3.3. The Optically Faint microJy Radio Source Population
There is now good evidence to suggest (e.g. Barger et al. 2000; Chapman et al. 2001) a close correspondence between the optically faint microJy radio sources and the faint (SCUBA) sub-mm source population. The bulk of the observational evidence suggests that these sources are located at cosmological distances, and are enveloped in thick, opaque dust. Since it is estimated that a substantial fraction of the total radiation in the Universe is emitted from these obscured systems, the detailed nature of these sources is a key topic in astronomy today. However, the study of these dusty systems is severely hampered by the fact that they are so difficult to detect in almost all wave-bands, except the sub-mm and radio domains. It is thought that the sub-mm emission is associated with FIR emission (again absorption and re-radiation of uv emission by dust) that is redshifted into the sub-mm wavelength range. However, the source of the original uv emission is unclear - it could be generated purely by massive stars in dense star forming regions, purely by embedded AGN, or some mixture of both phenomena. Similarly in the radio, it is not known whether the synchrotron emission is generated by massive star formation processes or by accretion associated with a central AGN. As we have seen, if massive star formation is responsible, radio flux densities imply SFRs ~ 1000 M/yr, and an inferred global SFR that is completely "at odds" with previous optical and ultraviolet studies (Haarsma et al. 2000, and references therein).
Distinguishing between the AGN and starburst phenomena in these systems is clearly of fundamental importance. In the radio, only VLBI provides sufficient angular resolution to distinguish between the two cases. In particular, radio emission generated by star formation processes should be resolved by current (sensitivity limited) VLBI observations. AGN, on the other hand, should remain considerably more compact and readily detectable.