Multi-density model of the ionised gas in NGC 253 using radio recombination lines

We have imaged the H92α (8.3 GHz), H75α (15 GHz), and H166α (1.4 GHz) Radio Recombination Lines (RRLs) from NGC 253 at resolutions of 4.5 pc (0.4) , 2.5 pc (0.2) and 53 pc (4.5) respectively. The H92α line arises from individual compact sources, most of which possess radio continuum counterparts. The line widths range from ∼200 km s for the sources near the radio nucleus to 70-100 km s for the extranuclear ones. These lines are emitted by gas at a density ∼10 cm. The remainder of the cm-wave RRLs arise in lower density gas (∼500 cm) with a higher area filling factor and with ten times higher mass. A third component of higher density gas (>10 cm) is required to explain the mm-wave RRLs.