GASP - XX. From the loose spatially resolved to the tight global SFR-mass relation in local spiral galaxies

Exploiting the sample of 30 local star-forming, undisturbed late-type galaxies in different environments drawn from the GAs Stripping Phenomena in galaxies with MUSE (GASP), we investigate the spatially resolved star formation rate-mass (Sigma(SFR)-Sigma(*)) relation. Our analysis includes also the galaxy outskirts (up to >4 effective radii, r(e)), a regime poorly explored by other Integral Field Spectrograph surveys. Our observational strategy allows us to detect Ha out to more than 2.7r(e) for 75 per cent of the sample. Considering all galaxies together, the correlation between the Sigma(SFR) and Sigma(*) is quite broad, with a scatter of 0.3 dex. It gets steeper and shifts to higher Sigma(*) values when external spaxels are excluded and moving from less to more massive galaxies. The broadness of the overall relation suggests galaxy-by-galaxy variations. Indeed, each object is characterized by a distinct Sigma(SFR)-Sigma(*) relation and in some cases the correlation is very loose. The scatter of the relation mainly arises from the existence of bright off-centre star-forming knots whose Sigma(SFR)-Sigma(*) relation is systematically broader than that of the diffuse component. The Sigma(SFR)-Sigma(tot gas) (total gas surface density) relation is as broad as the Sigma(SFR)-Sigma(*) relation, indicating that the surface gas density is not a primary driver of the relation. Even though a large galaxy-by-galaxy variation exists, mean Sigma(SFR) and Sigma(*) values vary of at most 0.7 dex across galaxies. We investigate the relationship between the local and global SFR-M-* relation, finding that the latter is driven by the existence of the size-mass relation.