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it is well established that an important component of the intergalactic medium ( igm ) in clusters and groups of galaxies is the hot gas , observed in x - rays and characterized by temperatures in the range @xmath05 - 10 kev , by a central density of @xmath010@xmath1 cm@xmath1 and by a density distribution approximated by a beta model ( cavaliere & fusco - femiano 1981 ) . in addition , magnetic fields are wide spread in clusters ( e.g. eilek 1999 ) , as deduced by rotation measure arguments , and relativistic electrons may be common ( sarazin & lieu 1998 ) . these two non - thermal components can be directly revealed in some clusters by the presence of diffuse extended radio sources , which are related to the intergalactic medium , rather than to a particular cluster galaxy . however , diffuse sources seem not to be a general property of the igm . the importance of these sources is that they represent large scale features , which are related to other cluster properties in the optical and x - ray domain , and are thus directly connected to the cluster history and evolution . in this paper , the observational properties of diffuse cluster sources and of their host clusters are outlined . intrinsic parameters are calculated with h@xmath3 = 50 km s@xmath4 mpc@xmath4 and q@xmath3 = 0.5 . the diffuse source coma c in the coma cluster ( fig . 1 , left panel ) , discovered 30 years ago ( willson 1970 ) , is the prototypical example of a cluster _ radio halo_. the radio halo is located at the cluster center , it has a steep radio spectrum ( @xmath5 ) and is extended @xmath0 1 mpc ( giovannini 1993 ) . another example of cluster - wide halo , associated with the cluster a  2255 , is shown in the right panel of fig . an additional diffuse source , 1253 + 275 , is detected at the coma cluster periphery , which might be connected to the cluster halo by a very low - brightness radio bridge ( giovannini 1991 ) . this source and similar diffuse sources located in peripheral cluster regions are referred to as _ radio relics _ in the literature . this name may be misleading , since it can also be used to indicate dying radio galaxies , without active nucleus , as b2  1610 + 29 in a  2162 ( parma et al . i would suggest _ peripheral halos _ , whereas ron ekers suggested _ radio flotsam_. since the interpretation of these sources is still unclear , i will use here the name _ relics _ , for homogeneity with the literature . radio halos and relics show low surface brightness ( @xmath6jy / arcsec@xmath7 at 20 cm ) and steep radio spectrum . their detection is limited by the surface brightness sensitivity coupled with the high resolution needed to separate such sources from the embedded discrete sources . because of their steep spectrum , they are better detected at lower frequencies . .clusters with well studied halos and relics [ cols="^,^,^,^,^,^,^,^,^ " , ]     caption . col . 1 : cluster name ; col . 2 : cluster redshift ; col . 3 : radio power of the diffuse source at 1.4 ghz ; col . 4 : largest linear size of the diffuse source ; col . 5 : cluster x - ray bolometric luminosity ; col 6 : cluster temperature obtained by averaging values in the literature ; col . 7 : projected distance of the diffuse source from the cluster center ; col . 8 : source classification , h = halo , r = relic .    until recently , the number of halos and relics was small , thus these sources were considered to be rare . this is no longer true . thanks to the better sensitivity of radio telescopes and to the existence of deep surveys , more than 30 clusters hosting diffuse sources are known today . for 18 of them ( see table 1 ) the presence of diffuse radio emission is well established and good radio data are available either from the literature or from new observations ( govoni in preparation ) . it is remarkable the existence in some clusters of more than one diffuse source . the sizes of halos are typically larger than 1 mpc . peripheral relics are elongated in shape , and the distribution of their largest sizes is not statistically different from that of halos on a kolmogorov - smirnov ( ks ) test .    the distribution of projected distances from the cluster center ( fig . 2 ) demonstrates that the diffuse sources are not located at random positions in the clusters , i.e. central halos are likely to be really at the cluster center and not simply projected onto it . radio powers are of the order of 10@xmath8 - 10@xmath9 w hz@xmath4 at 1.4 ghz . in a radio size - radio power diagram , the diffuse radio sources follow the same correlation of the radio galaxies ( e.g. ledlow 2000 ) , lying in the upper part of the plot . minimum energy densities in diffuse sources are between @xmath0 5 @xmath1010@xmath11 and 2 10@xmath12 erg cm@xmath1 . this implies that the pressure of relativistic electrons is much lower than that of the thermal plasma . equipartition magnetic fields are about @xmath00.1 - 1@xmath13 g . these values can be compared with independent estimates from rotation measure arguments and from inverse compton x - ray emission , to determine if