Origin of the metallicity distribution in the thick disc

¹ Jeremiah Horrocks Institute, University of Central Lancashire, Preston, PR1 2HE, UK
e-mail: msancho@uclan.ac.uk
² Departamento de Física Teórica, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain
³ E.A. Milne Centre for Astrophysics, Dept. of Physics & Mathematics, University of Hull, Hull, HU6 7RX, UK
⁴ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁵ School of Physics, University of Exeter, Exeter, EX4 4QL, UK
⁶ Yonsei University, Graduate School of Earth System Sciences-Astronomy-Atmospheric Sciences, Yonsei-ro 50, 120-749 Seoul, Republic of Korea
⁷ Laboratoire AIM Paris-Saclay, CEA/IRFU/SAp, Université Paris Diderot, 91191 Gif-sur-Yvette Cedex, France
⁸ Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
⁹ Department of Physics & Astronomy, University of Alabama, Tuscaloosa, AL 35487-0324, USA
¹⁰ National Radio Astronomy Observatory, PO Box 2, Green Bank, WV 24944, USA
¹¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 2 February 2015
Accepted: 13 December 2015

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, although 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 in 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 as seen in the radial gradients with scaleheight in the Milky Way.