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observations of both cool and hot intergalactic gas make it clear that heavy elements are by no means confined to galaxies . larson ( 1974 ) predicted that late in the galaxy formation process , galactic winds would carry heavy elements out into intergalactic space . soon afterwards x - ray spectra of intracluster gas confirmed that the gas was quite metal - rich . in recent years observations of absorption lines in qso spectra have elucidated the impact of supernovae on the diffuser gas that is found in the field , at least at high redshift . these observations imply that , from a redshift @xmath1 onwards , there does not seem to be a parcel of gas that has not felt the impact of spernovae , probably mediated by galactic winds .    while the ubiquity of the products of supernovae is beyond doubt , many aspects of the interaction of supernovae and intergalactic gas are highly uncertain . outstanding questions include : ( i ) in what types of galaxy were intergalactic metals manufactured ? ( ii ) which type of supernova ( core - collapse or ia ) has been most important , and at what epoch ? ( iii ) how important have supernovae been for the entropy budget of intergalactic gas ? most of the luminosity in the universe comes from spiral galaxies like the milky way . do such galaxies blow sn - driven winds ?    studies of the local ism show that a significant fraction of the pressure in the ism comes from cosmic - rays and the magnetic field . supernovae and fast stellar winds are the main energy sources of the ism , so the dynamical importance of cosmic rays and magnetic fields suggests that a significant fraction of the kinetic energy which these objects pump into the ism is chanelled into cosmic rays and magnetic fields . hence , assuming there to be @xmath2 supernova per century , the cosmic - ray reservoir is being energized at a rate @xmath3 . how does this power , which could build up the observed interstellar pressure within a disk @xmath4 in radius and @xmath5 thick within @xmath6 , manifest itself ? the local cosmic - ray energy spectrum is such that the energy is overwhelmingly contained in only mildly relativistic particles . such particles have enormous lifetimes because they are too fast to suffer much coulomb scattering and too slow to produce significant synchrotron radiation  which explains why the non - thermal radio luminosity of galaxies like the milky way is @xmath7 ( condon 1992 ) . hence , the energy imparted by spernovae can not be radiated either directly or by transfer to thermal particles , and is not simply building up within the disk . so it must drive expansion of the cosmic - ray plasma : the milky way must be blowing a wind that is working on the local igm at a rate @xmath8 . what volume within the local group would this wind have filled to the present epoch ? there are two ways of answering this question . if the wind were expanding into a vacuum , it would expand at @xmath9 and extend to @xmath10 in a hubble time . this gives us an upper limit on the radius of influence of an @xmath11 galaxy . a lower limit comes from assuming that the extragalactic baryons that are required by primordial nucleosynthesis theory were once distributed in galactic halos like dark matter , and the sn - driven wind raised its temperature by of order the virial temperature . we assume that @xmath12 and @xmath13 , so with baryons following dark matter in a singular - isothermal halo of circular speed @xmath14 , the baryonic mass interior to radius @xmath15 is given by @xmath16 . equating the energy required to heat this material by the virial temperature to the time - integral of the sn - power , we find , so @xmath17 is independent of @xmath14 . ] @xmath18 we see that even the present supernova rate within the milky way would impact the igm out to near the mid point to m31 , and in reality the sn rate has almost certainly been substantially larger in the past . recently , tripp , savage & jenkins ( 2000 ) have suggested that o@xmath19 absorption in quasar spectra points to a major reservoir of baryons in warm ( @xmath20k ) gas in galaxy groups . as pen ( 1999 ) points out , the thermal energy of this gas can not derive from gravity alone : without non - gravitational heating it would be so clumpy that its soft x - ray emission would violate the constraint imposed by the unresolved component of the x - ray background . it is tempting to conclude that galactic winds provide the required heat source ( see below ) . the most compelling evidence for the impact of supernovae comes from rich clusters of galaxies . arnaud et al ( 1992 ) show that for these objects gas mass and @xmath21-band luminosity are highly correlated  @xmath22 . at least in part the steepness of this correlation will arise for two reasons . first , more luminous clusters tend to have higher frations of early - type galaxies , which have lower @xmath23 per unit stellar mass , @xmath24 . second , lower - luminosity clusters are less