Compact symmetric objects—newborn radio galaxies?

Abstract We review the question of the age of Compact Symmetric Objects (CSOs); defined as lobe-dominated sources smaller than 1 kpc in overall size. We show that the evidence increasingly points to these objects being very young ( 4 yr old). Evidence from spectral aging, energy supply arguments and, most convincingly, from long term VLBI kinematic studies, is all consistent with the ‘youth’ scenario for CSOs. From VLBI kinematic studies hotspot advance speeds in CSOs are found to be 0.1 to 0.3 c and external densities estimated from ram pressure balance are ≈1 cm −3 . The separate question of the subsequent evolution of CSOs and whether they are the progenitors of classical double sources is, in contrast, not yet definitively answered. However it is found that the numbers of CSOs in flux limited samples is to first order what would be expected under such a scenario. The detailed differences in CSO population density between the data and model predictions might be resolved in various ways. Possibly not all CSOs evolve into large sources, or some sources show recurrent activity or, most likely, the simplest source evolution models need modification.