document: an important manifestation of the activity inside the disks of gas - rich galaxies is their highly structured hi distribution , marked by cavities , shells and supershells . 
   first discovered in the milky way ( heiles 1979 , 1984 ) , such features are now known to exist in a number of spiral galaxies ( e.g. lehnert & heckman 1996 , irwin & seaquist 1990 , puche et al . 
   1992 , brinks & bajaja 1986 ) . 
   exceptionally huge hi arcs and loops extending across several kiloparsecs have been identified with greater clarity in the hi images of a number of edge - on spirals , such as ngc  5775 ( irwin 1994 ) , ngc  4631 ( rand & van der hulst 1993 ) , ngc  3044 ( lee & irwin 1997 , hereafter li97 ) and ngc  3556 ( m 108 , king & irwin 1997 , hereafter ki97 ) . these have been interpreted as expanding supershells because of a loop - like or circular appearance in projection and either a persistence over a wide velocity range or , in a few cases , as some evidence for expansion in position - velocity space . 
   two main classes of explanations for the supershells posit the source of their kinetic energy to be , respectively , internal and external to the parent galaxy . 
   the internal source model involves starbursts , driving stellar winds ( or superwinds ) and subsequent supernova explosions ( e.g. lehnert & heckman 1996 ) . 
   the  chimney model " ( norman & ikeuchi 1989 ) , for example , attempts to explain disk - halo features and other halo gas via processes related to underlying star formation . 
   the association between extraplanar h@xmath1 filaments and star forming regions in the disk of ngc  891 and other correlations between halo emission and in - disk tracers of star formation ( dahlem et al . 
   1995 ; rand 1997 ) argue in favour of such models . if the presence of hi supershells is found to correlate with the existence of other halo gas , as might be expected in the chimney model , then stellar winds and supernovae are expected to be responsible for the hi supershells as well . 
   the main difficulty with the starburst model for hi supershells lies in the required input energies for the largest shells . using standard assumptions that the expanding supershells are in the post - sedov phase following an ` instantaneous ' injection of energy ( cf . 
   chevalier 1974 ) , hi supershells often require energy input from staggering numbers of spatially correlated supernova events . 
   this was realized early on for our own galaxy ( heiles 1979 , 1984 ) . for external edge - on galaxies , 
   since we are selectively observing only the largest shells , the energy deficit problem is exacerbated . in some cases , 
   hundreds of thousands of clustered supernovae are required ( e.g. ki97 , li97 ) , a conclusion which is not changed significantly if the energy is injected continuously over the lifetime of the shells . other evidence against star formation processes creating the hi shells is also emerging . 
   rhode et al . ( 1999 ) find no optical evidence for recent star formation in the numerous lower energy hi holes of holmberg ii and note that x - ray and fuv emission are also absent . 
   they conclude that supernovae have not played a part in the formation of the hi shells . 
   efremov et al . 
   ( 1998 ) outline numerous other examples in which there appears to be no relation between hi shells and star formation . 
   they , as well as loeb & perna ( 1998 ) , propose that the hi shells are produced , instead , by gamma ray bursts . 
   the alternative external source hypothesis invokes infall of massive gas clouds on to the galactic plane , as a result of gravitational interaction with neighbouring galaxies ( see tenorio - tagle & bodenheimer 1988 ) . 
   this resolves the energy problem since input energy is then a function of the mass and velocity of the infalling cloud . 
   evidence in favour of this hypothesis comes from observations of high velocity clouds ( hvcs ) around our own milky way and the signatures of interaction in m  101 ( van der hulst & sancisi 1988 ) and ngc  4631 ( rand & stone 1996 ) . 
   it does , however , require that the galaxy be in some way interacting with a companion or , at least , that sufficiently massive clouds be in the vicinity . 
   recent observations are revealing galaxies which are apparently isolated , yet harbour extremely large hi supershells . 
   two striking examples are the nearby , sb(s)cd galaxy , ngc  3556 ( ki97 ) and the sbc galaxy , ngc  3044 ( li97 ) . 
   both of these galaxies exhibit radio continuum halos extending to @xmath2 kpc from the galactic plane and have a number of supershells requiring energies up to a few @xmath3 10@xmath4 ergs . 
   these supershells are too large and energetic to have been produced by conventional clustered supernovae . at the same time 
   , there appears to be no evidence for interaction or nearby companions , either . 
   we propose here a new explanation for hi supershells . 
   that is , that they have been formed by radio jets which plow through the interstellar medium ( ism ) , accreting ism gas and sometimes inflating bubbles . 
   this allows for an internal energy source for the hi shells , provides a natural explanation for any spatial symmetries seen in the hi features , and also resolves the energy problem . in sect .  2 
   , we provide arguments in favour of jet inflated hi bubbles , sect .  3 presents the model , and sect . 
