Introduction

Recent advances in instrumentation have lead to a number of detections of diffuse, extended ionized gas outside of the standard galactic environment; for example, the extended disks of spiral galaxies (Bland-Hawthorn et al. 1997) and clouds in the Magellanic Stream (Weiner & Williams 1996). One particularly successful area of observation has been that of extended stellar and gaseous components surrounding radio-quiet and radio-loud active galaxies and quasars (e.g., Stockton & MacKenty 1987; Tadhunter et al. 1994; Hutchings et al. 1999). While these components vary greatly in both extent and morphology, the observations suggest that this material may be present in substantial quantities around such systems.

Traditional deep narrowband observations are expensive, in terms of both telescope time and monetary cost for interference filters for modern large-beam telescopes. In order to circumvent this, Fabry-Perot spectrometers are now being employed in a ``stare mode'' to detect diffuse emission-line gas over small fields and narrow wavelength regions. In this paper, we present deep H$\alpha$ observations of the extended ionized gas surrounding a quasar, MR 2251-178, using a new, etalon-based instrument which has been optimized for the detection of faint, extended emission-line gas, the Taurus Tunable Filter (TTF)¹. The flexibility of this instrument provides essentially monochromatic imaging at an arbitrary wavelength and bandpass. The high efficiency and tunable nature of the TTF allow the acquisition of multiple images at a range of wavelengths, providing kinematic information as well.