Evolution of Narrow Line Clouds

Summary

Cloud velocities in the NLR appear to be related mainly to the host galaxy (Whittle, 1992). This is compatible with evidence that the clouds are predominantly infalling (De Robertis & Shaw, 1990), with only a minor component of the line emission arising from jet induced emission. Either these infalling clouds contribute significantly to the BLR, connecting this to the NLR smoothly through an intermediate zone, or they are destroyed. We expect that the cloud geometry and dynamics will be reflected in the line emission ratios and profiles. We look here at the emission from clouds that are destroyed in an outflowing supersonic wind from the central nucleus which we assume to be in pressure balance with the ambient ISM (Smith, 1984; Mobasher & Raine, 1987)

We model the hydrodynamic evolution of two clouds each having an initial density of 10⁴ cm⁻³ and a temperature of 10⁴ K with a free-fall velocity of 2 × 10⁷ cm s⁻¹. The smaller cloud has a mass of 6.6 × 10⁻⁴ M_⊙ (r_c = 25 × 10¹⁵ cm) and the larger one a mass of 6.6 × 10² M_⊙ (r_c = 25 × 10¹⁷ cm). The clouds fall under gravity into a supersonic wind of Mach number 1.5 with a density that increases as r⁻². The initial distance from the continuum source is 10²¹ cm and its luminosity is ∼ 10⁴⁴ erg s⁻¹.

For the hydrodynamic simulation we employ a code utilising a first order Godunov scheme (Godunov, 1959) developed by R. Hillier of Imperial College London and adapted for astrophysical use at Leicester University by M. Dubai and P. Foulsham.