the southern lobe of PKS 1254-300 See text for details.

In document Optical emission lines in radio sources of intermediate power (Page 137-140)

of th e backflow were considerably faster. PK S 1254-300 lies a t th e centre of the cluster Abell 3532 (Abell, Corw in and Olowin 1989). A typical velocity of the host galaxy th ro u g h th e in tra-clu ster m edium is expected to be on th e order of a few x lO 3 k m s-1 to ~ 104 k m s- 1 . T he velocity im p arted via bouyancy to a p lasm a very m uch lower in density th a n th e surrounding m edium is on th e order of a few h u n d red kilom eters p er second. Hence if eith er one of these processes could noticeably deflect the backflow in a class II radio lobe (as ap p ears to be th e case for this o b ject) th en th e velocity of th e plasm a flow w ith respect to the surrounding m edium cannot be very m uch g reater th a n these typical velocities. We fu rth er note th a t C am eron (1988) has derived velocities of aro u n d 2000 k m s-1 for th e tails in PK S 2104-256 (see tab le 4.2) w ith which we have already draw n m orphological parallels. It is in terestin g to note th a t PK S 2104-256 also lies in th e core of a cluster of galaxies. It is generally believed th a t th e grow th of a class II radio source is governed by ram pressure balance betw een th e plasm a in th e lobe and th e ex tern al m edium . It m ay be tru e th a t th e presence of a high pressure ex tern al m edium in th e core of a cluster is responsible for th e lower speed of advance of th e radio lobes in PK S 1254-300 and PK S 2104-256. It m ay be in terestin g to consider w h eth er these two source are in term ed iate betw een class II radio sources an d th e class of wide angle tail radio sources comm only found in clusters of galaxies (O w en and R udnick 1976).

P K S 1 4 1 4 -2 1 2 : A co n to u r m ap of this source is p resen ted in figure 4.14. T h e sp ectral analysis is p resen ted in figures 4.15 and 4.16. This is a class II source w ith a peculiar surface b rightness d istrib u tio n in th e n o rth e rn lobe. T h e n o rth ern lobe has a brig h t h o tsp o t ap p ro x im ately 80 kpc from th e core an d on th e eastern edge of th e lobe. T his ap p ears to be a deflected je t w ith the h o tsp o t assym etrically disposed. This source m ay be sim ilar to PK S 2317-277 (see figure 4.28) w ith “backflow” continuing on from a deflected je t, or it m ay be a source w ith a new h o tsp o t w ithin an old lobe rem iniscent of th e “d en tist drill” m odel (Scheuer 1982) or th e double h o tsp o ts in Cygnus A (A lexander et al. 1984). In defining th e profiles across this lobe we ad o p ted th e form er in te rp re ta tio n , tak in g slices from the h o tsp o t tow ard th e n o rth along th e (presum ed) flow direction. A glance a t figure 4.15 shows o u r p resu m p tio n to have been erroneous. Figure 4.15g shows th a t th e plasm a actually “ages” from th e n o rth e rn tip of th e lobe. T h e tre n d of sp ectral index along th e flow is non-linear (see figure 4.15a). T h e situ atio n is obviously quite com plex an d we refrain from analysing th is lobe any further. It is certain, however th a t th e position of th e h o tsp o t is inconsistent w ith the a p p aren t plasm a flow direction an d we suggest th a t this is a case of a young h o tsp o t w ithin an older lobe. H igher resolution observations of this source would be m ost revealing.

00 30 0 0 30 0 0 30 0 0 30 0 0 14 1 4:

3, 6

1 4 1 4 - 2 1 2 I POL 1 5 1 4 . 9 0 0 MHZ 4 4 4 4 2 4 0 3 8 3 6 3 4 R I GH T ASCENSI ON PEAK FLUX - 6 . 0 3 6 3 E - 0 2 J Y / B E A M

H

Contour map of PKS 1414-212 at 1.4 GHz. Contour levels are at

9, 12, 15, 18, 24, 30, 36, 42, 48, 54 and 60 m Jy/beam .

1 4 1 4 —212 n o r th backflow .

In document Optical emission lines in radio sources of intermediate power (Page 137-140)