In order to understand the origin and evolution of radio galaxies , searches for the youngest such sources have been conducted .
Compact-medium symmetric objects ( CSO-MSOs ) are thought to be the earliest stages of radio sources , with possible ages of \la 10 ^ { 3 } yrs for CSOs ( < 1 kpc in size ) and 10 ^ { 4 } –10 ^ { 5 } yrs for MSOs ( 1–15 kpc ) .
From a literature selection in heterogeneous surveys , we have established a sample of 37 confirmed CSOs .
In addition , we only found three confirmed flat-spectrum MSOs in the literature .
The typical CSO resides on a z \la 0.5 galaxy , has a flat radio spectrum ( \alpha _ { thin } < 0.5 ; S _ { \nu } \propto \nu ^ { - \alpha } ) , is < 0.3 kpc in size , has an arm length ratio \leq 2 , and well-aligned ( \theta \leq 20 \degr ) opposite lobes with a flux density ratio \leq 10 .
In order to populate the 0.3–1 kpc size range ( large CSOs ) and also in order to find more flat-spectrum MSOs , we have built a sample of 157 radio sources with \alpha _ { 1.40 } ^ { 4.85 } < 0.5 that were resolved with the VLA-A 8.4 GHz .
As first results , we have ‘ rediscovered ’ nine of the known CSO/MSOs while identifying two new \sim 14 kpc MSOs and two candidate CSO/MSOs ( which only lack redshifts for final classification ) .
We were able to reject 61 of the remaining 144 objects from literature information alone .
In the series of papers that starts with this one we plan to classify the remaining 83 CSO/MSO candidates ( thanks to radio and optical observations ) as well as characterize the physical properties of the ( likely ) many 0.3–15 kpc flat-spectrum CSO/MSOs to be found .