Unusual integrated metallicity profile of our Milky Way

TL;DR

This study measures the integrated stellar metallicity profile of the Milky Way, revealing an unusual 'w'-shaped gradient that challenges existing galaxy formation models and suggests unique enrichment processes.

Contribution

First measurement of the Milky Way's integrated metallicity profile, revealing an atypical 'w'-shaped gradient not commonly seen in similar galaxies.

Findings

Milky Way's metallicity profile has a positive inner gradient and steep negative outer gradient.

The profile differs from other Milky Way-mass galaxies, indicating unique enrichment history.

Inner gradient may result from inside-out quenching, outer gradient remains unexplained.

Abstract

The heavy element abundance profiles in galaxies place stringent constraint on galaxy growth and assembly history. Low-redshift galaxies generally have a negative metallicity gradient in their gas and stars. Such gradients are thought to be a natural manifestation of galaxy inside-out formation. As the Milky Way is currently the only spiral galaxy in which we can measure temporally-resolved chemical abundances, it enables unique insights into the origin of metallicity gradients and their correlation with the growth history of galaxies. However, until now, these unique abundance profiles had not been translated into the integrated-light measurements needed to seamlessly compare with the general galaxy population. Here we report the first measurement of the light-weighted, integrated stellar metallicity profile of our Galaxy. We find that the integrated metallicity profile of the Milky Way has a '$\wedge$'-shape broken metallicity profile, with a mildly positive gradient inside a Galactocentric radius of 7 kpc and a steep negative gradient outside. This metallicity profile appears unusual when compared to Milky Way-mass star-forming galaxies observed in the MaNGA survey and simulated in the TNG50 cosmological simulation. The analysis of the TNG50 simulated galaxies suggests that the Milky Way's positive inner gradient may be due to an inside-out quenching process. The steep negative gradient in the outer disc, however, is challenging to explain in the simulations. Our results suggest the Milky Way may not be a typical spiral galaxy for its mass regarding metallicity distribution and thus offers insight into the variety of galaxy enrichment processes.