On the nature of bright compact radio sources at z>4.5
  High-redshift radio-loud quasars are used to, among other things, test the
predictions of cosmological models, set constraints on black hole growth in the
early universe and understand galaxy evolution. Prior to this paper, 20
extragalactic radio sources at redshifts above 4.5 have been imaged with very
long baseline interferometry (VLBI). Here we report on observations of an
additional ten z>4.5 sources at 1.7 and 5 GHz with the European VLBI Network
(EVN), thereby increasing the number of imaged sources by 50%. Combining our
newly observed sources with those from the literature, we create a substantial
sample of 30 z>4.5 VLBI sources, allowing us to study the nature of these
objects. Using spectral indices, variability and brightness temperatures, we
conclude that of the 27 sources with sufficient information to classify, the
radio emission from one source is from star formation, 13 are flat-spectrum
radio quasars and 13 are steep-spectrum sources. We also argue that the
steep-spectrum sources are off-axis (unbeamed) radio sources with rest-frame
self-absorption peaks at or below GHz frequencies and that these sources can be
classified as gigahertz peaked-spectrum (GPS) and megahertz peaked-spectrum
(MPS) sources.
