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Paper text:
radio galaxies display a wide range of structures in radio images .
the most common large - scale structures are called lobes : these are double , often fairly symmetrical , roughly ellipsoidal structures placed on either side of the active nucleus . a significant minority of low - luminosity sources exhibit structures usually known as plumes which are much more elongated .
some radio galaxies show one or two long narrow features known as jets coming directly from the nucleus and going to the lobes .
hence , the radio galaxies morphologically are divided into fri and frii types .
the fri objects ( ` edge - darkened doubles ' ) generally contain prominent , often two - sided jets , with the lobe emission trailing off into intergalactic space . whereas , the frii objects ( ` edge - brightened doubles ' , ` classical doubles ' ) contain hot spots in one or both lobes .
it seems that frii radio galaxies ( see fig .  [ fig:3c98 ] ) are usually isolated field sources and form a remarkably homogeneous set . on the other hand , the fri radio galaxies are much more varied .
they are not only found as isolated field sources , they are also often found in clusters @xcite .
for example , head - tail galaxies , which are fri objects , are almost always associated with clusters of galaxies and are characterized by a highly elongated radio structure with the associated optical galaxy at one end . a classical example ( see fig .
[ fig:3c129 ] ) is 3c129 ( a narrow - angle - tail ) which , along with its companion 3c129.1 ( a wide - angle - tail ) , is a member of an x - ray cluster . in the canonical picture ,
the characteristic shape is explained by the kinematics of the source , which is governed by the dominant gravitational force in the cluster and the properties of the beams and jets @xcite .
similarly , wide - angle - tail galaxies are intermediate between standard fri and frii objects , with collimated jets and sometimes hotspots , but with plumes rather than lobes , found at or near the centres of clusters . here again , the characteristic shape is mainly due to the dominant gravitational force in the cluster .
therefore , the general morphology of radio sources lends encouragement to ideas of confinement , or at least radio morphology partly governed by an external gas ( @xcite , and see hardcastle in this proceeding ) , but some objects persist in complicating the simple picture .
for example , b1059@xmath0169 ( fig .
[ fig : b1059 ] ) which shows x - shaped structure .
such sources are characterized by two axes , the ` wing ' axis oriented at an angle to the ` active ' axis , giving the total source an ` x ' shape .
these two sets of lobes usually pass symmetrically through the centre of the associated host galaxy , and the majority of these sources are frii and the rest are either fri or mixed .
these have been modelled via backflow from the lobes along with buoyancy - driven outflow .
precession of the radio axis also comes to mind .
they have also been put forth as derivatives of central engines that have been reoriented , perhaps due to a minor merger .
alternatively , they may also result from two pairs of jets that are associated with a pair of unresolved agns ( see  @xcite for a detailed summary ) .
the key point is that interpretation of radio source morphology , in general , is not necessarily simple .
* spectral ageing * under the standard model of a frii source evolution , the source grows longer as the jet pushes back the external medium .
if there s no significant re - acceleration within the lobes and no significant transport of particles since the last acceleration , then the central regions are ` older ' than the outer regions , in the sense that particles located in the central regions were processed through the hot spot shock at a time before those currently radiating at the ends of the radio source .
hence , the spectral index of the radiation from the central regions should have a steeper spectral index than that at the ends .
whereas , in the case of fri sources , e.g. , in almost all head - tail and wide - angle - tail galaxies , the radio spectrum has been found to steepen with distance along the tail and has been interpreted in terms of ageing of electron population .
finally , in several of the formation scenarios mentioned above for x - shaped sources , the wings are interpreted as relics of past radio jets and the active lobes as the newer ones .
hence , the wings are expected to show steeper spectra than the active lobes in standard models for electron energy evolution .
in recent years we have used the gmrt ( @xcite ) to study morphology of several radio galaxies at 240  mhz and 610  mhz , with a resolution of about 10  arcsec  and  5  arcsec , respectively .
the gmrt visibility data were analyzed using aips in the standard manner .
all details of the analysis procedures used are given in @xcite .
the gmrt has a hybrid configuration with 14 of its 30 antennae located in a central compact array with a size 1.1 km ( comparable to the vla d configuration ) and the remaining antennae distributed in a roughly ` y'-shaped configuration , giving a maximum baseline length of 25 km ( comparable to the vla b configuration ) .
hence , a single observation with the gmrt samples the ( @xmath1 ) plane on both short and long baselines , and can map detailed source structure with a reasonably good sensitivity .
i bring together two remarkable results obtained using gmrt .    * similar / dis - similar radio morphologies * it is remarkably unusual that any frii source has a morphology at low radio frequency ( less than a few hundred mhz ) that is different from its high radio frequency ( ghz ) morphology .
[ fig:3c98 ] shows 3c98 as an example showing similar radio morphologies at 240 mhz and 1.5  ghz .
in addition the 74  mhz and the 4.9  ghz radio morphologies are also identical .
the observations of some 3c radio galaxies at 151mhz and 1.4ghz @xcite show that the lobe lengths at these different frequencies are the same and they also suggest that the particles responsible for the low - frequency emission are entirely co - spatial with those responsible for the high - frequency emission .
this suggests that synchrotron - emitting particles of all energies permeate the lobe magnetic field in the same way , despite the fact that the higher energy particles have shorter radiative lifetimes than the lower energy ones .
this hints that low - frequency synchrotron emission fades ( nearly ) as rapidly as high - frequency synchrotron emission @xcite ; and so , are the observations really inconsistent with the idea that synchrotron cooling is not the dominant energy - loss mechanism for the synchrotron plasma ? @xcite note that by the time we get to 100 ghz there really are fairly obvious morphological differences in lobes , although the sensitivity of those observations is not great . whereas , the fri sources , in particular the head - tail galaxies and wide - angle - tail galaxies show signs of synchrotron cooling in spectral index images @xcite made using narrow frequency spacing ( 240 mhz and 1.4 ghz ) and often show the presence of steep spectrum diffuse emission at low radio frequencies , which is not seen at high radio frequencies ( fig .
[ fig:3c129 ] ) .    [ cols= " < , < " , ]
presently the gmrt observations have provided several high resolution , high sensitivity images , and it is now possible to probe , statistically , the morphological properties of radio sources .
i have presented above two unusual results , but it will still be important to obtain observations using gmrt of complete , unbiased samples of radio galaxies to draw general conclusions about the population of radio galaxies as a whole .
we thank the staff of the gmrt that made these observations possible .
gmrt is run by the ncra of the tifr .
i warmly thank my collaborators m.j .
hardcastle , r.p .
kraft and a.p . rao and especially a.l .
roy for a careful reading of this manuscript .
we have studied several radio galaxies at low radio frequencies using gmrt . our prime motivation to detect faint radio emission at very low frequencies due to low energy electrons .    
 our results provide evidence that there exists two classes of sources on morphological grounds . 
 the first class is explained by the simple picture of spectral electron ageing but in the second class the low - frequency synchrotron emission fades rapidly as high - frequency synchrotron emission . 
 in addition , in several sources , the spectra of low - surface - brightness features are flatter than the spectra of high - surface - brightness features , which suggests that either the simple picture of spectral electron ageing needs revision or we need to    the images and statistics , and the relevance of these results along with the role of gmrt in exploring several unknowns are presented .