Radial oxygen and argon abundance gradients of the thin and thicker disc of Andromeda from planetary nebulae

Andromeda (M 31) 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 M 31 disc PNe with chemical abundance measurements through the direct detection of the [OIII] 4363 Å line. This leads to 205 and 200 PNe with oxygen and argon abundance measurements respectively. We find that highand low-extinction M 31 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 ± 0.003 dex/kpc) and slightly negative radial argon abundance gradient (−0.005 ± 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 ± 0.006 dex/kpc) and argon (−0.018 ± 0.006 dex/kpc), similar to the gradients measured for M 31 HII regions. These abundance gradients are consistent with a major merger in the M 31 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 M 31 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 M 31 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 andM 31 have remarkably similar negative oxygen abundance gradient values.