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Paper text:
there exists compelling morphological and kinematical evidence that the activity in powerful radio galaxies is triggered by galaxy mergers and interactions .
this is also supported by the theoretical results ( kauffmann & haehnelt 2000 ) that the evolution of supermassive black holes is strongly linked to the hierarchical build - up of galaxies .
although these processes are likely to be more efficient and frequent at high redshifts , they are observed also in relatively `` nearby '' radio galaxies .
however , considerable uncertainties remain about the nature of the triggering events .
outstanding questions include : 1 ) is the activity triggered by major mergers between gas - rich galaxies or by minor accretions events ?
2 ) what is the relationship with other types of merging systems such as ulirgs ?
3 ) at what stage of the merger do the jets and associated activity occur ? 4 ) do all giant elliptical galaxies go through a radio phase as they evolve via galaxy interactions ?
it is now clear that some early - type galaxies contain a large amount of hi , in some cases even 10@xmath4 m@xmath1 ( e.g. oosterloo et al , 2001 and ref .
therein ) .
this gas is an important element for understanding the origin and the evolution of these galaxies .
large tails / arms of neutral hydrogen are a prototypical signature of a recent merger , while gas settled in large disk - like structures may indicate an older merger .
if radio galaxies have a similar origin , we may expect to find also there such hi  signatures .
the hi  properties could allow us to understand the temporal sequence between merger , starburst phase and onset of the radio activity .
it is , therefore , important to make the connection between the presence of a rich ism , the radio galaxies and the evolution of the population of giant elliptical galaxies in general .
the detection of hi  around radio galaxies would therefore give a powerful tool to answer some of the above questions .
so far we have found three radio galaxies where the neutral hydrogen is detected at large distances from the nucleus .
an other possible candidate is the radio galaxy 3c  234 studied by pihlstrm ( 2001 ) .
coma  a ( @xmath5 ) has a spectacular system of interlocking arcs and filaments detected in optical emission lines ( see fig .  1 and tadhunter et al . 2000 for details ) .
this ionised gas and the radio structure show a striking match that is suggestive of a complex interaction between the radio structure and a rich interstellar medium . using the wsrt and the vla we have detected hi   absorption in front of both radio lobes of coma a ( see fig .  1 and morganti et al .
this is a rare case where the absorption is not detected against the nuclear regions of a radio galaxy , but it is situated at large distances ( 30 kpc ) from the centre .
the kinematics of the neutral and ionised gas suggests that they are part of the same structure , likely a large - scale disk with the radio lobes expanding into this disk .
3c  433 is a radio galaxy ( @xmath6 ) with an unusual double - lobed radio morphology ( see fig .
we have observed 3c  433 using the vla ( c - array ) and we have found that at least part of the hi  absorption ( originally detected with the arecibo telescope , mirabel 1989 ) is situated against the southern radio lobe at about 40 kpc from the nucleus ( see fig .  2 ) .
the optical depth of the absorption is only about 2% . unlike coma
a , no ionised gas has been detected near the location of the hi  absorption .
3c  433 is a far - ir bright radio galaxy with a young stellar population component ( wills et al .
the presence of large - scale neutral hydrogen could be related to the particular stage of the evolution of this radio galaxy .
3c  433 is perhaps a relatively young radio galaxy .
b2  0648 + 27 ( @xmath7 ) is a compact radio galaxy and in this object we have detected neutral gas both in emission and in absorption ( see fig .  3 , wsrt observations ) .
the neutral gas is in a disk - like structure of about 100 kpc in size and contains about @xmath8 m@xmath1 of hi .
such a large amount of gas is usually believed to originate from a `` major merger '' , i.e. a merger of two large disk galaxies .
like 3c  433 , b2 0648 + 27 is a far - ir bright galaxy with a young stellar population component .
the relatively regular kinematics of the gas indicates that the merger must have happened more than @xmath2 yrs ago and therefore the radio activity ( usually estimated to last for few time @xmath9 yrs ) appeared at a late stage in the merger .
hi  in absorption is detected only in the centre , where the radio continuum is present , and has a kinematics similar to the molecular gas detected , in large amounts , in this object ( mazzarella 1996 ) .
large - scale ( up to 100 kpc ) gas structures have been found in three radio galaxies and they can be used to understand the origin and the evolution of these systems .
the presence of a young stellar population component in two of these objects will give further constrains on type and age of the merger .
extended hi  absorption ( observed against the ly@xmath10 emission ) has been found in a high fraction of high-@xmath3 radio galaxies ( van ojik et al .
this is considered an indication that high-@xmath3 radio galaxies are located in dense environments and helps in probing the effects of radio jet propagation in this medium .
although this may be happening more efficiently and frequently at high redshifts , in the low redshift radio galaxies described here we may witness a similar situation .
however , in the low redshift galaxies , we will be able to investigate in much more detail the relation between the thermal and non - thermal gas .
large - scale ( up to 100 kpc ) hi  structures have been found in three radio galaxies using the wsrt and the vla . in one case , 
 the hi  has been detected in emission while in the other two hi  absorption is detected against the radio lobes . in at least two of 
 the three studied radio galaxies the hi   appears to be distributed in a large disk and the large amount of neutral gas detected ( @xmath0 m@xmath1 ) indicates that it is resulting from mergers of gas - rich galaxies . the relatively regular structure and kinematics of these disks 
 suggest that the merger must have happened more than @xmath2 yrs ago , therefore supporting the idea that the radio activity starts late after the merger . in these low redshift radio galaxies 
 we may witness the processes that are more efficiently and frequently happening at high-@xmath3 .    
 # 1_#1 _ # 1_#1 _ =    # 1 1.25 in .125 in .25 in