**Table 7:** Observational and derived parameters of Components I, II and III.
Observational parameter	Component I	Observations	Comp.s II+III
		(low res.)	(expected)
H166 $\alpha$ at 1.4 GHz ( $\times$ 10^-23 W m^-2)	0.03-0.3	1.6	1.5 $\pm$ 0.4
H110 $\alpha$ at 4.8 GHz ( $\times$ 10^-23 W m^-2)	3.4-6.9	19.1	14.5 $\pm$ 4
H92 $\alpha$ at 8.3 GHz ( $\times$ 10^-23 W m^-2)	25.5	44.4	19 $\pm$ 3
H40 $\alpha$ at 99 GHz ( $\times$ 10^-23 W m^-2)	564	6270	5706
1.4 GHz continuum (mJy)	10	852	<842
4.8 GHz continuum (mJy)	11-27	1000	<990
8.3 GHz continuum (mJy)	25	800	<775
99 GHz continuum (mJy)	23-42	75	<52
Derived parameter	Component I		Component II
Density (cm^-3)	$\sim$ 10⁴		300-1000
Size (pc)	<10		30-60
$N_{\rm Lyc}$ ( $\times$ 10⁵² s^-1)	4		3-6

Column (1): Parameter; Col. (2): predicted value of the parameter, summed over the eight compact H92 $\alpha$ sources (Component I, see Sect. 5.2); Col. (3): observed value of parameter, from low resolution data; Col. (4): the difference between Col. 3 and Col. 2, which is modelled as Components II and III in Sect. 5.3.

Source LaTeX | All tables | In the text

	1 Introduction
	2 Observations
	3 Results
	4 The morphology and analysis of the extended H92 $\alpha$ RRL emission
	5 A multi-density model for the ionised gas in NGC 253
	6 Discussion
	7 Summary and conclusions
	References
	Online Material
	Appendix A: Radio continuum sources in the disk of NGC 253