A study of the central stellar populations of galaxies in SDSS-IV MaNGA:
  identification of a sub-sample with unusually young and massive stars
  This paper describes a search for galaxy centers with clear indications of
unusual stellar populations with an initial mass function flatter than Salpeter
at high stellar masses. Out of a sample of 668 face-on galaxies with stellar
masses in the range 10^10- 10^11 M_sol, I identify 15 galaxies with young to
intermediate age central stellar populations with unusual stellar population
gradients in the inner regions of the galaxy. In these galaxies, the 4000
Angstrom break is either flat or rising towards the center of the galaxy,
indicating that the central regions host evolved stars, but the H$\alpha$
equivalent width also rises steeply in the central regions. The ionization
parameter [OIII]/[OII] is typically low in these galactic centers, indicating
that ionizing sources are stellar rather than AGN. Wolf Rayet features
characteristic of hot young stars are often found in the spectra and these also
get progressively stronger at smaller galactocentric radii. These outliers are
compared to a control sample of galaxies of similar mass with young inner
stellar populations, but where the gradients in Halpha equivalent width and
4000 Angstrom break follow each other more closely. The outliers exhibit
central Wolf Rayet red bump excesses much more frequently, they have higher
central stellar and ionized gas metallicities, and they are also more
frequently detected at 20 cm radio wavelengths. I highlight one outlier where
the ionized gas is clearly being strongly perturbed and blown out either by
massive stars after they explode as supernovae, or by energy injection from
matter falling onto a black hole.
