Galaxy Dark Matter: Galaxy-Galaxy Lensing in the Hubble Deep Field

In this Letter we present calibrated observations of the average mass ofgalaxies with 22< I < 25 in the Hubble Deep Field. We measure the mean massprofile using the statistical gravitational lens distortion of faint bluebackground galaxies, based on 2221 foreground-background pairs. The observedweak lensing distortion is calibrated via full 3-D simulations and other HSTdata on a foreground cluster of known mass. Inside a projected radius of 5arcsec we find a 3\sigma detection of shear. Fitting to an isothermal massdistribution, we find a shear of 0.066^{+0.018}_{-0.019} at a radius of2\arcsec\ (\sim 8 ~ h^{-1} kpc). The (1\sigma) limits on the parameters for thetruncated isothermal model are \sigma_v =185^{+30}_{-35} km s^{-1}, r_{outer}\ge 15 ~ h^{-1} kpc, and r_{core}= 0.6^{+1.3}_{-0.6} ~ h^{-1} kpc. Thiscorresponds to an average galaxy mass interior to 20 h^{-1} kpc of5.9^{+2.5}_{-2.7}\times 10^{11} h^{-1} M_\odot. Inside 10 h^{-1} kpc, we findan average rest-frame V band mass-to-light ratio of 11.4^{+6.0}_{-6.7} h(M/L_V)_\odot.Comment: 11 pages with 3 encapsulated postscript figures, Submitted to ApJ