Why does the metallicity of stars decrease with increasing galactocentric radius and height above the galactic plane?

Question

Metallicity decreases with the Galactocentric radius and height above the Galactic plane (Bergemann et al. 2014; Duong et al. 2018). https://www.aanda.org/articles/aa/abs/2014/05/aa23456-14/aa23456-14.html

Answer 1

There's a combination of things going on.

Stars form in the disk and bulge of the Galaxy where most of the gas gathers. A lot of the chemical enrichment, particularly of "alpha elements" - Mg, Si, S, Ne then happens promptly (within 10-20 million years) from exploding core collapse supernovae. In such short lives, the progenitor massive stars have not moved far from their birth places, so the gas just gets more enriched in situ.

Iron-peak elements are formed mostly in type Ia supernovae. These exploding white dwarfs are older than the progenitors of core collapse supernovae and do migrate from their birth places. Nevertheless there is still some tendency for the enrichment to take place closer to the central bulge and Galactic plane and of course, gas that is above the Galactic plane tends to fall towards the Galactic plane in any case.

Stars that are found above the Galactic plane tend to be older, because the vertical speeds of stars are gradually increased by various tidal interactions during their lives. If they are older stars then they were formed from gas that was less enriched.

Very old, very metal poor stars may have formed before the gas disk formed and these are now more evenly, even spherically, distributed compared with "population I" stars in the disk.

The decrease with radius is again because if there are fewer progenitor stars being born at larger radius (the rate of star birth and the stellar density decreases with increasing radius) then there is less enrichment of the interstellar medium at larger radii.