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P/Po

Figure 21: The oxygen abundance gradient in NGC 3621 as a function of fractional isophotal radius p/p0. Data for HII regions are adopted from Ryder (1995, open diamonds) and Zarit-sky et al. (1994, filled diamonds). Arrows indicate lower limits, a typical error bar is shown at bottom left. Some HII regions were observed twice, in this case the corresponding symbols are connected. The dotted line denotes the least-squares fit to the data. Stellar abundances are superimposed (#9: Bresolin et al. 2001; #1: Przybilla 2002, dots). Note that oxygen abundances cannot be directly derived from the two available A-type supergiant spectra. Instead, the metallicity estimates are transformed to the given abundance scale, assuming O/H = M/H.

P/Po

Figure 21: The oxygen abundance gradient in NGC 3621 as a function of fractional isophotal radius p/p0. Data for HII regions are adopted from Ryder (1995, open diamonds) and Zarit-sky et al. (1994, filled diamonds). Arrows indicate lower limits, a typical error bar is shown at bottom left. Some HII regions were observed twice, in this case the corresponding symbols are connected. The dotted line denotes the least-squares fit to the data. Stellar abundances are superimposed (#9: Bresolin et al. 2001; #1: Przybilla 2002, dots). Note that oxygen abundances cannot be directly derived from the two available A-type supergiant spectra. Instead, the metallicity estimates are transformed to the given abundance scale, assuming O/H = M/H.

the ESO VLT to their limits. Spectra of 19 objects down to V « 22 mag were obtained in 10.7 hrs of integration time, confirming many as supergiants (Bresolin et al. 2001). Intermediate-resolution spectra can be quantitatively analysed once a reliable and comprehensive modelling is achieved (Fig. 13, see Przybilla et al. 2006a for thorough tests). However, medium-resolution spectroscopy implies a loss in detail and accuracy of the information that can be extracted from observation. We expect that metallicities can be determined to better than a factor ~2, i.e. at an accuracy similar to that achieved in published high-resolution studies. Our modelling is therefore highly competitive in extragalactic research.

An example is shown in Fig. 21, where data from two stars in NGC 3621 are compared to the oxygen abundance gradient as derived from H ii regions. Because individual lines are unresolved in the spectra, the estimated metallicity is transformed into O abundance by setting O/H = M/H - a good approximation, cf. Sect. 2.4. At first glance, excellent agreement is found, but see below.

Such studies can be extended to larger samples of objects covering the entire extent of galaxies. This allows the results from the only indicators used so far, luminous H ii nebulae, to be verified and extended. The most detailed study in this respect is on NGC 300, a spiral galaxy in the nearby Sculptor Group observed within the Araucaria project (Gieren et al. 2005), where in total 30 blue supergiants have so far been analysed (Bresolin et al. 2002; Urbaneja et al. 2005; Kudritzki et al. 2008).

Abundances from nebulae are in most cases derived by strong-line methods, i.e. empirical correlations between line ratios and abundance are used (like the R23-index for oxygen), as the spectral features necessary for a direct analysis are often not observed because of their weakness. It turns out that absolute abundances and the abundance gradient from nebulae depend strongly on the calibration. Several of the most-widely used calibrations from the literature fail to achieve consistency

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