We present first results from electronic Multi-Element Remotely Linked Interferometer Network ( e-MERLIN ) and electronic European VLBI Network ( e-EVN ) observations of a small sample of ultra-steep spectrum radio sources , defined as those sources with a spectral index \alpha < -1.4 between 74 MHz and 325 MHz , which are unresolved on arcsecond scales .
Such sources are currently poorly understood and a number of theories as to their origin have been proposed in the literature .
The new observations described here have resulted in the first detection of two of these sources at milliarcsecond scales and show that a significant fraction of ultra-steep spectrum sources may have compact structures which can only be studied at the high resolution available with very long baseline interferometry ( VLBI ) .