The Information Of The Milky Way From 2MASS Whole Sky Star Count: The
  Bimodal Color Distributions
  The J-Ks color distribution (CD) with a bin size of 0.05 magnitude for the
entire Milky Way has been carried out by using the Two Micron All Sky Survey
Point Source Catalog (2MASS PSC). The CDs are bimodal, which has a red peak at
0.8 < J-Ks < 0.85 and a blue peak at 0.3 < J-Ks < 0.4. The colors of the red
peak are more or less the same for the whole sky, but that of the blue peak
depend on Galactic latitude, (J-Ks ~ 0.35 at low Galactic latitudes and 0.35 <
J-Ks < 0.4 for other sky areas). The blue peak dominates the bimodal CDs at low
Galactic latitudes and becomes comparable with the red peak in other sky
regions. In order to explain the bimodal distribution and the global trend
shown by the all sky 2MASS CDs, we assemble an empirical HR diagram, which is
composed by observational-based near infrared HR diagrams and color magnitude
diagrams, and incorporate a Milky Way model. In the empirical HR diagram, the
main sequence stars turnoff the thin disk is relatively bluer, (J-Ks)0 = 0.31,
when we compare with the thick disk which is (J-Ks)0 = 0.39. The age of the
thin/thick disk is roughly estimated to be around 4-5/8-9 Gyr according to the
color-age relation of the main sequence turnoff. In general, the 2MASS CDs can
be treated as a tool to census the age of stellar population of the Milky Way
in a statistical manner and to our knowledge this is a first attempt to measure
the age.
