2. Observational data and astrophysical scenario
Observational data provide us with quite detailed information on the physical conditions of the narrow line emitting regions, in particular HST observations can now be used to determine the properties of individual NLR clouds: typically, densities are larger than cm-3, temperatures are of the order of K, and velocities are (Caganoff et al. 1991, Kraemer et al. 1998, Ferruit et al. 1999, Axon et al. 1998, Capetti et al. 1999).
These are the observational constraints that we try to match in our simulations. Results of simulations of a jet impinging on a uniform medium, with properties typical of the ISM, have shown that it is not possible to match, in this situation, the density values reported above (Steffen et al. 1997a, Rossi & Capetti 1998). We will therefore consider throughout the rest of the paper the case of a jet impinging on pre-existing inhomogeneities. We can identify such inhomogeneities with giant molecular clouds (GMCs), that typically populate spiral galaxies. These objects have typically mass , radius pc, and temperature K (Blitz 1993). The resulting particle densities span from a few up to about hundred particles per cm3.
A supersonic jet, of radius pc, that bore its way through the interstellar medium has a considerably good chance of impinging frontally upon a (much larger) GMC, and this is the case we will consider in our simulations. In any event, this latter case, i.e. the head-on collision with a large cloud, can be considered the most efficient case of interaction, for the compression, acceleration and heating of the NLR material.
As discussed below the effects of the jet/cloud interaction last for a time considerably longer than the cloud crossing time. Moreover, the jet crosses the tenuous inter-cloud regions at a much higher speed than while in a cloud. We therefore expect that more than one cloud will be interacting at any given time and they will display simultaneously the different evolutionary stages of the interaction.
© European Southern Observatory (ESO) 2000
Online publication: March 28, 2000