Radio spectra of bright compact sources at z>4.5
  High-redshift quasars are important to study galaxy and active galactic
nuclei (AGN) evolution, test cosmological models, and study supermassive black
hole growth. Optical searches for high-redshift sources have been very
successful, but radio searches are not hampered by dust obscuration and should
be more effective at finding sources at even higher redshifts. Identifying
high-redshift sources based on radio data is, however, not trivial. Here we
report on new multi-frequency Giant Metrewave Radio Telescope (GMRT)
observations of eight z>4.5 sources previously studied at high angular
resolution with very long baseline interferometry (VLBI). Combining these
observations with those from the literature, we construct broad-band radio
spectra of all 30 z>4.5 sources that have been observed with VLBI. In the
sample we found flat, steep and peaked spectra in approximately equal
proportions. Despite several selection effects, we conclude that the z>4.5 VLBI
(and likely also non-VLBI) sources have diverse spectra and that only about a
quarter of the sources in the sample have flat spectra. Previously, the
majority of high-redshift radio sources were identified based on their
ultra-steep spectra (USS). Recently a new method has been proposed to identify
these objects based on their megahertz-peaked spectra (MPS). Neither method
would have identified more than 18% of the high-redshift sources in this
sample. More effective methods are necessary to reliably identify complete
samples of high-redshift sources based on radio data.
