SDSS IV MaNGA - sSFR profiles and the slow quenching of discs in green
  valley galaxies
  We study radial profiles in H$\alpha$ equivalent width and specific star
formation rate (sSFR) derived from spatially-resolved SDSS-IV MaNGA
spectroscopy to gain insight on the physical mechanisms that suppress star
formation and determine a galaxy's location in the SFR-$\rm M_\star$ diagram.
Even within the star-forming `main sequence', the measured sSFR decreases with
stellar mass, both in an integrated and spatially-resolved sense. Flat sSFR
radial profiles are observed for $\rm log(M_\star/ M_\odot) < 10.5$, while
star-forming galaxies of higher mass show a significant decrease in sSFR in the
central regions, a likely consequence of both larger bulges and an inside-out
growth history. Our primary focus is the green valley, constituted by galaxies
lying below the star formation main sequence, but not fully passive. In the
green valley we find sSFR profiles that are suppressed with respect to
star-forming galaxies of the same mass at all galactocentric distances out to 2
effective radii. The responsible quenching mechanism therefore appears to
affect the entire galaxy, not simply an expanding central region. The majority
of green valley galaxies of $\rm log(M_\star/ M_\odot) > 10.0$ are classified
spectroscopically as central low-ionisation emission-line regions (cLIERs).
Despite displaying a higher central stellar mass concentration, the sSFR
suppression observed in cLIER galaxies is not simply due to the larger mass of
the bulge. Drawing a comparison sample of star forming galaxies with the same
$\rm M_\star$ and $\rm \Sigma_{1~kpc}$ (the mass surface density within 1 kpc),
we show that a high $\rm \Sigma_{1~kpc}$ is not a sufficient condition for
determining central quiescence.
