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B. MONTE CARLO ESTIMATES OF REDDENING AND UNDERLYING BALMER ABSORPTION IN REAL OBSERVATIONS

Here we provide examples of deriving reddening and underlying stellar absorption from emission line spectra. We take as a examples, the observations of SBS1159+545, SBS1415+437, and SBS1420+544 as reported in IT98. These three spectra are all reported with very high accuracy; the brightest lines are reported with errors of less than one percent. Since the emission line equivalent widths for all of the Balmer lines are required as input and since only the emission line equivalent width for Hbeta is reported, we have had to estimate these from the relative line strengths and the spectra shown in figures. The fractional uncertainties in the equivalent widths were assumed to be twice as large as the fractional uncertainties reported in the relative intensities.

We used the values reported in IT98 and calculated C(Hbeta) and aHI for the three targets. Our results are shown in Figure B-1. Here we display the originally observed Balmer line ratios, the corrected ratios reported in IT98 and our own solutions. There are two important points to note. First note that the IT98 corrected values for the Hgamma / Hbeta and Hdelta / Hbeta ratios are several sigma away from the theoretical values. In the case of SBS 1159+545, this is because these ratios were already higher than the theoretical ratios, and correcting for reddening and underlying absorption only increases the ratios. In the other two cases, the original ratios were very close to the theoretical ratios, but correcting the Halpha / Hbeta ratio for reddening has caused these ratios to exceed their theoretical ratios. Since the deviations from the theoretical line ratios are large (yielding generally high values of the chi2), we conclude that the uncertainties in the reported emission lines are underestimated.

Figure B1

Figure B-1. A comparison of the observed and corrected hydrogen Balmer emission line ratios for three blue compact galaxies from the sample of IT98. The open circles show the deviations of the original observations from the theoretical ratios in terms of the uncertainties in the final reported corrected values (the Halpha / Hbeta ratios for SBS 1415+437 and SBS 1420+544 are off scale at +33sigma and +25sigma respectively). The filled circles show the corrected values in the same manner. Note that in all three cases the corrected Hgamma / Hbeta and Hdelta / Hbeta ratios are several sigma away from the theoretical values. In the case of SBS 1159+545, this is because these ratios were already higher than the theoretical ratios, and correcting for reddening and underlying absorption only increases the ratios. In the other two cases, the original ratios were very close to the theoretical ratios, but correcting the Halpha / Hbeta ratio for reddening has caused these ratios to exceed their theoretical ratios. Since the deviations from the theoretical line ratios are large, we conclude that the uncertainties in the reported emission lines are underestimated.

The second point to note in Figure B-1 is the difference between our solutions are those of IT98. In all three cases, the IT98 solution yields very good agreement with theory in the Halpha / Hbeta ratio (better than ours) but not as good in the other lines. It would appear that the weighting scheme used by IT98 favors this line ratio more than would be called for by the relative errors in the line ratios.

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