MR 2251 - 178: a nearby QSO embedded in a giant H II envelope

The nearby QSO MR 2251-178 photographically shows a surrounding nebulosity of diameter approximately 27 kpc (H0=50 km s-1 Mpc-1). Spatially resolved spectrophotometry of the QSO and surrounding field reveals that the QSO lies in the nucleus of a galaxy having a gaseous component of very high ionization, high temperature (T=3x104K) and relatively low abundance and is surrounded by a giant HII envelope observed in (OIII) emission. The ionized matter in the envelope extends over a region of at least 230x60 kpc and is not associated with any nebulosity apparent on broad-band photographic plates. A clear rotation pattern is seen close to the QSO with a velocity gradient of 17 km s-1 kpc-1 out to 6 kpc. At large distances from the QSO nucleus, 50<r<170 kpc in the south-east direction, the rotation curve is flat. The total mass of MR 2251-178 within a radius of 170 kpc is (1.3x1012/sin2i)Msun. From the ionization equilibrium an upper limit of 0.2 cm-3 is derived for the density in the envelope and the mass of ionized gas is 2x1010<Msub(HII)<5x1011Msun. (author)