The first detection of weak gravitational shear in infrared observations: Abell 1689

We present the first detection of weak gravitational shear at infrared wavelengths, using observations of the lensing cluster Abell 1689, taken with the SofI camera on the ESO-NTT telescope. The imprint of cluster lenses on the shapes of the background galaxy population has previously been harnessed at optical wavelengths, and this gravitational shear signal enables cluster mass distributions to be probed, independent of whether the matter is luminous or dark. At near-infrared wavelengths, the spectrophotometric properties of galaxies facilitate a clean selection of background objects for use in the lensing analysis. A finite-field mass reconstruction and application of the aperture mass (M_ap) statistic are presented. The probability that the peak of the M_ap detection ((S)/(N)sim 5), arises from a chance alignment of background sources is only sim4.5times 10^-7. The velocity dispersion of the best-fit singular isothermal sphere model for the cluster is sigma_1D=1030⁺⁷⁰_-80 km {s^-1}, and we find a K-band mass-to-light ratio of sim40 M_sun/L_sun inside a 0.44 Mpc radius.