MACSJ1423.8+2404: Gravitational Lensing by a Massive, Relaxed Cluster of
  Galaxies at z=0.54
  We present results of a gravitational-lensing and optical study of MACS
,J1423.8+2404 (z=0.545, MACS, J1423). Our analysis uses high-resolution images
taken with the Hubble Space Telescope in the F555W and F814W passbands, ground
based imaging in eight optical and near-infrared filters obtained with Subaru
and CFHT, as well as extensive spectroscopic data gathered with the Keck
telescopes. At optical wavelengths the cluster exhibits no sign of substructure
and is dominated by a cD galaxy that is 2.1 magnitudes (K-band) brighter than
the second brightest cluster member, suggesting that MACS, J1423 is close to be
fully virialized. Analysis of the redshift distribution of 140 cluster members
reveals a Gaussian distribution, mildly disturbed by the presence of a loose
galaxy group that may be falling into the cluster along the line of sight.
Combining strong-lensing constraints from two spectroscopically confirmed
multiple-image systems near the cluster core with a weak-lensing measurement of
the gravitational shear on larger scales, we derive a parametric mass model for
the mass distribution. All constraints can be satisfied by a uni-modal mass
distribution centred on the cD galaxy and exhibiting very little substructure.
The derived projected mass of M(<65\arcsec [415 kpc])=(4.3\pm0.6)\times 10^{14}
M_sun is about 30% higher than the one derived from X-ray analyses assuming
spherical symmetry, suggesting a slightly prolate mass distribution consistent
with the optical indication of residual line-of-sight structure. The similarity
in shape and excellent alignment of the centroids of the total mass, K-band
light, and intra-cluster gas distributions add to the picture of a highly
evolved system [ABRIDGED]
