3. Dust extinction
The interstellar dust extinction, or reddening, was first evaluated using the H/H, H/H and H/H ratios, assuming their intrinsic values to be 2.75, 0.475, 0.264 respectively for an electron temperature of 20000 K and a density of 100 cm-3 (Osterbrock 1989). We used the extinction function
where I() is the intrinsic line intensity, F() is the observed flux at each wavelength and K() is the extinction function according to the galactic reddening law of Seaton (1979).
In order to correct from the effect of stellar absorption in the Balmer lines, we first assumed that their strength was the same for all the lines. We derived then the value for which consistent results for E(B-V) were obtained using the three different Balmer ratios. As the extent over which the flux was integrated is larger than the size of the ionizing star cluster, we corrected for the underlying stellar absorption only in the central area, i.e., over 3.8" (185 pc) centered on the maximum continuum emission, according to the images of Hunter & Thronson (1995). We found the underlying stellar absorption to be around 1.8 Å, close to the value of 2 Å used by Skillman & Kennicutt (1993) and Roy & Walsh (1987) and adopted this value (1.8 Å) for correction.
The variation of the extinction parameter E(B-V) along the slit is shown in Fig. 3a. It can be seen that a good agreement between the three computed values is obtained only in the central region (we have indeed forced this agreement by defining the strength of the absorption lines). Outside the central area, values obtained using H/H and H/H ratios are systematically lower than values obtained with H/H, and fall most of the time below zero. Artificially increasing the H flux by less than four percent erases this discrepancy, suggesting it could be (partially) related to small calibration errors dues to the Balmer absorption lines in the Feige 34 spectra.
Stasinska & Schaerer (1999) have shown that is partially excited by collisions in IZw 18, so that the ratio should be between 2.95 and 3.00 for at least the main body of the nebula, higher than for case B recombination. This effect would explain the discrepancy between the reddening values estimated using different line ratios. These authors conclude that the reddening affecting this ionized nebula should be practically equal to zero. Therefore, we have assumed no reddening at all along the slit in our calculations.
© European Southern Observatory (ESO) 2000
Online publication: March 21, 2000