Lens Modeling Abell 370: Crowning the Final Frontier Field with MUSE
  We present a strong lensing analysis on the massive cluster Abell 370 (A370;
z = 0.375), using a combination of deep multi-band Hubble Space Telescope (HST)
imaging and Multi-Unit Spectroscopic Explorer (MUSE) spectroscopy. From only
two hours of MUSE data, we are able to measure 120 redshifts in the Southern
BCG area, including several multiply-imaged lens systems. In total, we increase
the number of multiply-imaged systems with a secure redshift from 4 to 15, nine
of which are newly discovered. Of these, eight are located at z > 3, greatly
extending the redshift range of spectroscopically-confirmed systems over
previous work. Using these systems as constraints, we update a parametric lens
model of A370, probing the mass distribution from cluster to galaxy scales.
Overall, we find that a model with only two cluster- scale dark matter halos
(one for each BCG) does a poor job of fitting these new image constraints.
Instead, two additional mass clumps -- a central "bar" of mass located between
the BCGs, and another clump located within a "crown" of galaxies in the
Northern part of the cluster field -- provide significant improvements to the
fit. Additional physical evidence suggests these clumps are indeed real
features of the system, but with relatively few image constraints in the crown
region, this claim is difficult to evaluate from a modeling perspective.
Additional MUSE observations of A370 covering the entire strong-lensing region
will greatly help these efforts, further improving our understanding of this
intriguing cluster.
