The Information Of The Milky Way From 2MASS Whole Sky Star Count: The
  Structure Parameters
  The Ks band differential star count of the Two Micron All Sky Survey (2MASS)
is used to derive the global structure parameters of the smooth components of
the Milky Way. To avoid complication introduced by other fine structures and
significant extinction near and at the Galactic plane, we only consider
Galactic latitude |b| > 30 degree data. The star count data is fitted with a
threecomponent model: double exponential thin disk and thick disk, and a power
law decay oblate halo. Using maximum likelihood the best-fit local density of
thin disk is n0 = 0.030 +- 0.002 stars/pc^3. The best-fit scale-height and
length of the thin disk are Hz1 = 360+-10 pc and Hr1 = 3.7+-1.0 kpc, and those
of the thick disk are and Hz2 = 1020+-30 pc and Hr2 = 5.0+-1.0 kpc, the local
thick-to-thin disk density ratio is f2 = 7+-1%. The best-fit axis ratio, power
law index and local density ratio of the oblate halo are kappa = 0.55+-0.15, p
= 2.6+-0.6 and fh = 0.20+-0:10%, respectively. Moreover, we find some
degeneracy among the key parameters (e.g. n0,Hz1, f2 and Hz2). Any pair of
these parameters are anticorrelated to each other. The 2MASS data can be
well-fitted by several possible combinations of parameters. This is probably
the reason that there is a wide range of values for the structure parameters in
literature similar to this study. Since only medium and high Galactic latitude
data are analyzed, the fitting is very insensitive to the scale-lengths of the
disks.
