We have calculated the physical conditions throughout the merger Seyfert galaxy NGC 6240 by modelling the observed optical and infrared line ratios .
We have found that the optical spectra are emitted by clouds photoionised by the power-law radiation flux from the AGN ( or AGNs ) , and heated mainly by the shock accompanying the propagation of the clouds outwards .
The infrared line ratios are emitted from clouds ejected from a starburst which photoionises the gas by the black-body radiation flux corresponding to a stellar colour temperature of about 5 10 ^ { 4 } K. Both the flux from the AGN and the ionization parameters are low .
The most characteristic physical parameters are the relatively high shock velocities ( \geq 500 km s ^ { -1 } ) and low preshock densities ( \sim 40-60 cm ^ { -3 } ) of the gas .
The C/H , N/H , O/H relative abundances are higher than solar by a factor \leq 1.5 .
We suggest that those relative abundances indicate trapping of H into H _ { 2 } molecules rather than high metallicities .
Adopting an initial grain radius of 1 \mu m , the dust temperatures calculated in the clouds reached by the power-law radiation and by the black-body radiation are 81 K and 68 K , respectively .