| This thesis presents results of two studies for understanding the evolution of galaxies.; We have surveyed a 4.5 degree2 region centered on Leo I to explore its extended morphology. The Leo I giant candidates are selected by a photometric technique using the M, T2, and DDO51 filters. Our technique is nearly 100% reliable in selecting Leo I stars. Using Leo I giant candidates, we explore the two-dimensional distribution and find compelling evidence that Leo I has been tidally disrupted: (1) we find many Leo I giant candidates outside the King limiting radius, spread out predominantly to the east-west direction, (2) we find double breaks in the radial density profile and, (3) our isodensity contour plots show isophotal twists and hint of S-shaped morphology. We conclude that these features support a picture where Leo I has been tidally disrupted on two close passes close to the Milky Way. A Milky Way-bound Leo I implies a larger total mass for the Galaxy.; We present HST/STIS far- and near-ultraviolet photometry for globular clusters in M87. We detect 109 sources in total; 66 of them have optical counterparts while, surprisingly 43 are detected only in the UV. Our far-UV detection rate compared to the WFPC2 optical photometry is 63% for clusters with V0 < 23. Among M87 GCs in our sample, metal-poor clusters are found to have higher far-UV to optical flux ratios. Comparison with Milky Way globulars shows that the M87 clusters do not overlap with Milky Way clusters in the (V − I, FUV − V) diagram, whereas in UV color-color diagrams, the M87 clusters appear to represent extensions of the Milky Way GC sequences. In none of the UV-metallicity or UV color-color correlations do the M87 GCs overlap with elliptical galaxies, showing that these are two distinct types of stellar populations. We discuss various explanations for the UV-only sources and find that they are most likely UV-bright background galaxies. The multi-epoch near-UV observations show that besides the persistent UV-only sources, there are also transient sources as well as recurrent ones. The transients are probably classical novae, but the recurrent sources are not yet understood. |