   4 discusses the implications of this scenario . 
   seyferts are one class of disk galaxy for which several examples of the nucleus ejecting a radio jet pair have been found ( e.g. ulvestad & wilson 1984a , 1984b , kukula et al . 
   1995 , aoki et al . 
   likewise , several cases of jets occurring in normal spiral galaxies have been reported ( e.g. hummel et al . 1983 ) . 
   prominent examples include ngc  3079 ( de bruyn 1977 ) , ngc  5548 ( ulvestad et al . 
   1999 ) and circinus ( elmouttie et al . 1998 ) . 
   the total energy output from such nuclear activity can approach @xmath5 erg , assuming that the nuclear activity lasts @xmath0 years , and arises from accretion onto a supermassive black hole of @xmath6 at 10% of the eddington limit with the canonical 10% efficiency . in 
   normal " spiral galaxies , the central mass may be lower ; for example , the bipolar outflow from the sab(rs)cd galaxy , m  101 may be produced by a central @xmath7 10@xmath8 m@xmath9 black hole ( moody et al . 
   while jets are not always observed directly , the growing number of supermassive black holes inferred to be present in the nuclei of normal disk galaxies ( kormendy & richstone 1995 , van der marel 1999 ) , including late - type disks ( ho et al . 
   1997 ) , the milky way itself ( e.g. genzel et al . 
   1997 ) and gas - rich , large low - surface - brightness galaxies ( schombert 1998 ) , lend weight to the idea that many such galaxies may have undergone phases of nuclear activity accompanied with a bi - directional ejection of relativistic plasma jets , before entering a dormant nuclear phase . 
   recent studies have also revealed that the ejection of jets can take place at small angles to the large - scale galactic disk ( e.g. nagar & wilson 1999 ; kinney et al 2000 ) , plausibly leading to clear signatures of their dynamical interaction with the ism . 
   a classic case of such an interaction is the bow - shaped morphology of the radio lobe in the well known seyfert galaxy , ngc  1068 , which strongly suggests that the kiloparsec - scale radio jets are ploughing through the disk medium ( wilson & ulvestad 1987 , axon et al . 
   a dynamical interaction of the jets with the disk is also evident in the case of the spiral galaxy m  51 which is at the low end of nuclear activity scale . here 
   , the jets seem to have created a pair of ` radio lobes ' on opposite sides of the nucleus ; whereas one of the lobes is bow - shaped , the other one is actually resolved into a ring which is also detected in h@xmath1 emission ( ford et al . 
   another example is ngc  4258 whose vlbi jet has been ejected close to the galaxy disk ( see cecil et al . 
   1995 , herrnstein et al . 
   1997 , cecil et al . 
   large scale ( 14 kpc length )  anomalous arms " are also seen _ within _ the galactic disk and have been interpreted as manifestations of a larger - scale jet ( e.g. martin et al . 
   1989 , but see cox & downes 1996 ) . in m  101 , 
   the bipolar outflow is also roughly confined to the disk , with the outflow not expected to extend beyond a height of 400 pc ( moody et al . 
   additional support for the jet - disk interaction hypothesis in these disk galaxies comes from the detection of shock - excited optical emission lines associated with the radio lobes , as discussed in some of the references cited above . 
   it is further interesting to note that radio bubbles and shells of synchrotron plasma have been discovered within the lobes of a few radio galaxies . 
   two spectacular examples are 3c310 ( van breugel & fomalont , 1984 ) and hercules a ( dreher & feigelson 1984 ) . in hercules a , in addition , peculiar dark shells , about 3 kpc in size , have been found straddling the nucleus along the radio outflow axes ( baum et al . 
   for the dark shells , baum et al . prefer an explanation in terms of expanding bubbles of hot gas ejected from the active nucleus along the radio axis , in agreement with plasmon - like ejection from the core , though they do not rule out other possibilities . 
   these cases provide a clue that the radio bubbles / shells may have been ` puffed ' up due to localized instabilities in the radio jets , occurring at distances of a few kiloparsecs from the active nucleus ( e.g. morrison & sadun 1996 ) . 
   should such instabilities arise also in the jets ejected within disk galaxies ( sect . 
   2.1 ) , it is quite plausible that the resulting increase in pressure at those locations would inflate bubbles and shells out of the ambient ism material which is hi rich , in the case of spirals . 
   such features , puffed out of the disk during the brief phase of jet instability , would remain visible past the radiative lifetime of the synchrotron plasma against the radiative and expansion losses , which is typically of order @xmath10 years .        in ngc  3556 , 
   the two most prominent supershells are located symmetrically near the opposite extremities of the galaxy . 
   ki97 have carried out a detailed mapping of the hi and radio continuum emission from this nearly edge - on galaxy located 11.6 mpc away . 
   the superimposition of 2 channels of hi emission , symmetrically placed in velocity about systemic , are shown in fig . 
