The smallest radio sources: Implications for future space VLBI missions Original Research Article

The results of a high-precision Intra-Day Variability Survey carried out with the Australia Telescope Compact Array at 1.4, 2.4, 4.8 and 8.6 GHz reveal a significant population of sources which show strong Intra-Day Variability (IDV). We believe that the most likely interpretation for this IDV is scintillation in the Inter Stellar Medium, implying microarcsecond source angular sizes at cm wavelengths. However, if the IDV is intrinsic to the sources, then this implies source sizes several orders of magnitude smaller. On either interpretation, VLBI imaging of these sources will require sensitive cm-wavelength observations on baselines with Earth-Moon dimensions or longer.