Detection of a Large Population of Ultra Diffuse Galaxies in Massive
  Galaxy Clusters: Abell S1063 and Abell 2744
  We present the detection of a large population of ultra diffuse galaxies
(UDGs) in two massive galaxy clusters, Abell S1063 at $z=0.348$ and Abell 2744
at $z=0.308$, based on F814W and F105W images in the Hubble Frontier Fields
Program. We find 47 and 40 UDGs in Abell S1063 and Abell 2744, respectively.
Color-magnitude diagrams of the UDGs show that they are mostly located at the
faint end of the red sequence. From the comparison with simple stellar
population models, we estimate their stellar mass to range from $10^8$ to $10^9
M_\odot$. Radial number density profiles of the UDGs show a turnover or a
flattening in the central region at $r<100$ kpc. We estimate the total masses
of the UDGs using the galaxy scaling relations. A majority of the UDGs have
total masses, $M_{200} = 10^{10}$ to $10^{11}~M_\odot$, and only a few of them
have total masses, $M_{200} = 10^{11}$ to $10^{12}~M_\odot$. The total number
of UDGs within the virial radius is estimated to be N(UDG)$=770\pm114$ for
Abell S1063, and N(UDG)$=814\pm122$ for Abell 2744. Combining these results
with data in the literature, we fit the relation between the total numbers of
UDGs and the masses of their host systems for $M_{200}>10^{13} M_\odot$ with a
power law, N(UDG) $= M_{200}^{1.05\pm0.09}$. These results suggest that a
majority of the UDGs have a dwarf galaxy origin, while only a small number of
the UDGs are massive $L_*$ galaxies that failed to form a normal population of
stars.
