Ancient stellar populations in the outskirts of nearby grand-design spirals: Investigation of their star formation histories

Context. The main sequence (MS) of star-forming galaxies (SFGs) is the tight relation between the galaxy stellar mass (M-star) and its star formation rate (SFR) and was observed up to z similar to 6. The MS relation can be used as a reference for understanding the differences among galaxies, which are characterised by different rates of stellar production (starbursts, SFGs, and passive galaxies), and those inside a galaxy that is made up of different components (bulge, disk, and halo). To investigate peculiar features found in our galaxies sample in more depth, we focus here on their star formation history (SFH). Aims. The SFHs are a fundamental tool for revealing the galaxy path from the earlier stages of formation to the present time. The various phases of galaxy evolution are imprinted on the source spectrum globally and locally. Thus, we are able to interpret the dynamical origin of the spirals quantitatively and distinguish between in situ or ex situ formation processes. Methods. We performed a spectral energy distribution fitting procedure that accounted for the energetic balance between UV (observed) and far-IR (optically obscured) radiation on a sample of eight nearby face-on grand-design spiral galaxies from the DustPedia sample. This approach allowed us to study the spatially resolved MS of the sample and to recover the past SFH by accounting for attenuation due to the presence of dust. By exploiting the BAGPIPES code, we constrained the SFHs for each galaxy with a delayed exponentially declining model to derive their mass-weighted age (t(MW)). Results. The spiral galaxies in our sample have similar radial t(MW) trends overall. A central old region (t(MW) up to similar to 7 Gyr, consistent with the presence of a bulge for various systems) is followed by younger regions in which the disks are still forming stars (t(MW) similar to 4 Gyr). At larger distances from the centre of the galaxies, t(MW) increases mildly in general. Strikingly, in two galaxies (NGC4321 and NGC5194), we found a steep increase in t(MW)that reached levels similar to those of the bulge. These old stellar populations in the very galaxy outskirts, which are also detectable as "quenched rings" below the spatially resolved MS, is unexpected. We discuss their potential origin by considering the different gas phases (HI and H2) of the source with the most prominent quenched ring, NGC4321, and argue for two main possibilities: (1) some environmental effect (e.g. starvation) could affect the outer edge of the galaxies or (2) the circumgalactic medium of sources outside of high-density clusters might have stopped to supply pristine gas to the galaxy (e.g. if its specific angular moment is too high for being accreted).