these radio sources are at the equipartition . to derive the frequency of radio halos and relics we need systematic radio information on complete samples of clusters . giovannini ( 1999 ) used the nrao vla sky survey ( nvss ) to search for diffuse sources associated with clusters of galaxies from the sample of ebeling ( 1996 ) . this sample is complete down to an x - ray flux of 5 10@xmath14 erg @xmath15 s@xmath4 in ( 0.1 - 2.4 ) kev , for redshifts @xmath160.2 and for galactic latitude @xmath17b@xmath1820@xmath19 . moreover , it contains some clusters at higher redshift and at lower galactic latitude . the total detection rate of clusters with halos + relics in the complete sample is of 5% to 10% , and the relative occurence of halos and relics is similar ( taking into account the uncertain detections ) . the detection rate increases with the x - ray luminosity ( giovannini 2000 ) , being of @xmath040% in clusters with x - ray luminosity larger than 10@xmath20 erg s@xmath4 . in particular , the detection rate of central radio halos is more strongly dependent on the x - ray luminosity . the clusters hosting a diffuse source have a significantly higher x - ray luminosity than clusters without a diffuse source ( @xmath21 99.9% confidence level with a ks test ) . the detection rate in the high redshift sample is fully consistent with the previous results . these results are consistent with those of owen ( 1999 ) . an important property derived in the previous section is that the clusters hosting diffuse sources are more x - ray luminous and consequently they have a high temperature and a large mass . values larger than 1.5 10@xmath22 m@xmath23 are found for the total ( gravitating ) mass within 0.5 mpc , with gas fraction ranging from 10% to 20% . this is consistent with the serendipitous detection of halos observed during the attempts to detect sunyaev - zeldovich effect in massive high redshift clusters .    in previous studies ( e.g. feretti 1999 ) , radio halos have been found to be often associated with clusters showing indication of merger processes from x - ray and optical structure , and from x - ray temperature gradients . this is confirmed by statistical studies , according to the following arguments :    - x - ray images of clusters with halos and relics show the presence of substructures and distortions in the brightness distribution ( schuecker & bhringer 1999 ) , which can be interpreted as the result of subclumps interaction ;    - clusters with halos do not have a strong cooling flow ( e.g. feretti 1999 ) . this is further indication that a cluster has undergone a recent merger , as cooling flow and irregular cluster structure tend to be anticorrelated ( buote & tsai 1996 ) and a strong merger process is expected to disrupt a cooling flow ( peres 1998 ) ;    - the x - ray core radii of clusters with halos / relics in table 1 ( see fig . 3 , left panel ) are significantly larger ( @xmath2199% level using a ks test ) than those of clusters classified as single / primary by jones anf forman ( 1999 ) . according to the last authors , the large core radius clusters are multiple systems in the process of merging and hotter clusters tend to have larger core radii ;    - for the clusters of table 1 with optical and x - ray information , the values of spectroscopic @xmath24 are on average larger than 1 ( fig 3 , right panel ) , indicating the presence of substructure ( edge & stewart , 1991 ) ;    - clusters with halos and relics have larger distances to their next neighbours compared to ordinary clusters with similar x - ray luminosity , i.e. similar cluster mass ( schuecker and bhringer 1999 ) . the fact that they appear more isolated gives additional support to the idea that recent merger events in halo clusters lead to a depletion of the nearest neighbours .    in conclusion , there seems to be convincing evidence that diffuse sources are preferentially associated with high x - ray luminosity clusters with mergers . to our knowledge , however , not all the x - ray luminous merging clusters host a diffuse source , but this point needs further investigation . radio structures of halos show in many cases close similarity to the x - ray structures , suggesting a causal connection between the hot and relativistic plasma . an example is given in fig . 1 ( right panel ) , which shows that the central radio halo in a  2255 is elongated in the e - w direction as the x - ray gas . other examples are the radio halos in a  665 , a  1300 , a  2218 , a  2319 ( see feretti 1999 for comments on individual clusters ) and in 1e0658 - 56 ( liang 1999 ) . the correlation visible between the monochromatic radio power at 1.4 ghz and the bolometric x - ray luminosity for the clusters of table 1 ( fig . 4 ) implies a correlation between radio power and cluster temperature ( fig . 5 ) , also derived by liang ( 1999 ) . since the cluster x - ray luminosity and mass are correlated as well as the temperature and mass , it follows that the halo radio power also correlates with mass . the correlations must be taken into account in model of radio halo formation . it is of particular interest to understand whether the formation of a diffuse source is affected by the high cluster temperature or by the large cluster mass . a correlation seems to exist also between the largest radio size of diffuse sources and the cluster x - ray luminosity ( fig . 