Gas accretion as the origin of chemical abundance gradients in distant galaxies

TL;DR

This study provides observational evidence that cold gas accretion in early galaxies causes inverse metallicity gradients, supporting models where primordial gas inflows dilute central regions and drive galaxy growth.

Contribution

First direct measurement of chemical abundance gradients in galaxies at z~3, confirming cold flow accretion as a key process in early galaxy evolution.

Findings

Inverse metallicity gradients observed in three z~3 galaxies

Central regions show lower metallicity than outer regions

Results support cold flow models of galaxy growth

Abstract

It has recently been suggested that galaxies in the early Universe can grow through the accretion of cold gas, and that this may have been the main driver of star formation and stellar mass growth. Because the cold gas is essentially primordial, it has a very low abundance of elements heavier than helium (metallicity). As it is funneled to the centre of a galaxy, it will lead the central gas having an overall lower metallicity than gas further from the centre, because the gas further out has been enriched by supernovae and stellar winds, and not diluted by the primordial gas. Here we report chemical abundances across three rotationally-supported star-forming galaxies at z~3, only 2 Gyr after the Big Bang. We find an 'inverse' gradient, with the central, star forming regions having a lower metallicity than less active ones, opposite to what is seen in local galaxies. We conclude that the central gas has been diluted by the accretion of primordial gas, as predicted by 'cold flow' models.