Weak gravitational lensing around field galaxies in Hubble Space Telescope survey images

Using data from the Hubble Space Telescope (HST) Medium Deep Survey, a longterm key project, together with generically similar archived data, we have discovered evidence for weak gravitational 'shear' of background field galaxies (I = 22-26) in the vicinity of isolated, foreground galaxies (I = 15-22), especially those of early type. The statistical lensing is demonstrated by the slight preferential orientation of the major axes of the background galaxies in directions at right angles to the lines joining them to the foreground ones. The detected shear is at the level delta phi = 2.degrees 5 +/- 1.degrees 2 averaged over the background galaxies at about similar to 10 half-light radii from 400 foreground ellipticals, corresponding to a polarization of about 0.04. A positive but less significant result is also reported for foreground spirals. This result excludes the de Vaucouleurs model for the mass distribution of ellipticals but it is consistent with elliptical galaxies having a total mass-to-light ratio similar to 100 and extensive dark haloes.