Using Weak‐Lensing Dilution to Improve Measurements of the Luminous and Dark Matter in A1689

The E/SO sequence of a cluster defines a boundary redward of which a reliableweak lensing signal can be obtained from background galaxies, uncontaminated bycluster members. For bluer colors, both background and cluster members arepresent, reducing the distortion signal by the proportion of unlensed clustermembers. In deep Subaru and HST/ACS images of A1689 the tangential distortionof galaxies with bluer colors falls rapidly toward the cluster center relativeto the lensing signal of the red background. We use this dilution effect toderive the cluster light profile and luminosity function to large radius, withthe advantage that no subtraction of far-field background counts is required.The light profile declines smoothly to the limit of the data, r<2Mpc/h, with aconstant slope, dlog(L)/dlog(r)=-1.12+-0.06, unlike the lensing mass profilewhich steepens continuously with radius, so that M/L peaks at an intermediateradius, ~100kpc/h. A flatter behavior is found for the more physicallymeaningful ratio of dark-matter to stellar-matter, when accounting for thecolor-mass relation of cluster members. The cluster luminosity function has aflat slope, alpha=-1.05+-0.07, independent of radius and with no faint upturnto M_i'<-12. We establish that the very bluest objects are negligiblycontaminated by the cluster V-i'<0.2, because their distortion profile risestowards the center following the red background, but offset higher by ~20%.This larger amplitude is consistent with the greater estimated depth of thefaint blue galaxies, z~=2.0 compared to z~=0.85 for the red background, apurely geometric effect related to cosmological parameters. Finally, we improveupon our earlier mass profile by combining both the red and blue backgroundpopulations, clearly excluding low concentration CDM profiles.Comment: 17 pages, 21 figures, revised version in response to referee comments,(added some discussion, references), conclusions unchanged. Accepted for publication in Ap