There is quite a clear anticorrelation between the intrinsic peak frequency and the overall radio source size in Compact Steep Spectrum ( CSS ) and GHz Peaked Spectrum ( GPS ) radio sources as shown by O ’ Dea ( 1998 ) .
This feature is interpreted in terms of synchrotron self-absorption ( although free-free absorption may play a role as well ) of the radiation emitted by a small radio source which is growing within the inner region of the host galaxy .
This leads to the hypothesis that these objects are young and that the radio source is still developing/expanding within the host galaxy itself . Very young radio sources must have the peak in their radio spectra occurring above a few tens of GHz , and for this reason they are termed High Frequency Peakers ( HFPs ) .
These newly born radio sources must be very rare given that they spend very little time in this stage . H _ { o } = 100 km s ^ { -1 } Mpc ^ { -1 } and q _ { o } =0.5 are used throughout this paper .