Radial oxygen and argon abundance gradients of the thin and thicker disc
  of Andromeda from planetary nebulae
  Andromeda (M31) is the nearest giant spiral galaxy to our Milky Way (MW) and
the most massive member of our Local Group. We obtain a magnitude-limited
sample of M31 disc PNe with chemical abundance measurements through the direct
detection of the [OIII] 4363 \r{A} line. This leads to $205$ and $200$ PNe with
oxygen and argon abundance measurements respectively. We find that high- and
low-extinction M31 disc PNe have statistically distinct argon and oxygen
abundance distributions. In the radial range $2-30$ kpc, the older
low-extinction disc PNe are metal-poorer on average with a slightly positive
radial oxygen abundance gradient ($0.006 \pm 0.003$ dex/kpc) and slightly
negative radial argon abundance gradient ($-0.005 \pm 0.003$ dex/kpc), while
the younger high-extinction disc PNe are metal-richer on average with steeper
radial abundance gradients for both oxygen ($-0.013 \pm 0.006$ dex/kpc) and
argon ($-0.018 \pm 0.006$ dex/kpc), similar to the gradients measured for M31
HII regions. These abundance gradients are consistent with a major merger in
the M31 disc, with the majority of the low-extinction PNe being the older
pre-merger disc stars in the thicker disc, and the majority of the
high-extinction PNe being younger stars in the thin disc, formed during and
after the merger event. The chemical abundance of the M31 thicker disc has been
radially homogenized because of the major merger. Accounting for differences in
disc-scale lengths, we find the radial oxygen abundance gradient of the M31
thicker disc is amongst the most positive values observed till date in spiral
galaxies, flatter than that of the MW thick disc which has a negative radial
gradient. On the other hand, the thin discs of the MW and M31 have remarkably
similar negative oxygen abundance gradient values.