   1 a. the most obvious features are two loops of hi , producing extensions to the east and west along the major axis . fig . 
   1b shows the hi emission from these velocities superimposed on the radio continuum map , the latter taken from entirely independent observations . 
   the abrupt density drop - off in the radio continuum map occurs roughly where the optical galaxy ends ( fig . 
   1a inset ) . 
   the salient features of this system most relevant to the present study ( see fig . 1 ; ki97 ) are :    \(a ) two symmetrically placed giant hi loops are seen originating at a projected galactocentric radius of about 12 kpc on the east and west major axis . they appear to originate right at the opposite edges of the optical disk and extend , not perpendicular to the disk plane , but rather parallel to it and outwards . 
   \(b ) the giant hi loops or supershells are slightly bent into a z - symmetry . 
   \(c ) these hi loops are most obvious in velocity channels which are equally spaced about the systemic velocity of the galaxy ( though they extend over a larger velocity range , see ki97 ) . 
   \(d ) from their velocity dependent morphology , each of the loops shows evidence for shell expansion , but with only half of the shell present . in both cases , the receding side ( with respect to galaxy rotation ) is open . 
   \(e ) other smaller hi loops and extensions do exist at smaller galactocentric radii . 
   \(f ) the parameters of these two hi supershells are as follows : the eastern supershell has a mass , m = 5.9 @xmath3 10@xmath11 m@xmath9 , a radius , r = 3.2 kpc , an expansion velocity , @xmath12 = 51.7 km s@xmath13 , a kinematic age , @xmath14 = 6.0 @xmath3 10@xmath11 yr , a kinetic energy , e@xmath15 = 1.6 @xmath3 10@xmath16 ergs , and an implied input energy ( assuming instantaneous input from supernovae ) of e = 2.6 @xmath3 10@xmath4 ergs 
   . the parameters of the western supershell are : m = 2.3 @xmath3 10@xmath11 m@xmath9 , r = 1.85 kpc , @xmath12 = 41.4 km s@xmath13 , @xmath14 = 4.4 @xmath3 10@xmath11 yr , e@xmath15 = 3.9 @xmath3 10@xmath17 ergs , and e = 3.4 @xmath3 10@xmath18 ergs . 
   \(g ) the energy needed to create the eastern supershell alone would require a star cluster populated by @xmath19 ob stars , if supernovae and stellar winds are the drivers . 
   \(h ) the brighter and more spectacular eastern supershell has associated hi gas which extends as far as 30 kpc beyond the eastern edge of the optical disk and possibly much farther ( see fig . 
   1a inset ) . 
   the huge eastern hi extension appears to open up from one side of the eastern shell and extends fairly straight outwards in the radial , rather than the vertical direction as if formed in a short period of time in comparison to galactic rotation . 
   \(i ) the galaxy contains a large nonthermal radio halo with a scale height of @xmath2 kpc above the plane ( see also de bruyn & hummel 1979 , bloemen et al . 
   1993 ) . 
   \(j ) radio continuum emission appears to be associated with the hi loops , but is not spatially coincident with them . 
   the radio continuum emission extends farther out than the hi supershells , as if  funnelling " along or through the hi features ( ki97 ; fig . 
   \(k ) ngc  3556 has no obvious interacting companions . 
   the brightest galaxy in the vicinity is the 16th magnitude , mcg+09 - 19 - 001 , 25@xmath20 in size , undisturbed in appearance and @xmath7 12@xmath21 to the east of ngc  3556 . 
   this is most likely a background source . 
   ngc  5775 , an interacting galaxy , shows symmetrically placed hi features ( lee et al . 
   2000 ) in the sense that hi extensions occur on opposite sides of the major axis in which there appears to be an underlying disturbance ; these features occur at similar galactocentric radii , in projection ( see also fig . 3 of irwin 1994 which shows 3 of the extensions ) . 
   ngc  2613 ( chaves & irwin 2000 ) also shows six hi extensions which occur in pairs symmetrically on either side of the major axis ; two of the pairs occur near the ends of the optical disk . 
   another potentially very interesting example is m  31 . 
   blitz et al . ( 1999 ) show that two massive hi clouds exist symmetrically placed on opposite sides of this galaxy . 
   they bear a remarkable resemblance to extragalactic radio jets . 
   the two clouds are redshifted by @xmath22 km s@xmath13 with respect to the systemic velocity of m  31 . 
   conceivably , this could be explained by ram pressure sweeping as m  31 falls through the igm towards the milky way . 
