metallicity gradient

A galaxy's metallicity gradient is a measure of the rate at which metallicity changes across some direction through the galaxy. Typically, unless otherwise specified, a radial direction (center to distant edge) is what is intended, and any other usage would be specified, e.g., a vertical metallicity gradient. The term is commonly used for disk galaxies such as spiral galaxies like the Milky Way. The measure reveals something about the star formation history of the galaxy, e.g., the portion of the galaxy with higher metallicity presumed to have some star formation at the earlier date. The gradient may show higher metallicity within the center of the galaxy, or near the edge and a topic of research interest is whether and which galaxies are created (or later gained stars) with star formation beginning at the edge (such as from external clouds colliding with the galactic disk) or from the center (inside-out growth, such as from gravity drawing the interstellar medium into star-forming regions).