Parsec-scale morphology and spectral-index distribution in faint high-frequency peakers

We investigate the parsec-scale structure of 17 high-frequency peaking radio sources from the faint high-frequency peaker sample. Very Long Baseline Array (VLBA) observations were carried out at two adjacent frequencies, 8.4 and 15.3 GHz, both in the optically thin part of the spectrum, to obtain the spectral-index information. We found that 64 per cent of the sources are resolved into subcomponents, while 36 per cent are unresolved even at the highest frequency. Among the resolved sources, seven have a morphology and a spectral-index distribution typical of young radio sources, while in other four sources, all optically associated with quasars, the radio properties resemble those of the blazar population. The equipartition magnetic field of the single components are a few tens milliGauss, similar to the values found in the hotspots of young sources with larger sizes. Such high magnetic fields cause severe radiative losses, precluding the formation of extended lobe structures emitted at centimetre wavelengths. The magnetic fields derived in the various components of individual source are usually very different, indicating a non-self-similar source evolution, at least during the very first stages of the source growth.