2019MNRAS.486...42C

Carbon radio recombination lines (CRRLs) trace the interface between molecular and atomic gas. We present Giant Metrewave Radio Telescope observations of CRRLs, Cα244-Cα250 towards Cassiopeia A. We use a novel technique of stacking the emission lines in the visibility domain to obtain, for the first time, sub-pc resolution optical depth maps of these CRRLs. The emission shows a wide range of spatial and velocity structures, some of which are unresolved within our synthesis beam of 0.29 pc and velocity channel of 0.55 km s^–1. These variations in the emission optical depth and line width are indicative of inhomogeneity and fragmentation in the intervening Perseus Arm gas. We compare the distribution of the CRRL emission with that of diffuse and dense molecular gas using existing CO and H₂CO studies. We find that the CO emission in the -47 km s^–1 Perseus Arm component is primarily concentrated along an elongated structure detected in our CRRL maps, to the south of which lies the high-density molecular clumps traced by H₂CO. This spatial distribution of CRRL and molecular tracers is similar to what one would observe for a photodissociation region (PDR). In the other Perseus Arm component centred at -37 km s^–1, there is evidence for high column density (N_ H₂∼10²² cm^–2) molecular clumps embedded in diffuse CO and CRRL emission towards the centre of Cassiopeia A. We propose that the CRRL emissions coincident with molecular tracers originate from the line of sight integrated component of the C⁺ envelope of the molecular gas.