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studying the properties of the gaseous environments of radio galaxies and quasars on different scales are important for understanding the interactions of the radio jets with the external environment and the evolution of these sources . such studies could also provide useful insights towards understanding the triggering of radio activity , and examining consistency of these properties with the unified schemes for active galactic nuclei ( agn ) ( pihlstrm , vermeulen & conway 2003 ; gupta & saikia 2006b ) . a useful way of probing the cold neutral component of this gas is via 21-cm hi absorption towards radio sources , which range in size from the sub - galactic sized compact steep - spectrum ( css ) and gigahertz peaked - spectrum ( gps ) sources , to the large radio galaxies and quasars which are up to a few mpc in size . the css and gps objects ( o dea 1998 ) have been inferred to be young ( @xmath1110@xmath12 yr ) , while the larger sources could be older than @xmath810@xmath13 yr ( jamrozy et al . 2008 ; konar et al . 2008 ) . several hi absorption line studies have shown that css and gps objects tend to exhibit absorption lines more frequently than larger sources , with the lines being both blue- and red - shifted relative to the systemic velocity and exhibiting complex line profiles . the hi column densities also appear to be anticorrelated with the source sizes ( pihlstrm , conway & vermeulen 2003 ; gupta et al . 2006 ) , and are broadly consistent with the unified scheme for radio galaxies and quasars . the relationship between the hi column density and core prominence , which is being used as a statistical indicator of source orientation , appears to be consistent with the hi gas being distributed in a circumnuclear disk on a scale smaller than the size of the compact radio sources ( gupta & saikia 2006b ) . evidence of absorption arising from a circumnuclear disk - like structure has also been inferred from higher - resolution , very long baseline interferometric ( vlbi)-scale spectroscopic observations in several sources such as the css object j0119 + 3210 ( 4c+31.04 ) and the cores of a few larger sources , cyga , ngc4261 and hydraa ( e.g. conway & blanco 1995 ; taylor 1996 ; conway 1999 ; van langevelde et al . 2000 ) . a comparison of the hi absorption properties towards the cores of larger sources with those of the css and gps objects might provide insights towards understanding the evolution of the gaseous properties as the source ages . such a study indicates that the detection of hi in absorption towards the cores of larger sources is significantly smaller than for css and gps objects suggesting an evolution in the gaseous content of the host galaxies with source age ( chandola et al . , in preparation ) . there have also been suggestions that the torus / disk may be different in fri and frii sources , which is likely to be reflected in the hi absorption properties of these two classes of sources ( e.g. morganti et al . given the small number of detections at present , the difference in detection rate between fri and frii sources does not appear to be significant ( chandola et al . , in preparation ) . an important aspect in our understanding of agn is the episodic nature of their nuclear activity , and the physical processes that might be governing it ( saikia & jamrozy 2009 for a review ) . there appears to be a trend for a high rate of detection of hi absorption in sources with evidence of rejuvenated activity . these include the giant radio galaxy 3c236 which also exhibits evidence of star formation ( conway & schilizzi 2000 ) , the giant radio galaxy j1247 + 6723 with a gps core ( saikia , gupta & konar 2007 ) , the misaligned ddrg 3c293 ( beswick et al . 2004 ; emonts et al . 2005 ) , the well - studied southern radio galaxy centaurus a ( sarma , troland & rupen 2002 ; morganti et al . 2008 ) , 4c  29.30 ( chandola , saikia & gupta 2010 ) and cta  21 ( salter et al . 