IAUS379: Dynamical Masses of Local Group Galaxies

Under the current standard scenario, galaxies such as the Milky Way (MW) are thought to have formed through repeated mergers and accretions of small galaxies due to gravitational interactions. Since the relaxation timescale of these traces in phase space is as long as more than 10 billion years, it has been theoretically suggested that they may still exist in the halo. Advances in large-scale observations have ushered in the era in which such traces can be investigated observationally. The MW, to which we belong, is of particular interest because it is possible to directly observe stellar streams and substructures as they are destroyed by tidal forces as small galaxies merge and accrete into it. Currently while the structure and substructures in the halo within 20-30 kpc from the center of the MW is becoming better understood from the viewpoint of chemodynamics, the structure in the outer halo is still largely unresolved.
In this study we used the entire HSC-SSP data (~1400 square degrees), characterized by wide field of view (~1.8 square degrees) and deep photometry (i < 26 mag), to investigate the outer halo of the MW. In order to effectively detect the substructures in the outer halo, an isochrone-filter was created for the old, metal-poor stellar systems on the color-magnitude diagram. As a result, previously discovered substructures (e.g. the Orphan Stream) were detected, while new candidates were discovered. In this talk, we will discuss the origin of these substructures.