Understanding inverse metallicity gradients in galactic discs as a consequence of inside-out formation

By R. Schonrich, P. McMillan. 2016

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The early stages of a galaxy's evolution leave an imprint on its metallicity distribution. We discuss the origins and evolution of radial metallicity gradients in discs of spiral galaxies using an analytical chemical evolution model. We explain how radial metallicity gradients in stellar populations are determined by three factors: the radial metallicity profile of the star-forming interstellar medium (ISM), radial changes in the star formation history (in particular, inside-out formation) and radial mixing of stars. Under reasonable assumptions, inside-out formation steepens the negative ISM metallicity gradient, but contributes positively to the stellar metallicity gradient, up to inverting the metallicity profile to a positive d[Fe/H]/dR. This reconciles steep negative d[Fe/H]/dR in some high-redshift galaxies to generally flatter gradients in local observations. We discuss the evidence for inverse radial metallicity gradients (positive d[X/H]/dR) at high redshifts and the inverse relationship between azimuthal velocity and the metallicity (positive dVφ/d[Fe/H]) of stars for the Milky Way's thick disc. The former can be achieved by high central gas-loss rates and re-distribution processes, e.g. re-accretion of enriched material in conjunction with the inside-out formation and near-disc galactic fountaining. For the Milky Way-thick disc, we show that the positive dVφ/d[Fe/H] correlation points to comparable time-scales for inside-out formation, initial metal enrichment and SNIa enrichment. We argue that the original ISM metallicity gradient could be inferred with better data from the high-metallicity tail of the alpha-enhanced population. Including inside-out formation in our models changes the local vertical metallicity gradient by about −0.2 dex kpc−1, in line with local measurements.

Subfields: Astronomy and Astrophysics

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Citations in the corpus

Yule-Simpson’s paradox in Galactic Archaeology (2019, background): “…et al. 2014, Gaia-ESO - Recio-Blanco et al. 2014, LAMOST - Wang et al. 2019), explained by Minchev et al. (2014a) (see also Rahimi et al. 2014; Miranda et al. 2016; Ma et al. 2017; Schönrich & McMillan 2017) as the effect of inside-out disk formation and disk flaring2 of mono-age populations (Fig.”
The vertical metallicity gradients of mono-age stellar populations in the Milky Way with the RAVE and Gaia data (2017, background): “…the existence of a vertical metallicity gradient in a mono-age stellar population of the current Galactic disc stars indicates that a dynamical mechanism, such as radial mixing, built up the vertical metallicity gradient (e.g. Minchev et al. 2013; Kawata et al. 2017; Schönrich & McMillan 2017).”
Probing cosmic isotropy with a new X-ray galaxy cluster sample through the LX–T scaling relation (2020, background): “…central bulges of spiral galaxies, and the Milky Way specifically, the metallicity of the ISM is expected to be higher (Boissier & Prantzos 1999; Schönrich & McMillan 2017; A15, page 17 of 42 A&A 636, A15 (2020) Spina et al. 2017 than the metallicity in regions further away from the Galactic…”
Metallicity distributions of mono-age stellar populations of the Galactic disc from the LAMOST Galactic spectroscopic surveys (2018, background): “Our data shows negative radial [Fe/H] gradients for the stellar populations younger than 11 Gyr and negative radial [α/Fe] gradients for the oldest stellar populations , which is a consequence of the “inside-out” disc formation scenario (Matteucci & Francois 1989; Schönrich & McMillan 2017).”
The biggest splash (2019, methodology): “The thick disc shows a strong age gradient as a function of vφ in agreement with the picture presented in Schönrich & Mcmillan (2017), Kawata et al. (2018), and Minchev et al. (2019).”
The GALAH survey and Gaia DR2: dissecting the stellar disc’s phase space by age, action, chemistry, and location (2018, background): “Schönrich & McMillan (2017) argue that the effect arises because the α-rich disc grew rapidly in radial extent on a ∼Gyr time-scale coincident with the transition from Type II to Type Ia supernova enrichment.”
Constraints on Radial Migration in Spiral Galaxies - II. Angular momentum distribution and preferential migration (2018, unknown): “…see e.g. Sellwood & Binney 2002; Roškar et al. 2008; Schönrich & Binney 2009; Loebman et al. 2011; Minchev et al. 2012; Roškar, Debattista & Loebman 2013; Vera-Ciro, D’Onghia & Navarro 2016; Martinez-Medina et al. 2017; Peschken, Athanassoula & Rodionov 2017; Schönrich & McMillan 2017).”
Measuring Radial Orbit Migration in the Galactic Disk (2018, methodology): “We do not treat or model explicitly the vertical structure of the Milky Way disk, though there are of course vertical (populations) gradients in it (e.g., Ness et al. 2016), which are affected by radial migration (e.g., Kawata et al. 2017; Schönrich & McMillan 2017).”
De re metallica: the cosmic chemical evolution of galaxies (2018, background): “…in galaxies, both for the gas and the stars (Mollá et al., 1997; Chiappini et al., 2001; Naab & Ostriker, 2006; Mott et al., 2013; Spitoni et al., 2013; Ho et al., 2015; Kudritzki et al., 2015; Ascasibar et al., 2015; Spitoni et al., 2015; Schönrich & McMillan, 2017; Lian et al., 2018b).”
Confirming chemical clocks: asteroseismic age dissection of the Milky Way disc(s) (2017, background): “…for the Milky Way disc include inside–out and upside–down formation (e.g. Matteucci & Francois 1989; Brook et al. 2006; Bird et al. 2013; Ness et al. 2016; Schönrich & McMillan 2017), although there are suggestions that the old component of the disc formed outside–in instead (Robin et al. 2014).”