The HI-Rich Elliptical Galaxy NGC 5266

We present new HI images of the dust-lane elliptical galaxy NGC 5266 already known from single-dish observations to contain a large amount of HI. Our new data confirm that NGC 5266 contains ~2.4 x 10^10 m_sun (for H0 = 50 km/s/Mpc) of neutral hydrogen, i.e. more than most spiral galaxies of similar luminosity. The gas extends to ~8' each side of the nucleus, or 8 times the optical half-light radius R_e. Surprisingly, most of the HI extends almost orthogonal to the optical dust lane. A small fraction of the HI is associated with the dust lane and there are some hints of a faint warp connecting the two structures. The HI distribution is somewhat clumpy and asymmetric, but the overall velocity field in the inner 4' can be successfully modeled by assuming that the gas lies mainly in two perpendicular planes - in the plane of the dust lane in the central parts and orthogonal to this in the outer regions. Beyond the 4' radius, the gas has a different structure and may be in two tidal tails, or an edge-on ring. Measurement of the HI rotation curve is affected by asymmetries in the gas distribution, but the rotation velocity is at least 250 km/s at a radius of 4', and a flat rotation curve of ~270 km/s is consistent with the data. This would imply a value of M/L_B ~8 at ~4 R_e. If the outermost HI is in an edge-on ring, we estimate M/L_B ~16 at ~8 R_e. Comparing this with the value derived from optical observations for the inner region we find an increase of M/L_B by a factor ~2.7 at r~4 R_e, and by 5.3 at r~8 R_e. The large amount of neutral gas observed in NGC 5266 (M_HI/L_B~0.2) and the HI morphology, suggest that this object may have formed from the merger of two spiral galaxies. If so, NGC 5266 probably represents a relatively old merger remnant since most of the HI gas appears settled.