2018ApJ...863..155C

Recently a population of compact radio galaxies were classified as Fanaroff-Riley type 0 radio galaxies (FR 0s). The physical nature of FR 0s and the connection with the classical FR I and II galaxies are not currently well understood. Here, we report the radio properties of 14 FR 0s on parsec (pc) scales derived from their very long baseline interferometry (VLBI) imaging observations. All of the sources show compact structures. Four sources show relativistic beaming with Doppler-boosting factors ranging from 1.7 to 6. The brightness temperatures of the other 10 are below the equilibrium limit. Jet proper motions are determined in two sources that have multiple epoch data, between 0.23 c and 0.49 c, implying mildly relativistic jet flow. Low-amplitude flux density variation is found in J0943+3614 over a time period of 10 years. No significant variability are detected in the three other sources over timescales of a few years. The radio properties of the FR 0s inferred from the VLBI data resemble a GHz-peaked spectrum or compact steep-spectrum sources. Moreover, the diversity of their relativistic beaming indicators (brightness temperature, variability, jet proper motion) also imply that FR 0s might not be a homogeneous population of radio sources. Detailed studies of the low-power (P_1.4GHz< 10²⁴ W Hz^–1) FR 0 sources in the local universe additionally offer a promising opportunity to understand their connection to the FR Is.