   as mentioned above , the galaxy ngc  3556 puts to a severe test both of the standard explanations for the supershell phenomenon , namely : ( i ) localized starburst ( generating intense stellar winds , followed by multiple supernova explosions ) , and ( ii ) infall of massive gas clouds on to the galactic disk . 
   the major difficulties faced by these models , as highlighted in ki97 , are :    \(a ) * localized starbursts : * the energy needed to create the eastern supershell alone would require a star cluster populated by @xmath23 ob stars . 
   even for the recently discovered ` super star clusters ( ssc ) ' in some starburst galaxies , not more than @xmath24 ob stars are implied ( meurer et al . 1995 ) . 
   there is no indication of an ob association even remotely approaching the level of sscs at the locations of the supershells in ngc  3556 . 
   since the kinematic ages of the supershells are @xmath7 5 @xmath3 10@xmath11 yr , the starburst would have to have occurred within this time frame . yet 
   starburst durations and ob association ages are typically also of this order , so some evidence of the starburst might be expected to have survived . 
   no such evidence is presently found , however . 
   ngc  3556 does not show a markedly high supernova rate globally ( irwin et al . 1999 ) and independent low and high resolution radio continuum observations show no evidence for a source of energy at the bases of the supershells ( ki97 , irwin et al . 
   2000 ) . 
   \(b ) * infalling clouds : * several arguments have been advanced against this possibility ( ki97 ) . 
   firstly , this galaxy appears isolated , making the source of the putative clouds puzzlesome . secondly , to impart sufficient kinetic energy for the creation of the eastern supershell , an infalling cloud of mass @xmath25 would be required . a cloud this massive 
   would easily have been detected on the sensitive hi maps , yet no such cloud or clouds are seen . 
   ( an exception would be if the clouds had very narrow line widths , low fractal dimensions and were optically thick , in which case they could have been missed due to low filling factors . ) 
   thirdly , if an encounter with intergalactic clouds has occurred in the past , it would be necessary to postulate that two very massive clouds just happened to fall in parallel to the major axis at opposite ends of the galaxy , roughly at the same time , a scenario which seems implausible . 
   fourthly , we could instead suggest that a  rain " of hi clouds is infalling ( since other high latitude hi structures are also seen in this galaxy ) , including two which fell in along the major axis at either end . 
   however , in order for the rain to be no longer visible , the infall would have to be completed over a timescale which is less than or equal to the age of the supershells , i.e. a few 10@xmath11 years . 
   this is unlikely since infall timescales are of order a dynamical timescale which is typically a few 10@xmath26 yr .    while these arguments have been applied to ngc  3556 alone , the difficulty with internal energy from starbursting applies to a number of galaxies , as outlined in sect .  1 . 
   infalling clouds are certainly plausible , but run into difficulties for isolated galaxies such as ngc  3556 and ngc  3044 . barring new information coming to light on these galaxies , we therefore argue that a new model should be considered .      as mentioned earlier , the duality of the expanding hi supershells in ngc  3556 , together with their locations at the opposite edges of the disk marked by steep density gradients , their z - symmetric deviations from spherical symmetry , as well as their anomalously large energy requirements ( sect . 
   2.3.1 ) , together lead us to argue that the origin of these two supershells is linked to the jet phenomenon . in sect . 
   2 we have noted several manifestations of jet instabilities giving rise to bubble - shaped features , or shells , in the form of optical nebulosities or nonthermal radio emission in normal spirals and radio galaxies . in the present context of hi supershells , we examine the possibility of the supershell being inflated due to localised instabilities in the radio jets as they plough through the ism of the disk of ngc  3556 . 
   the present discussion should only be viewed as a feasibility check based on energy considerations , and not as a detailed quantitative model for the supershells . even in the case of ngc  3556 , 
   the standard mechanisms for the supershells ( sect . 
   1 ) may well have contributed at some level .    according to our proposal , 
   the two radio jets ejected close to the plane of the disk , during the active phase of the nucleus , undergo a rapid _ flaring _ as they encounter the region of large scale density decline near the outskirts of the galactic disk . consequently , both synchrotron plasma and the entrained thermal plasma deposited by the jets in these two regions get heated and the resulting high - pressure bubble of hot plasma undergoes a rapid expansion , sweeping the gas - rich ambient medium into the shape of the hi supershells . 
   an idealization to such a situation is the flaring of a jet crossing an ism / igm interface , which was first discussed in the context of radio galaxies analytically by gopal - krishna & wiita ( 1987 ) , followed by two - dimensional ( norman et al . 
   1988 , wiita et al . 
   1990 , zhang et al . 
   1999 ) and three - dimensional ( loken et al . 
