Detecting sub-lunar mass compact objects toward the Local Group galaxies

By monitoring a large number of stars in the Local Group galaxies, we can detect nanolensing events by sub-lunar mass compact objects (SULCOs) such as primordial black holes (PBHs) and rogue (free-floating) dwarf planets in the Milky Way halo. In contarst to microlensing by stellar-mass objects, the finite-source size effect becomes important and the lensing time duration becomes shorter (∼101−4s). Using stars with V < 26 in M33 as sources, for one-night observation, we would be able to detect 103−4 nanolensing events caused by SULCOs in the Milky Way halo with a mass of 10−9M⊙ to 10−7M⊙ for sources with S/N > 5 if SULCOs constitute all the dark matter components. Moreover, we expect 101−2 events in which bright blue stars with S/N > 100 are weakly amplified due to lensing by SULCOs with a mass range of 10−11M⊙ to 10−9M⊙. Thus the method would open a new window on SULCOs in the Milky Way halo that would otherwise not be observable.