Imaging H I in the lensing galaxy 2237+0305

Spectral-line observations of neutral hydrogen in the galaxy 2237+0305, which lenses the quasi-stellar object (QSO) Q2237+0305, are presented. In 1997 August, synthesis observations, undertaken with the National Radio Astronomy Observatory Very Large Array, detected H i emission from the lensing galaxy, and indicated a total H i mass of in the galaxy. The deprojected velocity width of the galaxy is 620±35 km s−1. The estimated dynamical mass of 2237+0305 implies an H i mass-to-(blue)-light ratio of ∼0.3 M⊙/L⊙. An r1/4 bulge fitted to the maximum circular speed yields a total bulge mass of . This is found to be identical to the total bulge mass determined by fitting an r1/4 bulge to the lensing data, which provide a precise value for the total mass in the region encircled by the quasar images. This result confirms that the bulge in 2237 + 0305 is maximal. The line-of-sight velocity dispersion for both fitted bulges is (145±8) km s−1, and is consistent with the observed stellar velocity dispersion corrected for observational effects, and with physical models of the system. The observations also show that 2237+0305 is a member of a loose group, comprising at least five other galaxies with radial velocities within 300 km s−1 of the lensing galaxy. Deep optical images of the faintest H i detection show that what was initially thought to be a rotating disc galaxy is two low-surface-brightness dwarf galaxies near the lens. The median projected separation of the galaxies in the group is , and the median radial displacement of the individual galaxy velocities from the adopted group radial velocity of 11 695 km s−1 is 108 km s−1. The long crossing time of the system, of the order of 5 Gyr, and the lack of evidence for tidal disturbance of the galactic H i discs imply that the group is loose, which, combined with the dynamical mass considerations, suggests a shallow and dark potential. While compact groups near lensing galaxies can produce shear in lensed or background images, it is shown that the shear effect of the group galaxies and the dark potential on the geometry of the ‘Einstein Cross’ is negligible.