   1995 , hooda & wiita 1996 , 1998 ) numerical simulations . the simulations by loken et al . 
   showed that intermediate power radio jets associated with wide - angle - tail ( wat ) sources undergo a rapid flaring upon crossing the ism / igm density discontinuity where a moderately supersonic jet becomes subsonic . 
   as shown by the numerical simulations , a density drop of a factor of just a few can cause such a flaring ( e.g. hooda & wiita 1996 ) . 
   observational evidence for the jet flaring comes from wide - angle - tail ( wat ) radio sources which are associated with the dominant members of rich clusters of galaxies ( e.g. odonoghue et al . 
   further evidence is provided , e.g. , by the recent radio / optical / x - ray study of the wat radio galaxy 3c465 in the abell cluster a2634 ( sakelliou & merrifield 1999 ) . particularly instructive for the present study 
   is the case of the active disk galaxy iras 04210 + 0400 ; the two @xmath27 kpc long radio jets associated with this disk galaxy are seen to flare up near the opposite optical boundaries of the galaxy ( holloway et al 1996 ) . 
   it may be noted that during their passage through the inner parts of the galactic disk , the jets are likely to encounter large ism density fluctuations . 
   however , these are less prone to disrupt the jets because of the higher jet thrust there , combined with the expected small spatial scale of the density fluctuations compared with the jets cross - section . 
   since the postulated radio jets in ngc  3556 are currently too weak for detection , a point which we re - address below , we shall adopt here the jet parameters inferred for another massive disk galaxy , ngc  4258 ( sect . 
   2.1 ) , and apply them to known conditions in ngc  3556 . 
   the jet velocity , as measured from the emission line gas , is _ at least _ 
   v@xmath28 = 2000 km s@xmath13 which , together with a density of @xmath29 = 0.02 @xmath30 g @xmath31 , where @xmath30 is the mass of the proton , correspond to a kinetic luminosity of l@xmath28 @xmath7 10@xmath32 erg s@xmath13 ( falcke & biermann 1999 ) . 
   assuming that the ram pressure in the jet dominates over the internal thermal pressure , then across the shock front we require @xmath33 , where @xmath34 is the ambient gas density , and @xmath35 is the shock velocity . taking @xmath34 = 0.26 @xmath30 g @xmath31 at the positions of the supershells ( ki97 ) , we obtain @xmath35 = 435 km s@xmath13 . 
   this is slightly higher than the value of @xmath7 300 km s@xmath13 estimated for ngc  4258 . 
   the shock velocity puts a minimum timescale on the duration of the jet in this model , since there must be sufficient time for the jet to propagate out to the locations of the supershells at 12 kpc galactocentric radius . 
   thus , the minimum duration of the jets is 2.7 @xmath3 10@xmath11 yr for the parameters in this illustration . 
   the mechanical power of the jet would then be @xmath36 = @xmath37 = 5 @xmath3 10@xmath38 erg s@xmath13 , where the effective jet radius , r has a typical value of 1 kpc ( e.g. cecil et al 1995 ) . 
   this is similar to the jet power estimated for the well known spiral galaxy m  51 , which is at the low end of the scale of nuclear activity ( cf . 
   ford et al 1985 ) . in ngc  3556 
   , the integrated mechanical luminosity of the jet over its minimum lifetime is then @xmath7 4 @xmath3 10@xmath4 erg . 
   this lower limit is adequate to account for the observed kinetic energy associated with the larger eastern supershell ( sect . 
   here we have assumed that the efficiency for converting the input energy into the kinetic energy of the shell is of order 1% as usually taken for the multiple supernova model . for a higher efficiency , the required jet power can be lower . 
   the expansion of a typical bubble in the ism should then proceed similarly to previously modelled scenarios , the difference being the source of input energy . 
   since the input energy is much higher than conventional supernovae , the bubbles are more likely to achieve blowout , providing a natural conduit through which cosmic rays ( including those supplied by the jets ) can escape into the halo . 
   thus , the presence of supershells is expected to correlate with the existence of a nonthermal radio halo . 
   the locations of the anchor points of the two supershells in ngc  3556 suggest that the postulated blasting of each supershell would have occurred at a galactocentric distance which is just past the peak of the hi rotation curve ( see fig . 
