Abundances and Gradients in M31 – A Chemical Study of Planetary Nebulae in the Substructures

Abstract

We present deep optical spectroscopy of seven planetary nebulae (PNe) in the substructures of M31, three in the Northern Spur and four associated with the Giant Stream. The spectra were obtained with the OSIRIS spectrograph on the 10.4 m GTC. The detection of the [O iii] λ4363 auroral line in all PNe of our sample enables reliable abundance determinations. Our targets have low N/O (<0.5) and He/H ratios, indicating that they are probably Type II PNe. The PNe in our sample have rather homogeneous oxygen abundances, with an average value of 8.56±0.10. Based on the abundances as well as the spatial and kinematical information of our targets, we speculate that the Northern Spur and the Giant Stream might have the same origin. We raise a hypothesis that the dwarf satellite M32 might be responsible for these two substructures. New observations have recently been made to assess this hypothesis.