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The Origin of the HII Envelope

Several scenarios have been put forth for the origin of the ionized gas envelope in MR 2251-178, including tidal debris from an interaction with galaxy G1 some  years ago, similar debris from a merging event, clouds expelled from the quasar and/or host galaxy, a cooling flow, or the ionized portion of a large HI envelope that is gravitationally bound to the quasar (e.g., [Nørgaard-Nielsen et al. 1986]; [Macchetto et al. 1990]). 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 two ``arms'' of ionized gas and a pervasive diffuse component. This symmetry, together with the organized large-scale kinematics, casts doubt upon the interaction model for creation of the ionized gas. Furthermore, the galaxy G1, the usual culprit quoted as an interacting companion to MR 2251-178, has a velocity redshifted by 1246 km s^-1 relative to the quasar ([Bergeron et al. 1983]), yet its spatial position corresponds to that of relatively blueshifted filaments. Published spectroscopy shows the emission lines from these filaments to be symmetric and significantly narrower than this (e.g., [Bergeron et al. 1983]), although we cannot rule out the presence of a very faint, high-velocity streamer extending out toward G1.

Nevertheless, the coarse kinematic structure of the extended ionized gas indicates that it represents a distinct component from the inner EELR region, since the latter appears to rotate in an opposite sense. This implies that the envelope most probably did not originate within the host galaxy of the quasar. A retrograde merging event, in which a small, gas-rich galaxy has been subsumed by the quasar, remains a possibility, however again the relatively well-ordered large-scale kinematics and azimuthal symmetry of the ionized envelope are problematic.

The cooling flow hypothesis appears equally unlikely. As pointed out by Macchetto et al. (1990), the large size and ordered kinematics of the envelope, and the off-centered position and peculiar systemic velocity of the quasar with respect to the underlying cluster, all argue against this scenario. By elimination, our observations therefore favor a model in which the extended ionized envelope resides within a large complex of HI gas centered about the quasar. If this is correct, a deep search for 21 cm HI line emission should reveal the massive neutral envelope and constrain its origin (e.g., captured intergalactic HI clouds, remnant accreting gas from galaxy formation, etc.).