Can the Steep Mass Profile of A1689 Be Explained by a Triaxial Dark Halo?

The steep mass profile of A1689 derived from recent detailed lensing observations is not readily reconciled with the low-concentration halos predicted by the standard cold dark matter (CDM) model. However, halo triaxiality may act to bias the profile constraints derived assuming a spherically symmetric mass distribution, since lensing relates only to the projected mass distribution. A degree of halo triaxiality is inherent to the CDM structure formation, arising from the collisionless nature of the dark matter. Here we compare the CDM-based model predictions of triaxial halo with the precise lensing measurements of A1689 based on the Advanced Camera for Surveys Hubble Space Telescope and Subaru data, over a wide range of 10 kpc ≲ r ≲ 2 Mpc. The model lensing profiles cover the intrinsic spread of halo mass and shape (concentration and triaxiality) and are projected over all inclinations when comparing with the data. We show that the model parameters are only weakly constrained and strongly degenerate mainly because of the lack of information along the line of sight. In particular, the limits on the concentration parameter become less restrictive with increasing triaxiality. Yet, by comparing the obtained constraints with expected probability distributions for the axis ratio and concentration parameters computed from numerical simulations, we find that ~6% of cluster-size halos in the CDM model can match the A1689 lensing observations at the 2 σ level, corresponding to cases in which the major axis of the halo is aligned with the line of sight. Thus, halo triaxiality could reduce the apparent discrepancy between theory and observation. This hypothesis needs to be further explored by a statistical lensing study for other clusters, as well as by complementary three-dimensional information derived using X-ray, kinematics, and Sunyaev-Zel'dovich effect observations.