Ionized Nebula around the Quasar MR 2251 - 178

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 (TTF) on the Anglo-Australian Telescope. Our observations reach a 2-σ detection level of ∼ 5 × 10 −18 erg 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 6 × 10 10 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.