Accretion, radial flows and abundance gradients in spiral galaxies

TL;DR

This paper develops analytical tools to model how angular momentum mismatches in accreting gas cause radial flows and influence chemical abundance gradients in spiral galaxy discs, with applications to the Milky Way.

Contribution

It introduces a novel method for solving metallicity evolution with radial flows and generalizes existing models to include angular momentum effects on accretion and gas dynamics.

Findings

Accreting gas in the Milky Way likely rotates at 70-80% of the disc's velocity.

The new method reduces computational complexity for modeling metallicity evolution.

Structural and chemical data can disentangle inside-out growth from radial flows.

Abstract

The metal-poor gas continuously accreting onto the discs of spiral galaxies is unlikely to arrive from the intergalactic medium (IGM) with exactly the same rotation velocity as the galaxy itself and even a small angular momentum mismatch inevitably drives radial gas flows within the disc, with significant consequences to galaxy evolution. Here we provide some general analytic tools to compute accretion profiles, radial gas flows and abundance gradients in spiral galaxies as a function of the angular momentum of accreting material. We generalize existing solutions for the decomposition of the gas flows, required to reproduce the structural properties of galaxy discs, into direct accretion from the IGM and a radial mass flux within the disc. We then solve the equation of metallicity evolution in the presence of radial gas flows with a novel method, based on characteristic lines, which greatly reduces the numerical demand on the computation and sheds light on the crucial role of boundary conditions on the abundance profiles predicted by theoretical models. We also discuss how structural and chemical constraints can be combined to disentangle the contributions of inside-out growth and radial flows in the development of abundance gradients in spiral galaxies. Illustrative examples are provided throughout with parameters plausible for the Milky Way. We find that the material accreting on the Milky Way should rotate at 70-80 per cent of the rotational velocity of the disc, in agreement with previous estimates.