   6 of ki97 ) . 
   the radially outward expansion of each supershell would then expose it to the velocity shear in the medium , exerting a side - ways wind pressure counter to the galactic rotation . 
   due to this , the half of the supershell towards the direction of the galactic rotation would be compressed and hence brightened , whilst the other half would be dragged out due to the velocity shear in the external medium and , consequently , dimmed . 
   such a deformation of the two supershells from spherical symmetry , in course of their expansion , would give rise to a z - symmetry , which is consistent with the observations ( sect .  2.3.1 ; ki97 ) . 
   the cooling time for the x - ray emitting sheaths around the jets of ngc  4258 , assuming unity filling factor , is only 4 @xmath3 10@xmath8 yr ( cecil et al . 1995 ) and therefore , once the jets have turned off , such a signature of their existence would disappear rapidly . 
   the lifetime of the synchrotron emitting particles in the jets is longer , typically from 10@xmath8 to 10@xmath11 yr . for ngc  4258 
   , it has been estimated to be between 1 to 5 @xmath3 10@xmath11 yr ( martin et al . 1989 ) . 
   however , shorter timescales are possible and will depend on local magnetic field strength and spectral index . 
   for instance , the magnetic field in the north - east radio jet in ngc  1068 , which extends to @xmath7 450 pc from the nucleus , is @xmath7 5 @xmath3 10@xmath39 gauss and the spectral index is -1 ( wilson & ulvestad 1987 ) . 
   this gives a synchrotron lifetime of only 1.5 @xmath3 10@xmath40 yr . 
   the particle lifetime is expected to be shorter in shocks where the magnetic field is compressed . 
   thus , we expect the radio jet in ngc  3556 to fade over a timescale less than the ages of the supershells . 
   similar arguments apply to the radio core which is expected to decay faster than the jets , once the nuclear activity has switched off , given the likelihood of a stronger magnetic field and flatter spectral index in the core . 
   even if some radio emission from a core were to persist after the activity ceases , it may be below the rms noise level of the available maps . 
   chary & becklin ( 1997 ) , for example , estimate the radio luminosity of the core of ngc  4258 , which is known to have a supermassive central object of mass 3.6 @xmath3 10@xmath11 m@xmath9 and a vlbi jet , to be l@xmath41 = 100 l@xmath9 . 
   the 2@xmath42 noise level of the radio maps of irwin et al . 
   ( 2000 ) corresponds to a radio power of 6.8 @xmath3 10@xmath43 w hz@xmath13 . 
   integrating over 10@xmath44 hz , this yields a radio luminosity of 180 l@xmath9 . 
   thus , even if ngc  3556 harbours a radio core of the same luminosity as ngc  4258 , it would not have been detected in the observations mentioned above . 
   in this study we have sketched a scenario whereby radio jets ejected during an active nuclear phase in the life of a large spiral galaxy could inflate out of the disk exceptionally large shells , i.e. supershells of neutral hydrogen gas . when applied to the scd galaxy , ngc  3556 
   , this model can account for each of the several features enumerated in sect . 
   2.3.1 . in the context of the hi supershells seen in this galaxy , this scheme appears to fare distinctly better than either of the two standard models for supershells which invoke either a starburst induced superwind or an infall of external gas clouds on to the galaxy disk . 
   thus , the jet - ism interaction scenario for disk galaxies could effectively supplement the other two mechanisms already proposed for the supershell phenomenon , with a greater relevance whenever the shells are extraordinarily large as well as energetic and occur at large galactocentric distances . 
   the energy requirement is no longer a major issue in the present model . 
   even a modest energy input from a 10@xmath11 m@xmath9 compact object at the galactic nucleus , such as that observed in ngc  4258 , is more than adequate to supply the input energy required for the supershells of ngc  3556 . 
   this model can therefore account for hi supershells which are difficult to form via conventional processes involving massive star formation , or those occurring in galaxies lacking potentially interacting companions . 
   larger supershells ( or parts thereof , given the probable development of instabilities ) may also be predicted , since known central masses and/or accretion rates may well surpass those considered here . 
   furthermore , the present model can account for the symmetric hi features in galaxies . 
   for instance , it is more likely that bubbles will be inflated by the propagating jets at galactocentric radii where the ism density has declined to sufficiently low values for blow - out . at the same time 
   , symmetry is not a generic outcome our model , since local shocks , dictated by the interaction of the jet with local density perturbations , can be important in determining where the bubbles will be inflated . 
   for example , known bubbles in the lobes of radio galaxies ( sect . 
   2.2 ) are not always seen to be equidistant on either side of the nucleus .    in our sample illustration ( sect . 
