Chandra X‐Ray ObservatoryObservation of A1689: New Determination of Mass Distribution and Comparison to Lensing Measurements

We present a new estimate of the projected X-ray mass of Abell 1689 observedwith Chandra, in an attempt to clarify the issue of whether or not there existsa discrepancy between X-ray and gravitational lensing mass estimates claimed inprevious investigations based on {\it Einstein}, {\it ROSAT} and {\it ASCA}observations. A particular attention is paid to examining if there is an offsetbetween X-ray centroid and central dominant cD galaxy, which may be anindicator of the presence of local dynamical activities of intracluster gas inthe central core and therefore, explain the discrepancy between X-ray andlensing mass estimates, if any. The unprecedentedly high spatial resolutionachieved by Chandra allows us to precisely localize the X-ray centroid of Abell1689, which appears to coincide perfectly with the central cD galaxy. Thisfact, along with the symmetry and regularity of the X-ray surface brightnessand temperature distributions, suggests that Abell 1689 is a fully-relaxedcluster. We thus employ hydrostatic equilibrium hypothesis to determine theprojected mass profile of Abell 1689, and compare it with the results obtainedby different lensing techniques available in the literature. Our analysisconfirms the existance of the discrepancy of a factor of $\sim2$ between X-rayand lensing mass estimates in the central region of $r\approx0.2$ Mpc, althoughthe two methods yield essentially consistent result on large radii. If theperfect coincidence between the X-ray center and the cD galaxy of Abell 1689detected by Chandra observation is not a projection effect, the central massdiscrepancy between X-ray and lensing measurements may pose a challenge to ourconventional understanding of dynamical evolution of the intracluster gas inthe central regions of clusters.Comment: 26 pages, 7 figures, Accepted for publication in the Astrophysical Journa