5 Flux and number density distributions

In Figs. 8 and 9 we show the flux density distribution and the number density distribution, respectively, of the HII regions across the face of the galaxy; the two plots are on linear scales, both in ordinate, intensity, and in abscissa, distance from the centre. To do this we divided the disc into rings using the values of PA and i of the galaxy (Table 1), but we have not shown the number density or the flux density in the inner ring, because the value in this small area was affected by this division. Two trends are clear in these plots: firstly the trend to lower flux, and number density, with increasing radius, which clearly reflects the standard radial decline in surface density of each major component of this disc galaxy; secondly the presence of notable peaks and troughs within this general behaviour. Two major peaks, at 5 arcsec (770 pc) and at 60 arcsec (9.3 kpc) from the centre are especially prominent. These represent the major star forming regions along the bar and at the end of the bar respectively, while the smaller peaks between 90-100 arcsec (13.9 - 15.5 kpc) correspond to peaks in the star formation along the prominent southern spiral arm. The underlying radial flux density distribution can be fitted by an exponential of form

$$f(r) = B \, {\rm e}^{-(r/h_{\rm H\alpha})}$$ (1)

from which we derive a value for the HH$\alpha$ scale length, $h_{\rm H\alpha} = 2.4~(\pm~0.3)$ kpc.