We analyse the structure and chemical enrichment of a Milky Way-like galaxy with a stellar mass of 2 \times 10 ^ { 10 } M _ { \odot } , formed in a cosmological hydrodynamical simulation .
It is disk-dominated with a flat rotation curve , and has a disk scale length similar to the Milky Way ’ s , but a velocity dispersion that is \sim 50 % higher .
Examining stars in narrow [ Fe/H ] and [ \alpha /Fe ] abundance ranges , we find remarkable qualitative agreement between this simulation and observations : a ) The old stars lie in a thickened distribution with a short scale length , while the young stars form a thinner disk , with scale lengths decreasing , as [ Fe/H ] increases .
b ) Consequently , there is a distinct outward metallicity gradient .
c ) Mono-abundance populations exist with a continuous distribution of scale heights ( from thin to thick ) .
However , the simulated galaxy has a distinct and substantive very thick disk ( h _ { z } \sim 1.5 kpc ) , not seen in the Milky Way .
The broad agreement between simulations and observations allows us to test the validity of observational proxies used in the literature : we find in the simulation that mono-abundance populations are good proxies for single age populations ( < 1 Gyr ) for most abundances .