Astron. Astrophys. 356, 73-82 (2000)

6. Conclusions

We have studied in detail the dynamics of the interaction of a jet with a large cloud pre-existing in the ISM in order to find the conditions for which it is possible to reproduce the main physical parameters of the NLR emitting material.

Following the suggestion by Steffen et al. (1997b) that the most relevant effects of the interaction arise when a jet hits dense massive clouds, we adopted a quite simplified geometry of a single gas condensation with can be astrophysically identified with a giant molecular cloud. As the interaction last for a time considerably longer than the cloud crossing time more than one cloud will be interacting at any given time and they will display simultaneously the different evolutionary stages of the interaction. Furthermore the characteristic jet-line structure of the NLR is thus reproduced. In any event, this case, i.e. the head-on collision with a large cloud, is the most efficient case of interaction, for the compression, acceleration and heating of the NLR material.

We concentrated our efforts on the exploration of the parameter plane (, since the other parameters, on which the simulation depends, have little influence on the properties of the optically emitting material. We have found that the condition for obtaining values of density, temperature and velocity in the observed range can be translated in a condition on the parameter , which is the ratio of the cloud crossing timescale to the radiative timescale () and which depends on our two fundamental parameters and . For small values of , radiation is inefficient and it is not possible to produce regions dense enough, while, on the other hand, for large values of , the cloud is too dense and the obtained velocities are too low. We have explored a range of cloud densities which can be considered typical of GMCs and, for this range, the jet velocities span an interval from km s^-1 to km s^-1.

The jet kinetic power corresponding to these combinations of parameters (for a jet density of 1 cm^-3) ranges from to erg s^-1, in general agreement with the estimates of Capetti et al. 1999 for Mrk 3. For jet density much lower than 1 cm ^-3, however, in order to match the observed NLR conditions we would need a correspondingly higher velocity and therefore untenable requirements on the kinetic power which grows with . We conclude that jets in Seyfert galaxies are unlikely to have densities much lower than 1 cm ^-3 and velocities higher than km s^-1, and therefore they are very different from their counterparts in radio-galaxies in which densities are much lower and velocities are relativistic.

Concerning radio-galaxies we can speculate that with lower jet densities and higher velocities, the gas postshock temperatures and radiative time would be increased with respect to the case of Seyfert galaxies and therefore the conditions for having efficient line emission would be more difficult to meet. In addition the different properties of the jet environment in the elliptical galaxies hosting radio-galaxies render encounters with gas condensations less likely to occur. This probably explains why the association between radio and line emission although often present in radio-galaxies (e.g. Baum & Heckman 1989) is not as strong as in Seyfert galaxies.

Finally, the study of the global dynamics allowed us to have estimates of the overall efficiency of the conversion of kinetic to high frequency radiative power in the shocks that form in the interaction between jet and ambient medium. We have found that the efficiency is increased by the presence of the cloud, its peak value is 0.1 - 2%, its typical value is much lower and it decreases with the jet power. These results lead us to the conclusion that radiation emitted in shocks can be only a small fraction of the overall ionization budget of the NLR, although it can have local and transient important effects.

© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000
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