Complex Structure of Galaxy Cluster A1689: Evidence for a Merger from X‐Ray Data?

Abell 1689 is a galaxy cluster at $z=0.183$ where previous measurements ofits mass using various techniques gave discrepant results. We present a newdetailed measurement of the mass with the data based on X-ray observations withthe European Photon Imaging Camera aboard the XMM-Newton Observatory,determined by using an unparameterized deprojection technique. Fitting thetotal mass profile to a Navarro-Frenk-White model yields halo concentration$c=7.2^{+1.6}_{-2.4}$ and $r_{200}=1.13 \pm 0.21 h^{-1} \mathrm{Mpc}$,corresponding to a mass which is less than half of what is found fromgravitational lensing. Adding to the evidence of substructure from opticalobservations, X-ray analysis shows a highly asymmetric temperature profile anda non-uniform redshift distribution implying large scale relative motion of thegas. A lower than expected gas mass fraction $f_{gas}=0.072 \pm 0.008$ (for aflat $\Lambda$CDM cosmology) suggests a complex spatial and/or dynamicalstructure. We also find no signs of any additional absorbing componentpreviously reported on the basis of the Chandra data, confirming the XMM lowenergy response using data from ROSAT.Comment: 14 pages, 10 figures, accepted by Ap