The Origin and Ionization of the HII Envelope

Our observations reveal a tremendous spatial extent for the emission in this system. We also observe a relatively high degree of symmetry in the envelope, including at least three ``arms'' of ionized gas. This symmetry, together with the organized large-scale kinematics, does not support an interaction model for creation of the ionized gas. The fact that galaxy G1 ($v_r = +1246$ km s$^{-1}$ relative to MR 2251-178) is positioned just north of the quasar, amongst relatively blueshifted filaments, strengthens this argument. Nevertheless, the coarse kinematic structure of the extended ionized gas indicates that it represents a distinct component from the inner counter-rotating EELR, suggesting that the envelope most probably did not originate within the quasar and/or host galaxy. Our observations appear to favor a model in which the extended ionized envelope resides within a large complex of HI gas centered about the quasar.

The source of the ionization of the extended component is difficult to constrain with our limited spectral information. Again, the relative symmetry of the envelope in our observations virtually eliminates ionization mechanisms associated with the quasar jet (PA $\sim 102$^o; Macchetto et al. 1990). One could conceive of a small population of in-situ hot stars which would provide sufficient ultraviolet flux to ionize the gas yet be faint enough to avoid detection in broadband observations (Macchetto et al. 1990). However, a significant amount of diffuse ionized gas is present in the nebula, suggesting an exterior ionization source. More importantly, the deep R-band imagery of Hutchings et al. (1999) exhibits a faint component extended along the N-S direction, markedly different from the NE-SW axis along which we observe strong emission in H$\alpha$. They do not detect an excess of continuum emission at the locations of any of the brighter H$\alpha$ knots in the envelope. We therefore concur with previous authors that the primary source of gas excitation is probably the quasar itself.

In summary, our H$\alpha$ observations of MR 2251-178 demonstrate that significant quantities of ionized gas may be present around luminous quasars at flux levels virtually undetectable by standard narrowband imaging techniques. The TTF promises to greatly impact such observational programs by increasing the depth to which diffuse line emission can be detected by a factor of at least 5-10.