Flat‐spectrum symmetric objects with ∼1 kpc sizes – I. The candidates

In order to understand the origin and evolution of radio galaxies, searches for the youngest such sources have been conducted. Compact symmetric objects (CSOs)/medium symmetric objects (MSOs) are thought to be the earliest stages of radio sources, with possible ages of ≲10 3  yr for CSOs ( <1 kpc in size) and 10 4 –10 5  yr 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 ≲ 0.5 galaxy, has a flat radio spectrum (α thin < 0.5; S ν ∝ν −α ) , is <0.3 kpc in size, has an arm length ratio ≤ 2, and well‐aligned (θ≤ 20°) opposite lobes with a flux density ratio ≤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 α 4.85 1.40 < 0.5 that were resolved with the Very Large Array A configuration (VLA‐A) 8.4 GHz. As first results, we have ‘rediscovered’ nine of the known CSO/MSOs while identifying two new ∼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.