Radial metallicity gradients of disk galaxies: The emission-line probes

The metallicity of stellar and interstellar probes thorough disk galaxies, including our own, is distributed non-uniformly, showing typically shallow radial metallicity gradients where the metallicity decreases radially, inside out. Ultimately, the chemical evolution models need to be constrained by observations to determine the existence and extent of gradient slope and evolution, which in turn are linked to the formation and evolution of the galaxy.
The use of emission-line probes to test metallicity gradients takes advantage of the brightness of the nebular emission lines, and of the fact that the two major nebular probes, planetary nebulae (PNe) and HII regions, probe different epochs since galaxy formation. The limitation is that direct abundances are within observational reach only in the Local Universe. On the Galactic side, additional complications are added to the problem by the uncertainty of the PN distance scale.
In this talk I review the radial metallicity gradients background and main observational results, the latest direct metallicity gradient determinations in the Galaxy and beyond, the Gaia solution to the PN distance problem, and the foreseen advances of this field with the next generation of observational capabilities.