What are the megahertz peaked-spectrum sources?
  Megahertz peaked-spectrum (MPS) sources have spectra that peak at frequencies
below 1 GHz in the observer's frame and are believed to be radio-loud active
galactic nuclei (AGN). We recently presented a new method to search for
high-redshift AGN by identifying unusually compact MPS sources. In this paper,
we present European VLBI Network (EVN) observations of 11 MPS sources which we
use to determine their sizes and investigate the nature of the sources with ~10
mas resolution. Of the 11 sources, we detect nine with the EVN. Combining the
EVN observations with spectral and redshift information, we show that the
detected sources are all AGN with linear sizes smaller than 1.1 kpc and are
likely young. This shows that low-frequency colour-colour diagrams are an easy
and efficient way of selecting small AGN and explains our high detection
fraction (82%) in comparison to comparable surveys. Finally we argue that the
detected sources are all likely compact symmetric objects and that none of the
sources are blazars.