   3.2 ) , the jets must exist for long enough to reach large galactocentric radii ( 3 @xmath3 10@xmath11 yr ) but need to inject energy over only a fraction of the lifetime of the supershells ( 2 @xmath3 10@xmath11 yr , or even less , if the efficiency exceeds 1% ) . 
   the jets presumably switch off thereafter , with the synchrotron emitting particles decaying in 10@xmath8 - 10@xmath11 yr . 
   it is difficult to predict whether jets and hi supershells should co - exist in spirals . 
   nuclear activity in galaxies is commonly believed to be a transient phenomenon , lasting for @xmath45 years ( e.g. eilek 1996 , scheuer 1995 ; soltan 1982 ; haehnelt & rees 1993 ; ho et al 1997 ) . 
   if agn activity occurs on timescales longer than a few @xmath3 10@xmath11 yr , this would suggest that there might be disk galaxies in which both phenomena should be observed at the same time . 
   however , such nuclear activity timescales are likely to be more representative of radio galaxies ; agn activity in spirals is probably shorter lived . 
   an important next step is to consider what jet parameters are required to reach large , or at least kpc - scale radii , in traversing the typical ism of a spiral galaxy .    if jets in spirals both recur and precess , it may be possible to inflate bubbles over a variety of galactocentric radii and azimuthal angles 
   . however , it is unlikely that the  frothy " nature of the hi seen in galaxies such as holmberg ii or the holes in m  31 could be produced by such jets directly without seeing some lingering evidence for the jets as well . 
   also , the jet phenomenon can not directly account for features which correlate with the star forming disk , such as some known radio and h@xmath1 halos . 
   if there is a connection , it is more likely to be indirect . 
   for example , once the expansion of the postulated jet - blown supershells is halted due to the gravitational pull of the host galaxy , their segments ( i.e. hi clumps ) would eventually shower back on to the galactic disk , giving rise to secondary shells , bubbles and cavities in the hi component of the disk . 
   the impact could also trigger sporadic bursts of star formation , especially in the outer disk , as seen , e.g. , in gas - rich low - surface - brightness galaxies ( cf . 
   oneil et al . 
   1997 ) which are thought to be the present - day remnants of quasars ( e.g. schombert 1998 ) . 
   thus , much after its cessation , the nuclear activity in gas - rich galaxies may continue to influence the evolution of their disks . 
   aoki k. , kosugi g. , wilson a. s. , yoshida m. 1999 , apj 521 , 565 axon d. j. , marconi a. , capetti a. , et al . 
   1998 , apj 496 , 75 baum s. a. , odea c. p. , de koff s. , et al . 
   1996 , apj , 465 , l5 blitz l. , spergel d. n. , teuben p. j. , hartmann d. , burton w. b. 1999 , apj 514 , 818 bloemen h. , duric n. , irwin j. a. 1993 , in 23rd international cosmic ray conference , eds . 
   d. a. leahy , r. b. hicks , d. venkatesan ( new jersey , world scientific ) , p. 279 brinks e. , bajaja e. 1986 , a&a 169 , 14 cecil g. , wilson a. s. , de pree c. 1995 , apj 440 , 181 cecil g. , de pree c. g. , greenhill l. j. , moran j. m. , dopita m. a. 1998 , aas 193.0710 chary r. , becklin e. e. 1997 , apj 485 , l75 chaves t. , irwin j. a. , in gas and galaxy evolution , asp conf . 
   ser . , ed . 
   j. e. hibbard , m. p. rupen , j. h. van gorkom , in press chevalier , r. a. 1974 , apj 188 , 501 cox p. , downes d. 1996 , apj 473 , 219 dahlem m. , lisenfeld u. , golla g. 1995 , apj 444 , 119 de bruyn a. g. 1977 , a&a , 58 221 de bruyn a. g. , hummel e. 1979 , a&a 73 , 196 dreher j. w. , feigelson e. d. 1984 , nature 308 , 43 efremov y. n. , elmegreen b. g. , hodge p. w. 1998 , apj 501 , l163 eilek j. a. 1996 , in energy transport in radio galaxies and quasars , asp conf . 
   100 , ed . p.e . 
   hardee , a.h . 
   bridel , j.a . 
   zensus , ( san francisco : astr . 
   pacific ) , 281 elmouttie m. , haynes r. f. , jones k. l. , sadler e. m. , ehle m. 1998 , mnras 297 , 1202 falcke h. , biermann p. l. 1999 , a&a 342 , 49 ford h. c. , crane p. c. , jacoby g. h. , lawrie d. g. , van der hulst j m. 1985 , apj 293 , 132 genzel r. , eckart a. , ott t. , eisenhauer f. 1997 , mnras 291 , 219 gopal - krishna , wiita p. j. 1987 , mnras 226 , 531 haehnelt m. , rees m. 1993 , mnras 263 , 168 heiles c. 1979 , apj 229 , 533 heiles c. 1984 , apjs 55 , 585 herrnstein j. r. , moran j. m. , greenhill l. j. , et al . 
   1997 , apj 475 , l17 , erratum , apj , 482 , l113 ho l. c. , filippenko a. v. , sargent w. l. w. , 1997 , apj 487 , 568 holloway a. j. , steffen w. , pedlar a. , et al . 