2010 ) . the well - known frii radio galaxy cygnus a , which has been shown to have two cycles of radio activity from radio and x - ray observations ( steenbrugge , blundell & duffy 2008 ; steenbrugge , heywood & blundell 2010 ) , also exhibits nuclear hi absorption ( conway 1999 ) .    in this paper we present hi observations with the giant metrewave radio telescope ( gmrt ) to localise and put constraints on the size of the hi absorber in the frii radio galaxy 3c321 , which has been reported earlier from arecibo observations ( mirabel 1990 ) . we also present radio continuum observations with the gmrt and the very large array ( vla ) of the diffuse plume of emission to explore whether this feature might be relic emission from an earlier cycle of activity . the radio galaxy 3c321 is an frii radio galaxy at a redshift of 0.0961 ( luminosity distance = 436 mpc ) so that 1 arcsec corresponds to 1.759 kpc in a universe with h@xmath14=71 km s@xmath0 mpc@xmath0 , @xmath15=0.27 and @xmath16=0.73 . our observations show that it has two prominent hot - spots at the outer edges with an overall angular size of @xmath8 286 arcsec , which corresponds to a projected linear size of @xmath8 503 kpc , similar to that found in the literature ( e.g. baum et al . 1988 ; lal , hardcastle & kraft 2008 ) . on smaller scales , a vla image with an angular resolution of @xmath81.4 arcsec by baum et al . ( 1988 ) , shows a compact core , a knot towards the north - west separated from the core by @xmath83.5 arcsec ( 6 kpc ) , and a somewhat diffuse but bright and prominent jet extending upto @xmath838 kpc . there is also evidence of a weaker counter - jet towards the south - east . a lower - resolution radio image has shown evidence of collimated emission extending all the way to the north - western hot - spot , and a diffuse plume of emission starting beyond the bright emission in the jet and extending for @xmath8140 kpc at an angle of @xmath830@xmath9 to the source axis ( evans et al . 2008 ) . optical observations show that the galaxy associated with the radio source has a prominent kpc - scale dust lane , and a fainter companion towards the north - west ( hurt et al . 1999 ; martel et al . 1999 ; roche & eales 2000 ; de koff et al . 2000 ) . the two galaxies are embedded in a common stellar envelope , and the velocity difference between the two galaxies is within @xmath8200 km s@xmath0 . optical spectroscopic and x - ray observations suggest that the companion galaxy also hosts an agn ( filippenko 1987 ; robinson et al . 2000 ; evans et al . 2008 ) . there are different regions of x - ray emission , with emission from both the hot - spots and regions offset from the radio peaks ( hardcastle et al . 2004 ; evans et al . hubble space telescope ( _ hst _ ) stis nuv and [ o ] images , along with the _ chandra _ and _ spitzer _ irac 4.5 @xmath17 m images show the complex distribution of gas in the circumnuclear region ( e.g. evans et al . 2008 ) . the results presented here are based on hi observations made with the gmrt , as well as on archival gmrt and vla data . the observing log for both the gmrt and vla observations is listed in table  1 . .the observation log . column 1 : the name of the telescope , and the array configuration for the vla observations ; column 2 : dates of the observations ; columns 3 and 4 : the frequency and bandwidth used in making the images . [ cols="^,^,^,^,^,^ " , ]     @xmath18 flux density has been estimated over same area as the vla images , and the total flux density of the plume is likely to be higher .      