Galaxy dark matter: Galaxy-galaxy lensing in the Hubble deep field

In this Letter we present calibrated observations of the average mass of galaxies with 22 <I <25 in the Hubble Deep Field. We measure the mean mass profile using the statistical gravitational lens distortion of faint blue background galaxies, based on 2221 foreground-background pairs. The observed weak lensing distortion is calibrated via full three-dimensional simulations and other Hubble Space Telescope data on a foreground cluster of known mass. Inside a projected radius of 5 '' we find a 3 sigma detection of shear. Fitting to an isothermal mass distribution, we find a distortion of 0.066(-0.019)(+0.018) at a radius of 2 '' (similar to 8 h(-1) kpc). The (1 sigma) limits on the parameters for the truncated isothermal model are sigma(upsilon)=185(-35)(+30) km s(-1), r(outer)greater than or equal to 15 h(-1) kpc, and r(core)=0.6(-0.6)(+1.3) h(-1) kpc. This corresponds to an average galaxy mass interior to 20 h(-1) kpc of 5.9(-2.7)(+2.5)x10(11) h(-1) M.. Inside 10 h(-1) kpc, we find an average rest-frame V-band mass-to-light ratio of 11.4(-6.7)(+6.0) h (M/L(V))..