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The H IX galaxy survey III: The gas-phase metallicity in HI eXtreme galaxies

This paper presents the analysis of optical integral field spectra for the HI eXtreme (HIX) galaxy sample. HIX galaxies host at least 2.5 times more atomic gas (HI) than expected from their optical R-band luminosity. Previous examination of their star formation activity and HI kinematics suggested that these galaxies stabilise their large HI discs (radii up to 94 kpc) against star formation due to their higher than average baryonic specific angular momentum. A comparison to semi-analytic models further showed that the elevated baryonic specific angular momentum is inherited from the high spin of the dark matter host. In this paper we now turn to the gas-phase metallicity as well as stellar and ionised gas kinematics in HIX galaxies to gain insights into recent accretion of metal-poor gas or recent mergers. We compared the stellar, ionised, and atomic gas kinematics, and examine the variation in the gas-phase metallicity throughout the stellar disc of HIX galaxies. We find no indication for counter-rotation in any of the components, the central metallicities tend to be lower than average, but as low as expected for galaxies of similar HI mass. Metallicity gradients are comparable to other less HI-rich, local star forming galaxies. We conclude that HIX galaxies show no conclusive evidence for recent major accretion or merger events. Their overall lower metallicities are likely due to being hosted by high spin halos, which slows down their evolution and thus the enrichment of their interstellar medium.