Tracing the Giant Outer Halo of the Mysterious Massive Disk Galaxy M104. I. Photometry of the Extended Globular Cluster Systems

M104 (NGC 4594, the Sombrero galaxy) is a mysterious massive early-type galaxy that shows a dominant bulge and a prominent disk. However, the presence of a halo in M104 has been elusive, and it is not yet known how M104 has acquired such a peculiar structure. Using wide (~2 deg²) and deep ugi images of M104 obtained with the CFHT/MegaCam, we detect a large number of globular clusters (GCs) found out to $R\approx 35^{\prime} $ (~100 kpc). The color distribution of these GCs shows two subpopulations: a blue (metal-poor) system and a red (metal-rich) system. The total number of GCs is estimated to be N _GC = 1610 ± 30 and the specific frequency to be S _N = 1.8 ± 0.1. The radial number density profile of the GCs is steep in the inner region at $R\lt 20^{\prime} $ and becomes shallow in the outer region at $20^{\prime} \lt R\lt 35^{\prime} $ . The outer region is dominated by blue GCs and is extended out to $R\approx 35^{\prime} $ . This shows clearly the existence of a giant metal-poor halo in M104. The inner region is composed of a bulge hosting a disk, corresponding to a metal-rich halo as seen in early-type galaxies. At least two clumps of blue GCs are found in the outer region. One clump is overlapped with a faint stellar stream located in the southwest, indicating that it may be a remnant of a disrupted dwarf galaxy. Our results imply that the metal-rich inner halo of M104 formed first via major mergers, and the metal-poor outer halo grew via numerous minor mergers.