Metallicity maps of the globular cluster system in the galaxy

Abstract

The spatial metallicity distribution in the Galaxy, as depicted by the globular cluster system, is analyzed. Two metallicity maps (on the ω - |Z| plane) corresponding to the halo and disk subsamples (Zinn 1985) have been obtained. The resulting pictures reveal the existence of a highly structured abundance pattern for the two subsystems. Variations of the metallicity with ω as well as with |Z| are clearly evidenced in both maps. However, the slope of these variations is very dependent on the direction chosen to estimate the gradient. This effect is particularly pronounced for the halo sample of globular clusters. Our results indicate that the metallicity distribution of the halo subsystem does not allow a simple description in terms of a single, direction independent gradient and that, possibly, this is the main cause of the long-standing controversy generated around the search for such a gradient. Superposed on this large-scale structure is the clearly visible presence of several metallicity patches of variable size. The most striking of them is a low-metallicity spot (LMS) located at the solar galactocentric radius, which extends up to 6 kpc away from the Galactic plane.