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ABSTRACT  In this paper the cosmic evolution of the space density of Fanaroff–Riley Class I (FR I) radio sources is investigated out to  z ∼ 1, in order to understand the origin of the differences between these and the more powerful FR IIs. High‐resolution radio images are presented of the best high‐redshift FR I candidate galaxies, drawn from two fields of the Leiden–Berkeley Deep Survey, and previously defined by Rigby, Snellen &amp; Best in Paper 1. Together with lower resolution radio observations (both previously published in Paper 1 and, for a subset of sources, also presented here) these are used to morphologically classify the sample. Sources which are clearly resolved are classified by morphology alone, whereas barely or unresolved sources were classified using a combination of morphology and flux density loss in the higher resolution data, indicative of resolved‐out extended emission. The space densities of the FR Is are then calculated as a function of redshift, and compared to both measurements of the local value and the behaviour of the more powerful FR IIs. The space density of FR I radio sources with luminosities (at 1.4 GHz) &gt; 10 25  W Hz −1  is enhanced by a factor of 5–9 by  z ∼ 1, implying moderately strong evolution of this population; this enhancement is in good agreement with models of FR II evolution at the same luminosity. There are also indications that the evolution is luminosity dependent, with the lower powered sources evolving less strongly.

A sample of mJy radio sources at 1.4 GHz in the Lynx and Hercules fields – II. Cosmic evolution of the space density of FR I radio sources