The temperature of extended gas in active galaxies : evidence for matter-bounded clouds

Abstract

We report measurements of the electron temperature at about a dozen locations in the extended emission-line regions of five active (Seyfert and radio) galaxies. Temperatures (T[O III] and T[N II]) have been determined from both the I([O III] λ4363)/I([O III] λ5007) and I([N II] λ5755)/I([N II] λ6583) ratios. T[O III] lies in the range (1.0 –1.7)x 10 4 K. We find a strong trend for T[O III] to be higher than T[N II], with the difference typically being ≈5000 K. Because the critical density for collisional de-excitation of the 1D2 level in N II is lower than that of the same level in O III, the deviations of the measured intensity ratios from those expected for T[O III] 5 T[N II] in the low-density limit are unlikely to result from collisional de-excitation. The measured values of T[O III] and the differences between T[O III] and T[N II] are very similar to those found in Galactic planetary nebulae. It is argued that the dominant form of energy input to the clouds is photoionization, but detailed modeling indicates that the temperature difference is too large to be accounted for in terms of photoionization of ionization-bounded clouds. We propose instead that both matter- and ionization-bounded clouds are present in the extended emission-line regions, with most of the [O III] emission originating from a hot zone in the matter-bounded clouds and essentially all the [N II] from the ionization-bounded clouds.