2020ApJ...897..106K

M64 is a nearby spiral galaxy with a Type III antitruncation component. To trace the origin of the Type III component, we present Hubble Space Telescope/Advanced Camera for Surveys (ACS) F606W/F814W photometry of resolved stars in a field located in the outer disk (2′ / . 5≲r≲6′ / . 5) of M64. At r~5′ / . 5 (7 kpc) to the east, we discover a new metal-poor globular cluster (R_eff=5.73±0.02 pc and M_V = -9.54 ± 0.09 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 ± 0.2, 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] ≃ -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 that has a much lower metallicity than the disk or bulge population.