The Surprisingly Steep Mass Profile of A1689, from a Lensing Analysis of Subaru Images

Subaru observations of A1689 (z=0.183) are used to derive an accurate,model-independent mass profile for the entire cluster, r<2 Mpc/h, by combiningmagnification bias and distortion measurements. The projected mass profilesteepens quickly with increasing radius, falling away to zero at r~1.0 Mpc/h,well short of the anticipated virial radius. Our profile accurately matchesonto the inner profile, r<200 kpc/h, derived from deep HST/ACS images. Thecombined ACS and Subaru information is well fitted by an NFW profile withvirial mass, (1.93 \pm 0.20)10^15 M_sun, and surprisingly high concentration,c_vir=13.7^{+1.4}_{-1.1}, significantly larger than theoretically expected(c_vir~4), corresponding to a relatively steep overall profile. A slightlybetter fit is achieved with a steep power-law model that has its 2D logarithmicslope -3 and core radius theta_c~1.7' (r_c~210 kpc/h), whereas an isothermalprofile is strongly rejected. These results are based on a reliable sample ofbackground galaxies selected to be redder than the cluster E/S0 sequence. Byincluding the faint blue galaxy population a much smaller distortion signal isfound, demonstrating that blue cluster members significantly dilute the truesignal for r~400 kpc/h. This contamination is likely to affect most weaklensing results to date.Comment: 5 pages, 3 figures, to appear in ApJ