Chemical Abundances of Planetary Nebulae in the Substructures of M31

TL;DR

This study uses deep spectroscopy of planetary nebulae in M31's substructures to analyze their chemical compositions, revealing potential common origins of the Northern Spur and Giant Stream, and suggesting M32's involvement.

Contribution

First detailed spectroscopic analysis of PNe in M31's substructures, linking their chemical properties to possible shared origins and the influence of M32.

Findings

PNe in M31's substructures show homogeneous oxygen abundances.

Progenitors of these PNe are likely older than 2 Gyr.

Evidence suggests the Northern Spur and Giant Stream may share a common origin.

Abstract

We present deep spectroscopy of planetary nebulae (PNe) that are associated with the substructures of the Andromeda Galaxy (M31). The spectra were obtained with the OSIRIS spectrograph on the 10.4 m GTC. Seven targets were selected for the observations, three in the Northern Spur and four associated with the Giant Stream. The most distant target in our sample, with a rectified galactocentric distance >100 kpc, was the first PN discovered in the outer streams of M31. The [O III] 4363 auroral line was well detected in the spectra of all targets, enabling electron temperature determination. Ionic abundances are derived based on the [O III] temperatures, and elemental abundances of helium, nitrogen, oxygen, neon, sulfur, and argon are estimated. The relatively low N/O and He/H ratios as well as abundance ratios of alpha-elements indicate that our target PNe might belong to populations as old as ~2 Gyr. Our PN sample, including the current seven and the previous three observed by Fang et al., have rather homogeneous oxygen abundances. The study of abundances and the spatial and kinematical properties of our sample leads to the tempting conclusion that their progenitors might belong to the same stellar population, which hints at a possibility that the Northern Spur and the Giant Stream have the same origin. This may be explained by the stellar orbit proposed by Merrett et al. Judging from the position and kinematics, we emphasize that M32 might be responsible for the two substructures. Deep spectroscopy of PNe in M32 will help to assess this hypothesis.