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Published online by Cambridge University Press: 07 August 2017
Electronographic plates have been obtained with the ESO 3,6 m telescope and the CFH telescope. One of the plates is reproduced figure 1. We have studied the regions surrounding the nucleus of 3C 120 (r<8″) and emitting in the continuum. The “jet” found by Wlérick et al. (1981) has been resolved into 3 condensations, A, B and D; another condensation C is located 7″ S-E of the nucleus; a filament F leaves the nucleus in the direction of condensation B. The 4 condensations have been measured in colours U B V and V′, this last band excluding the [OIII] emission lines. We have located each condensation with respect to the zones where gaz emission is observed and particularly two regions that we call E1 and E2. We have also located condensation A with respect to the radiojet and we have searched, in B and V colours, for polarization of this condensation. The results are the following:
1. The 4 condensations are faint: 20,0<B<20,8, corresponding Jo the absolute magnitudes: −16,1 < Mg<−15, 3, using H = 60 km s−1 Mpc−1. Colours of condensations A, B and D are similar: U-B∼−0,20; B-V∼0,87; condensation C is less blue: U-B∼0,30; B-V∼0,90.
2. The brightest condensation A is elongated nearby North-South, its southern part pointing toward S-E (Fig. 1). It is interesting to place A with respect to the radiojet (cf. Browne et al., 1982, Walker et al., 1987). The maximum of A is located 1″ North of the first knot of the jet but the southern part of A coincides with this knot. The maximum of the emission region E1 is located 1″ West of the maximum of A, but region overlaps a large part of A. Though no significant polarization has been found for A taken globally, we suggest that the flux of A consists of two parts: in the southern region of A, the flux could be synchrotron radiation corresponding to a spectral index radio-visible ∼0,7; the northern part of A would be a zone of star formation triggered by the impact of the jet on region E1; the situation is very similar to the one found with Minkowskis object (Van Breugel et al., 1985; Brodie et al., 1985), concerning the relative position of A and the jet, as well as the variation of polarization of this jet and its change of direction; on the other hand, A is not as blue as Minkowski's object and this is likely due to the fact that the number of young stars is smaller; the change of the polarization in a well localized part of the radio knot (Walker et al., 1987) suggests that the gazeous region E1 is flattened and that the jet crosses it obliquely.
3. Condensations B and D form a chain with A and they are located in a region where no emission is observed; they are made probably of stars older than the one of condensation A.
4. Brightness and colours of condensation C are similar to those of the nucleus of a spiral galaxy. We suggest that C is the residue of a galaxy captured by 3C 120; this agrees with the work of Heckman et al. (1986); these authors consider a galaxy merger as the origin of the activity of 3C 120 and, as they do not see any neighbouring galaxy, they state that the merger has nearly reached completion. The bright gazeous region E2 connects possibly C and the nucleus of 3C 120.
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