Matter-Bounded Photoionized Clouds

Abstract

The extended ionized gas in Seyfert and Radio-Galaxies is characterized by large values of the ratio HeII/Hβ, which exceeds the value predicted by the standard photoionization model in which the ionizing continuum consists of a power-law. This has lead to the suggestion of considering a matter-bounded (MB) component [3], [5], [2] for explaining such extreme values. We now find[l] that it is also possible to resolve the temperature problem[3] if the thickness and the ionization parameter of the MB is appropriately selected. Adopting a canonical power law (α=-1.3) and solar abundances (Z=l), we can account for the observed trends in excitation (represented for example by the ratio [O II]/[O III] in Fig. 1)) by varying the relative number of MB clouds (which emit the high excitation lines CIV, [Ne V], HeII… and most of [O III]) versus the number of ionization-bounded (IB) clouds (which emit [N II], [S II] [O II], [O I]…). We obtain a one-parameter sequence (solid line) which is function of the weight A_M/I of the MB component relative to the IB component. This A_M/I-sequence successfully reproduces the observed range in He II/Hβ. Note the failure of the traditional U-sequence (long dashed line). Fig. 2 indicates that we can also reproduce the ratio R_OIII= [O III]λ4363/[O III]λ5007 and therefore resolve the temperature problem. Interestingly, our model indicates a temperature difference of 5 000K between the IB component ([N II] temperature ≈ 10 000K) and the MB component ([O III] temperature ≈ 15 000K) while the traditional U-sequence predicts a difference of only 1 000 K. Such difference of 5 000 K has been reported[4] in the extended gas of Cygnus A.