the spectrum of the plume between 1511 and 4860 mhz , estimated from similar resolution vla images , is steep with a spectral index of @xmath80.8 . the flux density of the plume at 614 mhz has been estimated over same area as at the higher frequencies . the plume is seen over a smaller extent at 614 mhz compared with the vla images , and our estimate of the flux density at this frequency is @xmath8100 mjy . lal , hardcastle & kraft ( 2008 ) estimate the flux density of the plume which they term the ` north wing ' to be 32.1 and 17.2 mjy from their high- and low - resolution images at 610 mhz , and a spectral index of 0.30 between 1500 and 610 mhz . our flux density estimate is clearly significantly higher than those of lal et al . we imaged the archival gmrt data at the lower frequencies to determine the spectrum of the plume , but the images were not of satisfactory quality to give reliable estimates of the flux densities of the plume ( see for example the 240-mhz image of lal et al . the plume is also barely seen in the tifr gmrt sky survey ( tgss ; sirothia et al . , in preparation ) image at 148 mhz ; clearly better quality data are required to determine the low - frequency spectrum of the plume reliably . this would help determine any break in the spectrum and explore the possibility of whether the plume might be relic emission due to an earlier cycle of activity . although the present suggestion is that the plume is caused by the deflection of the jet ( e.g. evans et al . 2008 ) , the detailed spectrum of the plume would help examine the alternate possibility of this being relic emission . if the plume of emission is indeed due to an earlier cycle , the detection of hi in absorption would be consistent with the trend for a high incidence of absorption in sources with evidence of recurrent activity ( cf . saikia & jamrozy 2009 for a review ; salter et al . 2010 and references therein ) . the gmrt hi observations show that the absorption occurs towards the core component , with also a suggestion of absorption towards the knot in the jet 6 kpc north - west of the nucleus . this would indicate the size of the absorber to be @xmath19 6 kpc if it is a single absorber . the absorption profile towards the core has multiple components with a total column density of @xmath1(hi)=9.23@xmath210@xmath3(@xmath4/100)(@xmath7/1.0 ) @xmath6 . the column density towards the knot is @xmath1(hi)=7.54@xmath210@xmath3(@xmath4/100)(@xmath7/1.0 ) @xmath6 . no absorption is seen towards the hot - spots . radio continuum observations of lobes with similar resolution show that their spectra are straight up to 5 ghz , suggesting spectral ages of @xmath1026 myr for the lobes . the detection of x - rays from the hot - spots suggests that these are being continuously fed by the jets , consistent with their spectral ages . the full extent of the plume is not seen in our 614-mhz gmrt and also barely seen in the tgss image at 148 mhz . however , better quality data are required to determine the spectrum of the plume reliably over a large frequency range to try and examine a possible break in the spectrum and explore whether the plume might be relic emission from an earlier cycle of activity . we thank george privon for useful discussions and sharing his results on 3c321 with us , dave green , the reviewer for many useful comments which helped improve the presentation of the paper , and the staff of gmrt for their help with the observations . the gmrt is a national facility operated by the national centre for radio astrophysics of the tata institute of fundamental research . we thank the nrao for use of the archival data . the national radio astronomy observatory is a facility of the national science foundation operated under co - operative agreement by associated universities inc . this research has made use of the nasa / ipac extragalactic database ( ned ) which is operated by the jet propulsion laboratory , caltech , under contract with the national aeronautics and space administration . baars j.w.m . , genzel r. , pauliny - toth i.i.k . , witzel a. , 1977 , a&a , 61 , 99 baum s.a . , heckman t.m . , bridle a. , van breugel w.j.m . , miley g. k. , 1988 , apjs , 68 , 643 beswick r.j . , peck a.b . , taylor g.b . , giovannini g. , 2004 , mnras , 352 , 49 chandola y. , saikia d.j . , gupta n. , 2010 , mnras , 403 , 269 conway j.e . , 1999 , aspc , 156 , 259 conway j.e . , blanco p.r . , 1995 , apj , 449 , l131 conway j.e . , schilizzi r.t . , 2000 , proceedings of the 5th european vlbi network symposium , p. 123 de koff s. , et al . 