Structure of ACS Virgo Cluster Survey Galaxies and Morphology of Galactic Globular Clusters

• Main Authors: Chin-Wei Chen, 陳錦威
• Other Authors: Wen-Ping Chen
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/45065632953925661553

博士 === 國立中央大學 === 天文研究所 === 98 === 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.