**Table 4:** Best fit parameters for the nonthermal component (I Zw 18 at t=15 Myr).
Model	$l_{\rm nt}\tau_{\rm nt}(5~{\rm ~GHz})$	$\chi ^2$	$\chi _\nu ^2$
	(W Hz^-1 yr)
E	$2.4\times 10^{22}$	7.7	1.5
F	$3.0\times 10^{22}$	8.5	1.7
G	$2.4\times 10^{22}$	7.7	1.5
H	$3.0\times 10^{22}$	8.6	1.7

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