likely to attract infall , and more likely to suffer outflow of gas  renzini ( 1997 ) finds that systems with temperature in excess of @xmath25 do not lose gas . arnaud et al . note that @xmath26 is as tightly correlated with the luminosity from e and s0 galaxies as with total luminosity : @xmath27 and argue that the exponent in this relation can be taken to be unity , so that with @xmath28 for early type galaxies set to @xmath29 , @xmath30    from the fact that @xmath31 it is clear that much of the igm has never been in a galaxy . while the bulk of the gas may be primordial , the heavy elements in it are most certainly not . with @xmath29 the estimate of arnaud et al . ( 1992 ) becomes @xmath32 while renzini ( 1997 ) finds that for clusters hotter than @xmath33 @xmath34 independent of cluster temperature . again adopting @xmath29 , renzini s value of the fe abundance becomes @xmath35 the igm - abundances of several @xmath36-elements , o , ne , mg , and especially si , have been determined for many clusters ( mushotzky et al . when meteoric rather than solar - photospheric abundances are used as a point of reference , the igm proves to be only mildly @xmath36-enhanced  by a factor @xmath37 relative to solar ( brighenti & mathews , 1999 ) .    what does nucleosynthesis theory have to say about these mass fractions ? the picture is confused by uncertainties in the yields of fe and to a certain extent si , from core - collapse sne , and the rates ( current and past ) of type ia sne .    a key input into nucleosynthetic theory is the initial mass function ( imf ) . i shall assume that for @xmath38 this is of salpeter s form , since there is now significant evidence that the imf is universal ( which is surprising ) and lies near salpeter s form at larger masses . in particular , recent work based on hipparcos parallaxes ( binney , dehnen & bertelli , 2000 ) does not support the contention of scalo ( 1986 ) that the local imf is steeper than salpeter at @xmath38 , while kennicut , tamblyn & congdon ( 1994 ) argue that measurements of h@xmath36 fluxes from external disks rule out a steep imf .    for a salpeter imf extending between @xmath39 and @xmath40 we expect 0.0068 core - collapse sne per m@xmath41 of star formation , and @xmath42 of the initial stellar mass would by now have been returned to the ism . the average core - collapse sn is expected to inject @xmath43 of si and @xmath44 of fe into the ism ( gibson , loewenstein & mushotzky , 1997 ) , so from core - collapse sne we expect @xmath45 of si and @xmath46 of fe , where @xmath47 is the mass in stars before the onset of mass loss . these masses fall short of those observed [ eqs  ( [ mfea ] and ( [ mfer ] ) ] by a factor of order 2 . the calculation ignores the existence of intergalactic stars ( mendez et al . , 1996 ) , but it also ignores the retention of heavy elements within galaxies . in practice these two omissions will roughly cancel , and a more elaborate calculation would leave us significantly short of heavy elements . the resolution of this shortfall is controversial . some argue for a flatter than salpeter imf ( david , 1997 ; gibson et al . , 1997 ) . others argue that the missing heavies were produced by type ia supernovae ( renzini et al . , 1993 ; ishimaru & arimoto , 1997 )  after all more than half of the fe in the solar neighbourhood is thought to come from type ia sne . a type ia sn injects @xmath48 of si and @xmath49 of fe into the ism ( thielemann , nomoto & hashimoto , 1993 ) , so these objects inject four times as much fe as si , whereas core - collapse sne inject at least as much si as fe . consequently , the observed ratio of si to fe masses in the igm ( @xmath50 ) constrains the importance of type ia sne . the big problem with imposing this constraint at the present time is uncertainty in the ratio of mean yields of si and fe from core - collapse suprnovae : if as , is perfectly plausible , core - collapse supernovae produce significantly less fe than si , one will need a good number of type ia sne to bring the overall ratio @xmath51 down to @xmath50 as observed . the current rate of type ia sne can in principle be determined in two ways : ( i ) direct observation ( with still depressingly uncertain results ) , and ( ii ) from models of isolated elliptical galaxies , which require the heating rate to decline towards the current epoch in such a way that galaxies of appropriate luminosity and ambient igm pressure are now suffering cooling catastrophes in increasing numbers ( loewenstein & mathews , 1987 ; ciotti et al . , 1991 ; binney & tabor , 1995 ) . the rates in times past , which will surely differ from the present one , are well - nigh impossible to determine or predict . if rich clusters were fashioned by gravity alone , they should evolve self - similarly . kaiser ( 1991 ) pointed out that it is then almost inevitable that there should be more x - ray luminous clusters in the past than now . this conclusion follows because the luminosity of an individual cluster scales