We use the most extensive integral field spectroscopic map of a local galaxy , NGC 628 , combined with gas and stellar mass surface density maps , to study the distribution of metals in this galaxy out to 3 effective radii ( R _ { e } ) .
At each galactocentric distance , we compute the metal budget and thus constrain the mass of metals lost .
We find that in the disc about 50 % of the metals have been lost throughout the lifetime of the galaxy .
The fraction of metals lost is higher in the bulge ( \sim 70 % ) and decreases towards the outer disc ( \sim 3 R _ { e } ) .
In contrast to studies based on the gas kinematics , which are only sensitive to ongoing outflow events , our metal budget analysis enables us to infer the average outflow rate during the galaxy lifetime .
By using simple physically motivated models of chemical evolution we can fit the observed metal budget at most radii with an average outflow loading factor of order unity , thus clearly demonstrating the importance of outflows in the evolution of disc galaxies of this mass range ( log ( M _ { \star } / M _ { \odot } ) \sim 10 ) .
The observed gas phase metallicity is higher than expected from the metal budget and suggests late-time accretion of enriched gas , likely raining onto the disc from the metal-enriched halo .