A new scaling relation for HII regions in spiral galaxies: unveiling the   true nature of the mass-metallicity relation
  We demonstrate the existence of a -local- relation between galaxy surface mass density, gas metallicity, and star-formation rate density using spatially-resolved optical spectroscopy of HII regions in the local Universe. One of the projections of this distribution, -the local mass-metallicity relation- extends over three orders of magnitude in galaxy mass density and a factor of eight in gas metallicity. We explain the new relation as the combined effect of the differential radial distributions of mass and metallicity in the discs of galaxies, and a selective star-formation efficiency. We use this local relation to reproduce -with remarkable agreement- the total mass-metallicity relation seen in galaxies, and conclude that the latter is a scale-up integrated effect of a local relation, supporting the inside-out growth and downsizing scenarios of galaxy evolution. 
F. F. Rosales-Ortega, S. F. S\'anchez, J. Iglesias-P\'aramo, A. I.   D\'iaz, J. M. V\'ilchez, J. Bland-Hawthorn, B. Husemann...
astro-ph.CO