A study of outer disk stellar populations of face-on star-forming
  galaxies in SDSS-IV MaNGA: causes of H$\alpha$ deficiency
  Integral field unit (IFU) spectra of face-on star-forming galaxies from the
MaNGA survey are stacked in radial bins so as to reach a S/N high enough to
measure emission lines and Lick indices out to 2.5-3 R_e. Two thirds of
galaxies have stellar populations in the outer disks that are older, more metal
poor and less dusty than in the inner disks. Recent bursts of star formation
have occurred more frequently in the outer disk, but extinction-corrected
Halpha equivalent widths are significantly lower at fixed D_n(4000) in these
regions. I examine the properties of a subset of galaxies with the the most
H$\alpha$ deficient outer disks. These regions contain young stellar
populations that must have formed within the last 0.5 Gyr, but
extinction-corrected Halpha values well below the values predicted for a
standard Kroupa IMF. The Halpha deficient galaxies have flat D_n(4000) and
Hdelta_A profiles with little radial fluctuation, indicating that star
formation has occurred extremely uniformly across the entire disk. The
H$\alpha$ line profiles indicate that the ionized gas kinematics is also very
regular across the disk. The main clue to the origin of the Halpha deficiency
is that it sets in at the same radius where the dust extinction abruptly
decreases, suggesting a mode of star formation deficient in massive stars in
quiescent, HI-dominated gas. Finally, I have carried out a search for galaxies
with signatures of unusual Halpha kinematics and find that 15% of the sample
exhibit evidence for significant ionized gas that is displaced from the
systemic velocity of the disk.
