Origin of the Metallicity Distribution in the Thick Disc

Abstract

Aims. Using a suite of cosmological chemodynamical disc galaxy simulations, we assess how (a) radial metallicity
gradients evolve with scaleheight; (b) the vertical metallicity gradients change through the thick disc; and (c) the
vertical gradient of the stellar rotation velocity varies through the disc. We compare with the Milky Way to search for
analogous trends. Methods. We analyse five simulated spiral galaxies with masses comparable to the Milky Way. The simulations span a range of star formation and energy feedback strengths and prescriptions, particle- and grid-based hydrodynamical implementations, as well as initial conditions/assembly history. Disc stars are identified initially via kinematic decomposition, with a posteriori spatial cuts providing the final sample from which radial and vertical gradients are inferred.
Results. Consistently, we find that the steeper, negative, radial metallicity gradients seen in the mid-plane flatten with
increasing height away from the plane. In simulations with stronger (and/or more spatially-extended) feedback, the
negative radial gradients invert, becoming positive for heights in excess of !1 kpc. Such behaviour is consistent with
that inferred from recent observations. Our measurements of the vertical metallicity gradients show no clear correlation
with galactocentric radius, and are in good agreement with those observed in the Milky Way’s thick disc (locally).
Each of the simulations presents a decline in rotational velocity with increasing height from the mid-plane, albeit the
majority have shallower kinematic gradients than that of the Milky Way.
Conclusions. Simulations employing stronger/more extended feedback prescriptions possess radial and vertical metallicity
and kinematic gradients more in line with recent observations. The inverted, positive, radial metallicity gradients seen in the simulated thick stellar discs originate from a population of younger, more metal-rich, stars formed in-situ, superimposed upon a background population of older migrators from the inner disc; the contrast provided by the former increases radially, due to the inside-out growth of the disc. A similar behaviour may be responsible for the same
flattening seen in the radial gradients with scaleheight in the Milky Way.