The Stellar Number Density Distribution in the Local Solar Neighborhood
  is North-South Asymmetric
  We study the number density distribution of a sample of K and M dwarf stars,
matched North and South of the Galactic plane within a distance of 2 kpc from
the sun, using observations from the Ninth Data Release of the Sloan Digital
Sky Survey. We determine distances using the photometric parallax method, and
in this context systematic effects exist which could potentially impact the
determination of the number density profile with height from the Galactic plane
--- and ultimately affect a number density North-South asymmetry. They include:
(i) the calibration of the various photometric parallax relations, (ii) the
ability to separate dwarfs from giants in our sample, (iii) the role of stellar
population differences such as age and metallicity, (iv) the ability to
determine the offset of the sun from the Galactic plane, and (v) the correction
for reddening from dust in the Galactic plane, though our stars are at high
Galactic latitudes. We find the various analyzed systematic effects to have a
negligible impact on our observed asymmetry, and using a new and larger sample
of stars we confirm and refine the earlier discovery of Widrow et al. of a
significant Galactic North-South asymmetry in the stellar number density
distribution.
