The dependence of O 23 on the degree of ionization of the nebula is partially responsible for the large scatter found in the O 23 vs metallicity relation. The peculiar position of IZw18 in the plots is due to its very low metallicity. The data points corresponding to IZw18 deviate slightly from linearity. The two fits are shown in the upper panel of Figure 3 by a solid and a dashed line respectively. The corresponding residuals are shown in the lower panel as a function of the oxygen abundance.
Solid dots correspond to the linear fit while open circles correspond to the quadratic one.
Empirical calibration of oxygen abundance through the sulphur abundance parameter S 23 . Figure 4 shows the relation between S 23 and O 23 . These objects are 48 out of the 196 listed in Table 1 and therefore constitute about a quarter of the total sample. Most of them are circumnuclear star forming regions, HII regions in inner galactic discs and HII regions in Virgo cluster galaxies. Relation between O 23 and S 23 for the objects in Table 1 .