2000 , apjs , 129 , 33 emonts b.h.c . , morganti r. , tadhunter c.n . , oosterloo t.a . , holt j. , van der hulst j.m . , 2005 , mnras , 362 , 931 evans d.a . , et al . 2008 , apj , 675 , 1057 filippenko a.v . , 1987 , iaus , 124 , 761 giovannini g. , feretti l. , gregorini l. , parma p. , 1988 , a&a , 199 , 73 gupta n. , saikia d.j . , 2006a , mnras , 370 , l80 gupta n. , saikia d.j . , 2006b , mnras , 370 , 738 gupta n. , salter c.j . , saikia d.j . , ghosh t. , jeyakumar s. , 2006 , mnras , 373 , 972 hurt t. , antonucci r. , cohen r. , kinney a. , krolik j. , 1999 , apj , 514 , 579 hardcastle m.j . , harris d.e . , worrall d.m . , birkinshaw m. , 2004 , apj , 612 , 729 jamrozy m. , konar c. , machalski j. , saikia d.j . , 2008 , mnras , 385 , 1286 konar c. , jamrozy m. , saikia d.j . , machalski j. , 2008 , mnras , 383 , 525 lal d.v . , hardcastle m.j . , kraft r.p . , 2008 , mnras , 390 , 1105 martel a.r . , et al . 1999 , apjs , 122 , 81 miley g. , 1980 , ara&a , 18 , 165 mirabel i. f. , 1990 , apj , 352 , l37 morganti r. , oosterloo t.a . , tadhunter c.n . , van moorsel g. , killeen n. , wills k.a . , 2001 , mnras , 323 , 331 morganti r. , tadhunter c.n . , oosterloo t.a . , 2005 , a&a , 444 , l9 morganti r. , oosterloo t. , struve c. , saripalli l. , 2008 , a&a , 485 , l5 odea c.p . , 1998 , pasp , 110 , 493 pihlstrm y. , conway j. , vermeulen r. , 2003 , pasa , 20 , 62 robinson t.g . , tadhunter c.n . , axon d.j . , robinson a. , 2000 , mnras , 317 , 922 roche n. , eales s.a . , 2000 , mnras , 317 , 120 saikia d.j . , jamrozy m. , 2009 , basi , 37 , 63 saikia d.j . , gupta n. , konar c. , 2007 , mnras , 375 , l31 sarma a.p . , troland t.h . , rupen m.p . , 2002 , apj , 564 , 696 salter c.j . , saikia d.j . , minchin r. , ghosh t. , chandola y. , 2010 , apj , 715 , l117 steenbrugge k.c . , blundell k.m . , duffy p. , 2008 , mnras , 388 , 1465 steenbrugge k.c . , heywood i. , blundell k.m . , 2010 , mnras , 401 , 67 tadhunter c. , wills k. , morganti r. , oosterloo t. , dickson r. , 2001 , mnras , 327 , 227 taylor g.b . , 1996 , apj , 470 , 394 vermeulen r. c. , et al . 2003 , a&a , 404 , 861 vermeulen r.c . , labiano a. , barthel p.d . , baum s.a . , de vries w.h . , odea c.p . , 2006 , a&a , 447 , 489 van langevelde h.j . , pihlstrm y.m . , conway j.e . , jaffe w. , schilizzi r.t . , 2000 , a&a , 354 , l45
we report the results of giant metrewave radio telescope ( gmrt ) observations of hi absorption towards the frii radio galaxy 3c321 ( j1531 + 2404 ) , which is associated with an active galaxy interacting with a companion . 
 the absorption profile towards the radio core is well resolved and consists three components , of which the two prominent ones are red - shifted by 186 and 235 km s@xmath0 relative to the optical systemic velocity . the neutral hydrogen column density towards the core is estimated to be @xmath1(hi)=9.23@xmath210@xmath3(@xmath4/100)(@xmath5/1.0 ) @xmath6 , where @xmath4 and @xmath7 are the spin temperature and covering factor of the background source respectively . 
 we also present radio continuum observations of the source with both the gmrt and the very large array ( vla ) in order to understand the properties of a plume of emission at an angle of @xmath830@xmath9 to the source axis . 
 this feature appears to have a steep high - frequency spectrum . 
 the current hotspots and jet are active and seen in x - ray emission . 
 the spectral ages of the lobes are @xmath1026 myr . 
 we discuss the possibility that the plume could be relic emission due to an earlier cycle of activity . 
 [ firstpage ]    galaxies : active  galaxies : nuclei  galaxies : individual : 3c321  radio continuum : galaxies  radio lines : galaxies