A New Metal-poor Globular Cluster and Resolved Stars in the Outer Disk
  of the Black Eye Galaxy M64: Implication for the Origin of the Type III Disk
  Break
  M64 is a nearby spiral galaxy with a Type III anti-truncation component. To
trace the origin of the Type III component, we present Hubble Space
Telescope/Advanced Camera for Surveys $F606W/F814W$ photometry of resolved
stars in a field located in the outer disk ($2.5' \lesssim r \lesssim 6.5'$) of
M64. At $r\approx 5.5'$ (7 kpc) to the east, we discover a new metal-poor
globular cluster ($R_{\rm eff}=5.73\pm0.02$ pc and $M_V=-9.54\pm0.09$ mag),
M64-GC1. This is the first globular cluster found in M64. The color-magnitude
diagram (CMD) of the resolved stars in M64-GC1 is well matched by 12 Gyr
isochrones with [Fe/H] $=-1.5\pm0.2$, showing that this cluster belongs to a
halo. The CMD of the resolved stars in the entire ACS field shows two
distinguishable red giant branches (RGBs): a curved metal-rich RGB and a
vertical metal-poor RGB. The metal-rich RGB represents an old metal-rich
([Fe/H] $\approx -0.4$) disk population. In contrast, the CMD of the metal-poor
RGB stars is very similar to the CMD of M64-GC1, showing that the metal-poor
RGB represents a halo population. The radial number density profile of the
metal-rich RGB stars is described by an exponential disk law, while the profile
of the metal-poor RGB stars is described by a de Vaucouleurs's law. From these,
we conclude that the origin of the Type III component in M64 is a halo which
has a much lower metallicity than a disk or bulge population.
