  The main sequence (MS) of star-forming galaxies (SFGs) is the tight relation
between the galaxy stellar mass and its star formation rate (SFR) and was
observed up to z ~ 6. The MS relation can be used as a reference for
understanding the differences among galaxies, characterised by different rates
of stellar production (starbursts, SFGs, and passive galaxies), and those
inside a galaxy made up of different components (bulge, disk, and halo). To
investigate peculiar features found in our sample galaxies, we focus here on
their star formation history (SFH). We performed a spectral energy distribution
fitting procedure that accounted for the energetic balance between UV and
far-IR radiation on a sample of eight nearby face-on 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 exploiting the BAGPIPES code, we
constrained the SFHs for each galaxy with a delayed exponentially declining
model to derive their mass-weighted age (tMW). A central old region (tMW up
to~7Gyr, consistent with the presence of a bulge for various systems) is
followed by younger regions in which the disks are still forming stars
(tMW~4Gyr). At larger distances, tMW increases mildly in general. Strikingly,
in two galaxies (NGC4321 and NGC5194), we found a steep increase in tMW that
reached levels similar to those of the bulge. These old stellar populations in
the very galaxy outskirts are unexpected. We discuss their potential origin by
considering the different gas phases of the source with the most prominent
quenched ring, NGC4321, and argue for two main possibilities: 1) some
environmental effect (e.g. starvation) 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).
