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The Ionization of the HII Envelope

The high excitation level of the brighter knots around MR 2251-178 (e.g., [Bergeron et al. 1983]; [Nørgaard-Nielsen et al. 1986]; [Macchetto et al. 1990]) requires an energetic source of ionization. In-situ ionization by a faint hot stellar component is unlikely to be dominant for a number of reasons. Deep broadband imagery of MR 2251-178 reveals a morphology very unlike that of our narrowband observations. The R-band image of Hutchings et al. (1999) exhibits a faint component extended along the N-S direction, markedly different from the symmetric spiral pattern that we observe in H. Moreover, Hutchings et al. (1999) do not detect an excess of continuum emission at the locations of any of the brighter H knots in the envelope. Perhaps even more damaging to the stellar ionization scenario is our detection of a large amount of diffuse ionized gas outside of the brighter knots. We therefore favor an external source of ionization.

The presence of a significant amount of ionized gas at large angles relative to the jet axis (PA ; [Macchetto et al. 1990]) seems to rule out ionization mechanisms directly associated with the quasar jet (e.g., shocks). We are therefore left with the possibility that the ionization of the nebula is sustained by the quasar radiation field. As described in detail in previous studies (e.g., [Bergeron et al. 1983]; [Nørgaard-Nielsen et al. 1986]; [Macchetto et al. 1990]), the power radiated by the quasar can easily account for the high ionization level of the nebula. However, the relative symmetry of the envelope requires that the ionizing radiation is escaping the quasar symmetrically with respect to our line of sight. The large-scale radiation field from MR 2251-178 therefore shows no sign of anisotropy or alignment with the radio axis, contrary to expectations from unified models of active galactic nuclei that rely on orientation effects (e.g., [Barthel 1989]; [Antonucci 1993]; [Urry & Padovani 1995]). This symmetry of the radiation field, if typical of all quasars, may have important consequences on our understanding of the impact that quasars have on the intergalactic environment (e.g., proximity effect of Ly clouds; [Carswell et al. 1982]; [Murdoch et al. 1986]).