Bounds on intergalactic compact objects from observations of compact radio sources

Abstract

We investigate the constraints that can be imposed on the cosmic density of uniformly distributed intergalactic compact objects in the mass range 10^7^ <~M/M_sun_ <~ 10^9^ using VLBI observations of compact radio sources. A compact object at a sufficiently small angular separation from a compact radio source produces a deamplified and inverted secondary image. No such cases are known. Using published maps of compact radio sources and assuming an Einstein-de Sitter cosmology, we exclude a density parameter for mass in this range {OMEGA}_M_> 0.4 at the 99.7% confidence level. Observations using the VLBA should allow one to exclude or discover {OMEGA}_M_ compact objects in the mass range 10^5^<~M/M_sun_ <~10^8^.