  Using a representative sample of 14 star-forming dwarf galaxies in the local
Universe, we show the existence of a spaxel-to-spaxel anti-correlation between
the index N2 (log([NII]6583/Halpha)) and the Halpha flux. These two quantities
are commonly employed as proxies for gas-phase metallicity and star formation
rate (SFR), respectively. Thus, the observed N2 to Halpha relation may reflect
the existence of an anti-correlation between the metallicity of the gas forming
stars and the SFR it induces. Such an anti-correlation is to be expected if
variable external metal-poor gas fuels the star-formation process.
Alternatively, it can result from the contamination of the star-forming gas by
stellar winds and SNe, provided that intense outflows drive most of the metals
out of the star-forming regions. We also explore the possibility that the
observed anti-correlation is due to variations in the physical conditions of
the emitting gas, other than metallicity. Using alternative methods to compute
metallicity, as well as previous observations of HII regions and
photoionization models, we conclude that this possibility is unlikely. The
radial gradient of metallicity characterizing disk galaxies does not produce
the correlation either.