   1996 , mnras 279 , 171 hooda j. s. , wiita p. j. 1996 , apj 470 , 211 hooda j. s. , wiita p. j. 1998 , apj 493 , 81 hummel e. , van gorkom j. h. , kotanyi c. g. 1983 , apj 267 , l5 irwin j. a. 1994 , apj 429 , 618 irwin j. a. , seaquist e. r. 1990 , apj 353 , 469 irwin j. a. , english j. , sorathia b. 1999 , aj 117 , 2102 irwin j. a. , saikia d. j. , english j. 2000 , aj 119 , 1592 king d. , irwin j. a. 1997 , new astr . 2 , 251 ( ki97 ) kinney a. l. , 2000 , apj 
   ( july 2000 issue ) kormendy j. richstone d. 1995 , araa 33 , 581 kukula m. j. , pedlar a. , baum s. a. , odea , c. p. 1995 , mnras 276 , 1262 lee s .- w . , irwin j. a. 1997 , apj 490 , 247 ( li97 ) lee s .- w . 
   , irwin j. a. , dettmar r .- j . , 
   2000 , in prep . 
   lehnert m. d. , heckman t. m. 1996 , apj 462 , 651 loeb a. , perna r. , 1998 , apj 503 , l35 
   loken c. , roettiger k. , burns j. o. , norman m. 1995 , apj 445 , 80 martin p. , roy j .- 
   , noreau l. , lo k. y. 1989 , apj 345 , 707 meurer g. r. , heckman t. m. , leitherer c. , et al . 
   1995 , aj 110 , 2665 moody j. w. , roming p. w. a. , joner m. d. , et al . 
   1995 , aj 110 , 2088 morganti r. , killeen n. e. b. , ekers r. d. , oosterloo t. a. 1999 , mnras 307 , 750 morrison p. , sadun a. 1996 , mnras 278 , 265 nagar n. m. , wilson a. s. 1999 , apj 516 , 97 norman m. l. , burns j. o. , sulkanen m. 1988 , nature 335 , 146 norman c. a. , ikeuchi s. 1989 , apj 345 , 372 odonoghue a. a. , eilek j. a. , jones j. m. 1993 , apj 408 , 428 oneil k. , bothun g. d. , schombert j. , cornell m. e. , impey c. d. 1997 , aj 114 , 2448 puche d. , westpfahl d. , brinks e. , roy j .- r . 
   1992 , aj 103 , 1841 rand r. j. 1997 , in the interstellar medium in galaxies , ed . 
   j. m. van der hulst ( dordrecht : kluwer ) , 105 rand r. j. , stone j. m. 1996 , aj 111 , 190 rand r. j. , van der hulst j. m. 1993 , aj 105 , 2098 , erratum , aj , 107 , 392 rhode k. l. , salzer j. j. , westpfahl d. j. , radice l. a. 1999 , aj 118 , 323 sakelliou i. , merrifield m. r. 1999 , mnras 305 , 417 scheuer p. a. g. 1995 , mnras 277 , 331 schombert j. 1998 , aj 116 , 1650 soltan a. 1982 , mnras 200 115 tenorio - tagle g. , bodenheimer p. , 1988 
   , ara&a 26 , 145 ulvestad j. s. , wilson a. s. 1984a , apj 278 , 544 ulvestad j. s. , wilson a. s. 1984b , apj 285 , 439 ulvestad j. s. , wrobel j. m. , roy a. l. , et al . 1999 , apj 517 , l81 van breugel w. , fomalont e. b. 1984 , apj 282 , l55 van der marel r. p. , 1999 
   , aj 117 , 744 van der hulst t. , sancisi r. 1988 , aj 95 , 1354 wiita p. j. , rosen a. , norman m. l. 1990 , apj 350 , 545 wilson a. s. , ulvestad j. s. 1987 , apj 319 , 105 zhang h .- m . , koide s. , sakai j .- 
   1999 , pasj 51 , 449
summary: taking a clue from the pair of hi supershells found in the scd galaxy ngc  3556 ( m  108 ) , we propose a new mechanism for the origin of hi supershells in gas - rich massive galaxies . in this scenario , 
   the two supershells were inflated out of the neutral hydrogen disk due to the localised flaring of a pair of radio lobes formed by the jets ejected from the nucleus during an active phase about @xmath0 years ago , but have faded away by now . 
   it is shown that the salient features of this supershell pair , such as their symmetrical locations about the galactic centre , the anomalously large energy requirements , the large galacto - centric distances , as well as the z - symmetric hemispherical shapes , find a more natural explanation in terms of this scenario , as compared to the standard models which postulate either a massive starburst , or the infall of external gas clouds . 
   other possible implications of this hypothesis are briefly discussed .