly be understood otherwise than through a field that increase almost simultaneously in strength with the star formation rate . a final argument is that the large scale shear deformations induced by the later accretion of large subclusters may amplify the field even further . thus we can not exclude @xmath75 g fields ; the force balance certainly allows them . the wind bubbles and the associated magnetic fields should at some stage decouple from the galaxies they emanated from , simply by magnetic reconnection which is fastest near the galaxies : the alfv@xmath111n velocity is approximately independent of the meridional distance s in the wind , whereas the distance between oppositely directed field lines that emanate from the galaxy is obviously the smaller the smaller s is . thus , after the termination of the starburst , the magnetized bubbles and the stellar component of the remaining early type galaxies should acquire independent identities and their dynamics should decouple . the topologically disconnected structure of the mean magnetic field in the cluster has also some bearing on the evolution of magnetic field strengths during the development of cooling flows towards the cluster center ( @xcite ) . instead of building up global magnetic pressure gradients and tension forces such a subsonic and sub - alfv@xmath111nic flow allows optimal internal segregation of high and low field regions on galaxy - galaxy seperation scales . thus we should expect that the field strength towards the cluster center increases by less than by isotropic compression , @xmath126 , even though at the compressed spatial scales reconnection will be more effective . the two - thirds law should hold only for the overall cluster gas compression discussed earlier . a question of direct importance for the interpretation of faraday rotation measurements is the scale of reversal changes of these fields . field reversals are a natural consequence of the suggested field structure , given the field pattern in the interstellar media of the starburst galaxies . if the milky way can serve as a guide , then this pattern is determined by the 100 pc scale of the parker instability ( parker 1966 ) . from the radius @xmath127 to distances @xmath128 this scale projects like the ratio @xmath129 . using the above estimates this implies a field reversal scale @xmath130 kpc in clusters , rather well in line with observational estimates which indicate reversal scales of @xmath131 to @xmath132 kpc ( @xcite ; @xcite ) .      from the foregoing arguments there is hardly any need for a contemporary `` turbulent icm dynamo '' . however , the ongoing accretion will perturb this mean intracluster field randomly in space and time , maintaining a turbulent magnetic fluctuation field that develops smaller and smaller spatial scales . due to the topology of the mean field , the largest turbulent scale is given by the distance between galaxies . the accretion will probably span the range from single field galaxies falling into the cluster to accreting massive subclusters . as long as this accretion process remains important , one has to expect that the cluster rings with it . at the largest scale the relative magnetic fluctuation level should approach unity ( see subsection 2.3 ) . aharonian , f. a. , hofmann , w. , konopelko , a. k. , & vlk , h. j. 1997 , astropart . phys . , 6 , 343 ; 369 arnaud , m. , rothenflug , r. , boulade , o. , vigroux , l. , & vangioni - 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we propose a mechanism for the early generation of the mean intracluster magnetic field in terms of magnetized galactic winds . 
 these winds are the result of starburst phases of the cluster galaxies , assumed to produce the predominant population of early type galaxies in mergers of gas - rich progenitors . after further cluster contraction 
 typical field strengths are @xmath0 gauss . 
 this estimate may increase to the level of @xmath1 gauss if more extreme galactic parameters , and subsequent shear amplification of the field are considered . 
 the topology of the field is one of almost unconnected wind bubbles with parker - type spiral field configurations over scales of the distance between galaxies . 
 further cluster accretion , that continues chaotically in space and time up to the present , will perturb these `` large - scale '' mean fields on smaller or at best comparable spatial scales . 
 the small scale fields in the resulting turbulent fluctuation spectrum should be able to confine relativistic particles over times longer than the age of the universe . 
 the nonthermal particle content of galaxy clusters should therefore also have a `` cosmological '' hadronic component generated during the early starburst phase of the member galaxies . already by itself 
 it implies a nonthermal energy fraction of about 10 percent for the intracluster gas which should then be detectable by future gamma - ray telescopes .