The chemistry of planetary nebulae in the outer regions of M31

TL;DR

This study analyzes the chemical composition of planetary nebulae in M31's outer regions, revealing complex interactions, mergers, and a significant population of solar-metallicity stars formed about 2 billion years ago.

Contribution

It extends the oxygen abundance gradient study of M31 to 100 kpc, showing diverse metallicities and kinematics, indicating a complex merger history.

Findings

Outer PNe have diverse oxygen abundances, including solar levels.

Some PNe exhibit kinematics inconsistent with the disk, suggesting complex origins.

Evidence of a starburst event ~2 Gyr ago in M31's outer regions.

Abstract

We present spectroscopy of nine planetary nebulae (PNe) in the outskirts of M31, all but one obtained with the 10.4m GTC telescope. These sources extend our previous study of the oxygen abundance gradient of M31 to galactocentric radii as large as 100 kpc. None of the targets are bona fide members of a classical, metal-poor and ancient halo. Two of the outermost PNe have solar oxygen abundances, as well as radial velocities consistent with the kinematics of the extended disk of M31. The other PNe have a slightly lower oxygen content ([O/H] ~ -0.4) and in some cases large deviations from the disk kinematics. These PNe support the current view that the external regions of M31 are the result of a complex interaction and merger process, with evidence for a widespread population of solar-metallicity stars produced in a starburst that occurred ~2 Gyr ago.