Local anticorrelation between star formation rate and gas-phase metallicity in disc galaxies

Using a representative sample of 14 star-forming dwarf galaxies in the local Universe, we show the existence of a spaxel-to-spaxel anticorrelation between the index N2≡log([N II]λ6583/Hα) and the Hα flux. These two quantities are commonly employed as proxies for gas-phase metallicity and star formation rate (SFR), respectively. Thus, the observed N2 to Hα relation may reflect the existence of an anticorrelation between the metallicity of the gas forming stars and the SFR it induces. Such an anticorrelation 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 anticorrelation 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 disc galaxies does not produce the correlation either.© 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.
We thank Sebastian Sanchez for providing some of the data used in Section 4.2, Enrique Perez-Montero for help with the use of HIIC, and Matteo Miluzio for discussions and support during the early stages of the analysis. We also thank an anonymous referee whose comments help us improving the reliability of our argumentation. MF gratefully acknowledges the financial support of the Fundacao para a Ciencia e Tecnologia (FCT - Portugal), through the research grant SFRH/BPD/107801/2015. This work has been partly funded by the Spanish Ministry of Economy and Competitiveness, projects Estallidos AYA2013-47742-C04-02-P and AYA2016-79724-C4-2-P, as well as AYA2014-58861-C3-1-P.