1999ApJ...524L..83S

We report the results of deep Hα imaging of the ionized gas surrounding the low-redshift (z=0.0638) quasar MR 2251-178 using the Taurus Tunable Filter on the Anglo-Australian Telescope. Our observations reach a 2 σ detection level of ∼5x10^–18 ergs.s^–1.cm^–2.arcsec^–2, more than an order of magnitude deeper than conventional narrowband images previously published on this object. Our data reveal a spiral complex that extends more or less symmetrically over ∼200 kpc, making it the largest known quasar nebula. The total mass of ionized gas is 6x10¹⁰ M_☉ (upper limit), a large fraction of which is in a very faint, diffuse component. The large and symmetric extent of the gaseous envelope favors a model in which the filamentary and diffuse emission arises from a large cloud complex, photoionized by the bright quasar. A crude kinematic analysis reveals relatively smooth rotation, suggesting that the envelope did not originate with a cooling flow, a past merger event, or an interaction with the nearby galaxy G1.