Morphology

Our observations detect H$\alpha$ emission throughout the central arcminute of the field. We confirm two primary ionized gas components in MR 2251-178: The first consists of strong H$\alpha$ emission directly surrounding the quasar out to a radius of $\sim 20$ kpc, slightly elongated along the axis of the quasar's radio jet (PA $\sim 102$^o; Macchetto et al. 1990). This component has been identified as an ``extended emission-line region'' (EELR) of the quasar host galaxy (Mulder & Valentijn 1992).

The second H$\alpha$ component consists of a much larger network of filamentary emission. Diffuse emission is visible surrounding the galaxy on all sides, out to a radius of at least 50 kpc; knots and filaments of ionized gas are visible to radii up to 120 kpc towards the northeast, southeast, and southwest. The morphology of the filaments is very similar to that of spiral arms, with an ``arm'' clearly visible in each of these three directions. This remarkable symmetry has not been seen in previous, i.e., less deep, imaging observations.