as the product @xmath52 of mass times density , and with @xmath53 the usual spectral index of the primoridial fluctuations , @xmath54 for @xmath55 , while @xmath56 , so the characteristic luminosity will be roughly independent of @xmath57 . the number density of freshly collapsed objects will , on the other hand , be much higher at high @xmath57 . hence there should have been many more luminous clusters in the past , in contradiction with observation . the obvious way of suppressing the x - ray luminosities of early clusters is to have them form from preheated gas , which will either not be trapped by early potential wells , or will be trapped at lower density than in the unheated case . kaiser estimates the entropy boost required by observing that the gas density at the core of a rich cluster is about @xmath58 times the current mean density , or the mean density at @xmath59 . so we could get the gas onto the required adiabat by heating it to current cluster temperatures @xmath60k at @xmath59 . alternatively , we could heat it to @xmath61 ^ 210^{7.5}\,$]k at redshift @xmath62 .    pen s lower limit on the entropy of gas in groups that was discussed above implies a similar energy budget . the current maximum density of the gas is only an order - of - magnitude greater than the current cosmic mean density , so two order of magnitude smaller than the maximum density of cluster gas , and the temperature would be at @xmath63k an order - of magnitude lower than the temperature of cluster gas . so the gas would be on a slightly higher adiabat . on the other hand , it could be put onto that adiabat rather later and therefore at a smaller cost in energy . do sne provide the requisite energy ? the energy release per cosmic baryon is @xmath64 where @xmath65 is the number of baryons per unit mass , @xmath66 is the energy and @xmath67 is the mass of iron produced by a single sn . since core - collapse sne produce the least fe per unit energy , we will maximize the energy release per fe nucleus synthesized if we assume that type ia sne are unimportant . using equations ( [ mgas ] ) and ( [ mfer ] ) to evaluate equation ( [ esn ] ) under this assumption , and then equating the result to the thermal energy per baryon , @xmath68 , of a plasma of molecular weight @xmath69 , we find the plasma temperature to be @xmath70 this is clearly an upper limit on the achievable temperature because it is assumes core - collapse sne to be dominant while ignoring radiative losses , which are liable to be significant for many core - collapse sne , because they tend to go off in dense gas clouds . my judgment is that the enegy budget is too tight , even if we assume that the gas is heated as late as @xmath71 , but others may disagree . agn are strong candidates for providing the energy if sne can not do the job ( begelman , this volume ) . as one moves down the mass scale through virial temperatures below @xmath72 , the ratio of gas mass to stellar mass declines ( renzini , 1997 ) . one interpretation of this result is ( binney , 1980 ; whitmore , gilmore & jones , 1993 ) that intracluster gas is the material from which disks are formed in the field , and that its early enrichment to @xmath73 by early - type galaxies is the counterpart of the pre - enrichment of the galactic disk by the bulge , which ostriker & thuan ( 1975 ) argued is the correct solution of the g - dwarf problem . whereas in rich clusters late - infalling , high angular - momentum gas was shocked to the virial temperature before it could form an accretion disk around a spheroid , in systems like the local group disks did form , and the floor metallicity , @xmath73 , mirrors the current metallicity of intracluster gas . in this picture , the declining fraction of mass in the igm as one moves to poorer and poorer clusters simply reflects the fact that a larger and larger fraction of the original igm has settled into disks .    the argument i have given that star - forming @xmath11 galaxies blow powerful winds suggests that all @xmath11 galaxies , cluster and field alike , lose most of their heavy elements into deep space . if this is so , conventional galaxy evolution models require substantial revision . moreover , in this picture overwhelming quantities of metal - enriched baryons are stored in deep space . more than one tentative line of argument now suggests that this is indeed the case . within a few years we will probably know for sure .
an energetic argument implies that a galaxy like the milky way is blowing a powerful wind that carries away most of the heavy elements currently synthesized and has impacted the igm out to at least @xmath0 . 
 rich clusters of galaxies appear to be closed systems in which most heavy elements are ejected from galaxies . 
 more supernovae are required than the yield of core - collapse sne from a salpeter imf . 
 x - ray observations imply that the igm in groups and clusters as been strongly preheated . 
 sne probably can not supply the required energy , which must come from agn .