Thin disc, thick disc and halo in a simulated galaxy

Abstract

Within a cosmological hydrodynamical simulation, we form a disc galaxy with\nsub- components which can be assigned to a thin stellar disc, thick disk, and a\nlow mass stellar halo via a chemical decomposition. The thin and thick disc\npopulations so selected are distinct in their ages, kinematics, and\nmetallicities. Thin disc stars are young (<6.6 Gyr), possess low velocity\ndispersion ({\\sigma}U,V,W = 41, 31, 25 km/s), high [Fe/H], and low [O/Fe]. The\nthick disc stars are old (6.6<age<9.8 Gyrs), lag the thin disc by \\sim21 km/s,\npossess higher velocity dispersion ({\\sigma}U,V,W = 49, 44, 35 km/s),\nrelatively low [Fe/H] and high [O/Fe]. The halo component comprises less than\n4% of stars in the "solar annulus" of the simulation, has low metallicity, a\nvelocity ellipsoid defined by ({\\sigma}U,V,W = 62, 46, 45 km/s) and is formed\nprimarily in-situ during an early merger epoch. Gas-rich mergers during this\nepoch play a major role in fuelling the formation of the old disc stars (the\nthick disc). This is consistent with studies which show that cold accretion is\nthe main source of a disc galaxy's baryons. Our simulation initially forms a\nrelatively short (scalelength \\sim1.7 kpc at z=1) and kinematically hot disc,\nprimarily from gas accreted during the galaxy's merger epoch. Far from being a\ncompeting formation scenario, migration is crucial for reconciling the short,\nhot, discs which form at high redshift in {\\Lambda}CDM, with the properties of\nthe thick disc at z=0. The thick disc, as defined by its abundances maintains\nits relatively short scale-length at z = 0 (2.31 kpc) compared with the total\ndisc scale-length of 2.73 kpc. The inside-out nature of disc growth is\nimprinted the evolution of abundances such that the metal poor {\\alpha}-young\npopulation has a larger scale-length (4.07 kpc) than the more chemically\nevolved metal rich {\\alpha}-young population (2.74 kpc).\n