4 Diameter distribution

In Fig. 7 we present the integral diameter distribution: the number of regions with diameters greater than a given value as a function of diameter. In previous studies (van den Bergh 1981; Hodge 1987; Cepa & Beckman 1989; Ye 1992; Knapen et al. 1993; Rozas et al. 1996b) it has been found that the diameter distribution of the HII regions can be well fitted by an exponential of form $N(\gt D)= N_{\rm 0}$ $\exp(-D/D_{\rm 0}$) where $D_{\rm 0}$ is a characteristic diameter, and $N_{\rm 0}$ is an (extrapolated) characteristic value for the total number of regions. From the observations represented in Fig. 7 we obtain the values $N_{\rm 0}=5000 \pm 300$ and $D_{\rm 0}=110~\pm~2$ pc assuming H₀=75  km s^-1 km s^-1 Mpc^-1 (and $D_{\rm 0}=167~\pm~2$ for H₀=50  km s^-1 km s^-1 Mpc^-1). We have given two values for $D_{\rm 0}$, the first being a more realistic estimate, while the second is included to compare explicitly with previously published values, where 50 km s^-1 km s^-1 Mpc^-1 for H₀ has been used. The value for $N_{\rm 0}$ is within the range found for previously observed late type spirals, as given in the papers cited in the present paragraph. $D_{\rm 0}$ has the value predicted (within its error bars) for a galaxy of its measured absolute luminosity according to the observational plot of $D_{\rm 0}$ versus luminosity in Hodge (1987).

  

Figure 7: Integral diameter distribution function of all HII regions of NGC 7479. The straight line indicates the best fit

  

Figure 8: Flux density distribution of all the HII regions of NGC 7479 as a function of the deprojected distance to the centre of the galaxy

  

Figure 9: Number-density distribution of all the HII regions of NGC 7479 as a function of deprojected distance from the centre