| dc.description.abstract | Star count is one of the basic methods to study the structure of our Galaxy. The Two Micron All Sky Survey (2MASS) offers a better view of the global distribution of stars in our Galaxy than optical survey, because the extinction of near infrared is only 10% of that in the visible. We analyze the data of 2MASS Point Source Catalog for the whole sky at one degree resolution by differential star count. Except in the vicinity of the Galactic Center, the star count-apparent magnitude relation is well approximated by a single power law in the magnitude between 9 to 15, and the power law indices are roughly the same (~0.3) in the whole sky. The average power law indices of the whole sky in J, H, Ks bands are 0.310±0.028, 0.311±0.024, 0.327±0.026, respectively.
We fit the data by a simple double exponential disk for the Galaxy density distribution with a single power law luminosity function (LF). We neglect extinction in the fitting process. We consider main sequence stars from O3 type to M4 type only. The power law indices of LF in the glactic plane in the J, H, Ks bands are 1.83, 1.8, 1.79, respectively.
We adopt the least square method to find the parameters of number density profile of the Galaxy. The scale height, scale length, number density near the Sun and the position of the Sun above the mid-plane are about 400 pc, 2.35 kpc, 0.06 pc^(-3), 28 pc, respectively. We also fit the data by a self-gravitating disk density distribution, and compare the results with those from double exponential disk density profiles. After subtracting the basic model from the data, the residual shows some prominent features, such as, LMC, SMC, NGC 5139, NGC 5272, flare in the direction towards the anti-galactic center.
We deduce the mass function of the whole sky from the LF. The mass function in the range above one solar mass for J, H, Ks bands are -1.76, -1.6, -1.55, respectively. These are somewhat steeper than the initial mass function found by Salpeter 1955, which is -1.35. | en_US |