6 ) , and goes in the direction of more x - ray luminous clusters hosting larger diffuse sources . all the correlations are more evident for radio halos than for relics . another link between radio and x - ray is represented by the detection of hard x - ray emission produced by the ic scattering of relativistic electrons with the microwave background photons . the hard x - ray emission detected in the coma cluster ( fusco - femiano 1999 ) and in a  2256 ( fusco - femiano 2000 ) has been interpreted in this way . it is evident from the observational results that central halos are strictly related to the presence of recent mergers . the cluster merger can provide energy for the electron reacceleration and magnetic field amplification . however , also the high x - ray luminosity is relevant for the formation of halos . this seems to imply that the dynamical history of the cluster , i.e. the formation process of a massive hot cluster is crucial to trigger a halo . this scenario would explain why not all merging clusters host a halo ( at least 50% of clusters show mergers , jones & forman 1999 , while less than 5% of clusters show central halos ) . brunetti ( 2000 ) suggested a two - phase model , successfully applied to coma c , which includes : i ) a phase of injection of relativistic electrons in the cluster volume from starburst galaxies , agns , and strong shocks during the cluster formation ; ii ) a phase of particle reacceleration by recent mergers . the brunetti model can be generalized to the other halos , and it would account for the presence of halos in x - ray luminous clusters , since the hot massive clusters should have had a more efficient injection phase . it still debated if radio halos and relics have a common origin and evolution , or if they should be considered as different classes of sources . several clusters host both a central and a peripheral halo , thus favouring a common origin and nature . peripheral relics , like halos , are associated with merging clusters . however , the clusters are less luminous in x - ray and the radio - x - ray correlations are weaker . a current hypothesis is that radio relics have a different nature , and may be reaccelerated by shock waves of an ongoing merger event , or from shocks during the structure formation of the universe ( ensslin 2000 ) . i would like to thank the scientific and local organizing committee for organizing such an enjoyable and useful conference . i am grateful to isabella gioia and gabriele giovannini for helpful discussions . brunetti g. , setti g. , feretti l. , giovannini g. , 2000 ,  submitted buote d.a , tsai j.c . , 1996  458 , 27 cavaliere a. , fusco - femiano r. , 1981 ,  100 , 194 ebeling h. , , 1996 ,  281 , 799 edge a.c . , stewart g.c . , 1991 ,  252 , 428 eilek j. , 1999 , in _ diffuse thermal and relativistic plasma in galaxy clusters _ , h. bhringer , l. feretti , p. schuecker eds . , mpe report no . 271 , p. 71 ensslin t. , 2000 , this meeting feretti l. , 1999 in _ diffuse thermal and relativistic plasma in galaxy clusters _ , h. bhringer , l. feretti , p. schuecker eds . , mpe report no . fusco - femiano r. , , 1999 ,  513 , l21 fusco - femiano r. , , 2000 ,  letters submitted giovannini g. , feretti l. , stanghellini c. , 1991 ,  252 , 528 giovannini g. , feretti l. , venturi t. , kim k .- , kronberg p.p . , 1993 ,  406 , 399 giovannini g. , tordi m. , feretti l. , 1999 , new astronomy 4 , 141 giovannini g. , feretti l. , govoni f. , 2000 , this meeting jones c. , forman w. , 1999 ,  511 , 65 ledlow m.j . , owen f.n . , eilek j.a . , 2000 , in _ life cycles of radio galaxies _ , j. biretta eds . , new astronomy reviews , in press liang h. , 1999 , in _ diffuse thermal and relativistic plasma in galaxy clusters _ , h. bhringer , l. feretti , p. schuecker eds . , mpe report no . 271 , p. 33 owen f. , morrison g. , voges , w. , 1999 , in _ diffuse thermal and relativistic plasma in galaxy clusters _ , mpe report no . parma p. , de ruiter h.r . , fanti c. , fanti r. , 1986 ,  64 , 135 peres c.b . , , 1998 ,  298 , 416 sarazin c.l . , lieu r. , 1998 ,  494 , l177 schuecker p. , bhringer h. , 1999 , in _ diffuse thermal and relativistic plasma in galaxy clusters _ , mpe report no . 271 , p. 43 willson m. , 1970 ,  151 , 1
the number of known diffuse radio sources in clusters of galaxies has grown in recent years , making it possible to derive statistical properties of these sources and of the hosting clusters . 
 we show that diffuse sources are associated with x - ray luminous clusters which have undergone recent merger processes . 
 the radio and x - ray structures are often similar , and correlations are found between radio and x - ray parameters . 
 this is indication of a link between the diffuse relativistic and thermal plasma in clusters of galaxies . 
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