Spectroscopy of PNe in Sextans A, Sextans B, NGC 3109 and Fornax

Abstract

Planetary nebulae (PNe) and HII regions provide a probe of the chemical enrichment and star formation history of a galaxy from intermediate ages to the present day. We present the first results of NTT spectroscopy of HII regions and/or PNe in four nearby dwarf galaxies: Sextans A, Sextans B, NGC 3109, and Fornax. For all PNe and some of the HII regions in these galaxies we have obtained elemental abundances via the classic Te-method based on the detection of the [OIII] 4363 line. The oxygen abundances in three HII regions of Sextans A are all consistent within the individual rms uncertainties. The oxygen abundance in the PN of Sextans A is however significantly higher. This PN is even more enriched in nitrogen and helium, implying its classification as a PN of Type I. The presumably unaffected PN abundances of S and Ar are well below those in the HII regions, indicating a lower metallicity at the epoch of the PN progenitor formation. For two HII regions in Sextans B, the oxygen abundances do not differ within the rms uncertainties. The third one is, however, twice as metal-rich, providing evidence for the inhomogeneity of the current metallicity distribution in Sextans B. For the PN in Sextans B we measured an O/H that is consistent with that of the low-metallicity HII regions. For NGC 3109 our preliminary results indicate that the oxygen abundances of PNe and HII regions are all within a small range of $\pm$0.15 dex. For the PN in Fornax, Ne, Ar and S abundances suggest that the ISM metallicity was $\sim$0.3 dex lower at the epoch of the PN progenitor's formation, compared to the O/H value derived for the PN (abridged).