P-MaNGA: Gradients in Recent Star Formation Histories as Diagnostics for
  Galaxy Growth and Death
  We present an analysis of the data produced by the MaNGA prototype run
(P-MaNGA), aiming to test how the radial gradients in recent star formation
histories, as indicated by the 4000AA-break (D4000), Hdelta absorption
(EW(Hd_A)) and Halpha emission (EW(Ha)) indices, can be useful for
understanding disk growth and star formation cessation in local galaxies. We
classify 12 galaxies observed on two P-MaNGA plates as either centrally
quiescent (CQ) or centrally star-forming (CSF), according to whether D4000
measured in the central spaxel of each datacube exceeds 1.6. For each galaxy we
generate both 2D maps and radial profiles of D4000, EW(Hd_A) and EW(Ha). We
find that CSF galaxies generally show very weak or no radial variation in these
diagnostics. In contrast, CQ galaxies present significant radial gradients, in
the sense that D4000 decreases, while both EW(Hd_A) and EW(Ha) increase from
the galactic center outward. The outer regions of the galaxies show greater
scatter on diagrams relating the three parameters than their central parts. In
particular, the clear separation between centrally-measured quiescent and
star-forming galaxies in these diagnostic planes is largely filled in by the
outer parts of galaxies whose global colors place them in the green valley,
supporting the idea that the green valley represents a transition between
blue-cloud and red-sequence phases, at least in our small sample. These results
are consistent with a picture in which the cessation of star formation
propagates from the center of a galaxy outwards as it moves to the red
sequence.
