Subaru Weak-Lensing Survey of Dark Matter Subhalos in the Coma Cluster :
  Subhalo Mass Function and Statistical Properties
  We present a 4 deg^2 weak gravitational lensing survey of subhalos in the
very nearby Coma cluster using the Subaru/Suprime-Cam. The large apparent size
of cluster subhalos allows us to measure the mass of 32 subhalos detected in a
model-independent manner, down to the order of 10^-3 of the virial mass of the
cluster. Weak-lensing mass measurements of these shear-selected subhalos enable
us to investigate subhalo properties and the correlation between subhalo masses
and galaxy luminosities for the first time. The mean distortion profiles
stacked over subhalos show a sharply truncated feature which is well-fitted by
a Navarro-Frenk-White (NFW) mass model with the truncation radius, as expected
due to tidal destruction by the main cluster. We also found that subhalo
masses, truncation radii, and mass-to-light ratios decrease toward the cluster
center. The subhalo mass function, dn/dln M_sub, in the range of 2 orders of
magnitude in mass, is well described by a single power law or a Schechter
function. Best-fit power indices of 1.09_-0.32^+0.42 for the former model and
0.99_-0.23^+0.34 for the latter, are in remarkable agreement with slopes of
~0.9-1.0 predicted by the cold dark matter paradigm. The tangential distortion
signals in the radial range of 0.02-2Mpc/h from the cluster center show a
complex structure which is well described by a composition of three mass
components of subhalos, the NFW mass distribution as a smooth component of the
main cluster, and a lensing model from a large scale structure behind the
cluster. Although the lensing signals are 1 order of magnitude lower than those
for clusters at z~0.2, the total signal-to-noise ratio, S/N=13.3, is comparable
to, or higher, because the enormous number of background source galaxies
compensates for the low lensing efficiency of the low lensing efficiency of the
nearby cluster.
