2003ApJ...591L..83I

A significant fraction of nonbaryonic or baryonic dark matter in galactic halos may consist of massive compact objects (MASCOs) with mass M=10¹-10⁴ M_☉. Possible candidates for such compact objects include primordial black holes or remnants of primordial (Population III) stars. We propose a method for directly detecting MASCOs in extragalactic halos, using the VLBI techniques with extremely high resolution that would be achieved by the next-generation mission of the VLBI Space Observatory Program such as VSOP-2. If a galactic halo comprising a large number of MASCOs produces multiple images of a background radio-loud QSO by gravitational lensing, then a high-resolution radio map of each macrolensed image should reveal microlensing effects by MASCOs. To assess their observational feasibility, we simulate microlensing of the radio-loud, four-image lensed QSO, B1422+231, assuming angular resolution of ∼0.01 mas. MASCOs are represented by point masses. For comparison, we also simulate microlensing of B1422+231 by singular isothermal spheres. We find that the surface brightness of the macrolensed images shows distinct spatial patterns on the scale of the Einstein radius of the perturbers. In the case of point-mass perturbers, many tiny dark spots also appear in the macrolensed images associated with a decrease in the surface brightness toward the fringe of the original QSO image, whereas no such spots are available in the SIS models. Because such spatial patterns in each macrolensed image cannot be linearly mapped to those in other macrolensed images if they are relevant to lensing perturbers, it is fairly easy to discriminate them from intrinsic substructures within a QSO. Based on the size, position, and magnified or demagnified patterns of images, we shall be able to determine the mass and density profile of an individual MASCO as well as its spatial distribution and abundance in a galactic halo.