SDSS-IV MaNGA: Integral-field kinematics and stellar population of a
  sample of galaxies with counter-rotating stellar disks selected from about
  4000 galaxies
  We present the integral-field kinematics and stellar population properties of
64 galaxies (61 are Early-Type galaxies, ETGs) with Counter-Rotating stellar
Disks (CRD) selected from about 4000 galaxies in the MaNGA survey, based on
evidence of counter-rotation or two velocity dispersion peaks in the kinematic
maps. For 17 CRDs, the counter-rotating components can also be separated
spectroscopically. The frequency of CRDs in MaNGA is <5% for ellipticals, <3%
for lenticulars and <1% for spirals (at 95% confidence level), consistent with
previous estimates. We produced age and metallicity maps, and compared the
stellar population properties to those of the general ETGs population. We found
that CRDs have similar trends in age and metallicity to ETGs, but are less
metallic at low masses, and show flatter age and steeper metallicity gradients,
on average. A comparison of the velocity fields of the ionized gas and the
stars reveals that in 33 cases the gas corotates with either the inner (15
cases) or outer (18 cases) stellar disk, in 9 cases it is misaligned. In most
cases the gas corotates with the younger disk. Evidence of multimodality in the
stellar population is found in 31 galaxies, while the 14 youngest and least
massive galaxies show ongoing star formation; 14 galaxies, instead, exhibit
unimodality, and are the oldest and most massive. As a general result, our work
indicates that CRDs form primarily via gas accretion in retrograde rotation
with respect to a pre-existing stellar disk.
