| 摘要: | 「數星」是研究本銀河系結構的基本方法之ㄧ。因為消光在近紅外波段是可見光的十分之ㄧ，二微米巡天觀測 (2MASS) 提供我們對本銀河系比較理想的視野。我們拿二微米巡天觀測的點源目錄 (Point Source Catalog)，以一度的解析度，對全天利用「微分星球計數」 (Differential Star Count) 進行資料分析。除了銀心附近的天區以外，在視星等9~15等間的星數─視星等的關係是個羃次關係，且全天的羃次指數沒有太大的差異〈~0.3〉。J、H、Ks三個波段全天平均的羃次指數分別為0.310±0.028、0.311±0.024、0.327±0.026。 我們利用資料去擬合簡單雙指數衰減銀盤密度分布和單一羃次指數的光度函數。在擬合過程中，我們忽略了消光。針對光度函數的參數，我們只考慮從主序O3型態到M4型態的星。在銀盤方位J、H、Ks波段光度函數的羃次指數分別是1.83、1.8、1.79。 我們採取「最小平方差法」定出密度分布參數。標尺高、標尺長、太陽附近廠星的數目密度、太陽位於銀盤上的高度分別大約是400 pc、2.35 kpc、0.06 pc^(-3)、28 pc 。以及使用資料擬合重力束縛銀盤密度分布，並跟雙指數密度分布的結果做比較。扣掉基本的銀河模型之後，剩下一些顯著的特徵，像大小麥哲倫雲、球狀星團NGC 5139、NGC 5272，還有位於反銀心方向的喇叭狀結構 (flare) 等等。 我們從光度函數導出全天的質量函數。J、H、Ks波段的質量函數在大於一個太陽質量的範圍裡分別是-1.76、-1.6、-1.55。這些質量函數都比Salpeter 在1955年找出的初始質量函數〈-1.35〉來得稍為陡峭一點。 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. |
