| 摘要: | 本論文對不同尺度的星球系統進行了結構的分析以及研究。第一章中，我們以多波段(ugriz)、廣角的斯隆數位巡天(Sloan Digital Sky Survey)資料，分析了室女座星系團(Virgo Cluster)中，100個早期星系的結構以及光度參數。這些早期星系，顏色偏紅，亮度在g波段差了約1000倍。我們開發了一套工具，可自動拼貼影像(mosaic images)並扣除其天光背景，並將場源抹除(mask)，最後以無母數分析(non-parametric approach)量測星系的形狀、星等、等效半徑、以及等效表面亮度。同時我們也以瑟西克模型(Sersic Model)擬合星系的表面亮度分佈以及平均顏色。對大多數的星系而言，本分析的結果與文獻相當吻合。然而對於一些較亮(質量較大)的星系，我們發現了其結構參數因不同量測方法之故，仍欠缺一致性。利用我們的光度資料，可對近年來廣為使用的室女座星系團目錄(Virgo Cluster Catalog)提供了額外的星等誤差估計:σ(BT)~0.2 mag。我們的分析顯示了：1.這100個星系的結構參數---表面亮度分佈曲率、等效半徑、平均表面亮度---連續且均勻地隨著星等而分佈﹔2.矮星系(dwarf galaxy)的軸比分佈函數與巨型橢圓星系近似。最後我們也發現這些早期星系的顏色，隨著亮度有著非線性但均勻的變化。較暗、質量較小的矮星系的顏色星等關係則分佈較為零散，顯示了其較為複雜的恆星形成史。 本論文的第二個部份，是對於銀河系中116個球狀星團的形狀分析結果。我們利用近紅外全天巡天(2 Micron All-Sky Survey)資料庫，並以恆星計數的方式，估計球狀星團的軸比以及方向角。我們的結果顯示了大部分的球狀星團，並非完美的圓形，而是輕微扁平，軸比約為0.87。靠近銀核的星團有著各種的橢圓率，其中較扁的星團則傾向於指向銀何中心，顯示了銀核潮汐作用對這些星團的影響。遠離銀核的星團，形狀則有變圓的趨勢，但有些在銀暈裡的星團，形狀呈扁平。這些遠離銀核且較扁的星團，其中有三個已知跟星流(stellar stream)有關或是有較特殊的動力演化史：NGC5897已知在其周圍有南北向分佈的潮汐結構﹔NGC6838剛穿過銀河盤面不久﹔IC4499則分佈於麥哲倫流(Magellanic Stream)上。除了星團本身的形狀，我們也對星團周圍是否有尺度較大的結構感到好奇，例如 潮汐尾、潮汐流。我們發現了31個星團有可能的潮汐結構，其中四個有趣的例子是：NGC6366經歷過強大的潮汐撕裂作用(tidal stripping) ﹔NGC2808則已知有多重主星序﹔NGC3201以及NGC6397則於數百萬年穿過銀河盤面。我們對球狀星團的形狀分析，將有助於瞭解銀河系內的質量分佈以及合併史(merging history)。 Structural studies on stellar systems in multi-scale are presented in this thesis. In the first chapter, I present a photometric and structural analysis of 100 ACS Virgo Cluster Survey (ACSVCS) galaxies based on homogeneous, multi-wavelength (ugriz), wide-field SDSS (DR5) imaging. These early-type galaxies, which populate the red sequence in the Virgo Cluster, span a factor of nearly ~10^3 in g-band luminosity. We describe an automated pipeline that generates background-subtracted mosaic images, masks field sources, and measures shapes, total magnitudes, effective radii and effective surface brightnesses using a model-independent approach. A parametric analysis of the surface brightness profiles is also carried out to obtain Sersic-based structural parameters and mean galaxy colors. We compare the galaxy parameters to those in the literature, including those from the ACSVCS, finding good agreement in most cases, although the sizes of the brightest, and most extended, galaxies are found to be most uncertain and model dependent. Our photometry provides an external measurement of the errors on total magnitudes from the widely-used Virgo Cluster Catalog to be sigma(B_T) ~ 0.2 mag. The distribution of axial ratios of low-mass (``dwarf") galaxies bears a strong resemblance to the one observed for the higher-mass (``giant") galaxies. The global structural parameters for the full galaxy sample --- profile shape, effective radius and mean surface brightness --- are found to vary smoothly and systematically as a function of luminosity, with unmistakable evidence for changes in structural homology along the red sequence. As noted in previous studies, the ugriz galaxy colors show a non-linear but smooth variation over a ~7 mag range in absolute magnitude, with an enhanced scatter for the faintest systems that is likely the signature of their more diverse star-formation histories. In the second part of the thesis, I present the morphological analysis on stellar systems in much smaller scales in respect to the galaxy cluster. The axial ratios and position angles of 116 Galactic globular clusters (GCs) are presented, based on the analysis of the spatial distribution of 2MASS point sources. In general, Galactic GCs are slightly flattened in shape, with a median axial ratio of 0.87. The GCs close to the Galactic bulge have various degrees of flattening, and those exhibiting obvious flattening tend to have their elongation pointing toward the Galactic center, manifesting the tidal effect from the bulge. In comparison, GCs away from the Galactic center tend to be spherical. A few GCs in the halo are also found to have elongated shape. Three notable cases of highly flattened clusters were previously known to be associated with stellar streams or particular dynamical history: NGC,5897 is known to have a stellar tidal tail extending to its equatorial North and South; NGC,6838 had a recent encounter with the Galactic plane; IC,4499 is spatially close to, and collimated with, the ``Magellanic Stream'. We also found 31 GCs associated with possible clumpy structures in stellar distributions, as diagnosed by possible cluster members selected by colors and magnitudes. Four such cases of stellar debris are presented:~NGC,6366 is experiencing heavy tidal stripping, and NGC,2808 has been known to have multiple main sequence branches. NGC,3201 and NGC,6397, like the flattened cluster, NGC,6838, also have their recent passage through the Galactic plane. Our sample is derived from the most comprehensive analysis of the morphology of Galactic GCs, and should provide a valuable database to probe the mass distribution, or merging history of the Milky Way galaxy. |
