  Exploiting the fundamentally achromatic nature of gravitational lensing, we
present a lens model for the massive galaxy cluster SMACSJ0723.3-7323
(SMACSJ0723, z=0.388) that significantly improves upon earlier work. Building
on strong-lensing constraints identified in prior Hubble Space Telescope (HST)
observations, the mass model utilizes 21 multiple-image systems, 17 of which
were newly discovered in Early Release Observation (ERO) data from the James
Webb Space Telescope (JWST). The resulting lens model maps the cluster mass
distribution to an RMS spatial precision of 0.32'' and is publicly available.
Consistent with previous analyses, our study shows SMACSJ0723.3-7323 to be well
described by a single large-scale component centered on the location of the
brightest cluster galaxy. However, satisfying all lensing constraints provided
by the JWST data, the model point to the need for the inclusion of an
additional, diffuse component west of the cluster. A comparison of the galaxy,
mass, and gas distributions in the core of SMACSJ0723 based on HST, JWST, and
Chandra data reveals a concentrated regular elliptical profile along with
tell-tale signs of a recent merger, possibly proceeding almost along our line
of sight. The exquisite sensitivity of JWST's NIRCAM reveals in spectacular
fashion both the extended intra-cluster-light distribution and numerous
star-forming clumps in magnified background galaxies. The high-precision lens
model derived here for SMACSJ0723-7323 demonstrates the unprecedented power of
combining HST and JWST data for studies of structure formation and evolution in
the distant Universe.
