Gas inflow and metallicity drops in star-forming galaxies

Gas inflow feeds galaxies with low-metallicity gas from the cosmic web, sustaining star formation across the Hubble time. We make a connection between these inflows and metallicity inhomogeneities in star-forming galaxies, by using synthetic narrow-band images of the Ha emission line from zoom-in AMR cosmological simulations of galaxies with stellar masses of M-* similar or equal to 10(9) MD at redshifts z = 2-7. In 50 per cent of the cases at redshifts lower than 4, the gas inflow gives rise to star-forming, Ha-bright, off-centre clumps. Most of these clumps have gas metallicities, weighted by Ha luminosity, lower than the metallicity in the surrounding interstellar medium by similar to 0.3 dex, consistent with observations of chemical inhomogeneities at high and low redshifts. Due to metal mixing by shear and turbulence, these metallicity drops are dissolved in a few disc dynamical times. Therefore, they can be considered as evidence for rapid gas accretion coming from cosmological inflow of pristine gas.