Planck early results XIV: ERCSC validation and extreme radio sources
  Planck's all sky surveys at 30-857 GHz provide an unprecedented opportunity
to follow the radio spectra of a large sample of extragalactic sources to
frequencies 2-20 times higher than allowed by past, large area, ground-based
surveys. We combine the results of the Planck Early Release Compact Source
Catalog (ERCSC) with quasi-simultaneous ground-based observations, as well as
archival data, at frequencies below or overlapping Planck frequency bands, to
validate the astrometry and photometry of the ERCSC radio sources and study the
spectral features shown in this new frequency window opened by Planck. The
ERCSC source positions and flux density scales are found to be consistent with
the ground-based observations. We present and discuss the spectral energy
distributions (SEDs) of a sample of "extreme" radio sources to illustrate the
richness of the ERCSC for the study of extragalactic radio sources. Variability
is found to play a role in the unusual spectral features of some of these
sources.
