| This thesis focuses primarily on how two important processes---galaxy mergers and gas accretion from the surrounding intergalactic medium---affect the evolution of galaxies. Using post-starburst, or E+A, galaxies as a marker sample that undergoes a rapid transition from gas-rich star-forming galaxies to quiescent, passively-evolving E/S0s, we study what triggers E+A evolution and what E+A galaxies will become after the fading of their young stellar population. With high resolution HST WFPC2/ACS imaging, we investigate their small and large scale properties, including their detailed morphologies, bulge fractions, color gradients, scaling relationships, and newly formed star-clusters. 70% of E+A galaxies show disturbances and tidal features indicating a merger origin and all their properties are either consistent with those of E/S0s or, if left to evolve passively, will become like those of early-types.;Using cosmological simulations, we study hydrogen and helium gravitational cooling radiation from gas accretion by young galaxies, finding that observing optically thin cooling lines such as He II lambda1640 and H I Halpha is critical in understanding the nature of galaxies forming via gas-accretion. To obtain an unbiased sample of Lyalpha blobs that will allow us to follow-up their optically thin Halpha lines in the NIR, we conduct a blind, wide-field, narrow-band imaging survey for Lyalpha blobs. After searching over 4.82 deg2, we discover four blobs that we spectroscopically confirm to lie at z = 2.3. The properties of these blobs are diverse: two blobs are X-ray-detected and have broad optical emission lines (e.g., C IV) characteristic of AGN. The other 50% of blobs are not X-ray or optically-detected as AGN down to similar limits. The number density of the four blobs is extremely low, ∼3 x 10-6 h370 Mpc-3, comparable to that of galaxy clusters at similar redshifts. The two X-ray undetected blobs are separated by only 70" (550 kpc) and have almost identical redshifts (corresponding to ≲ 360 kpc along the line-of-sight), suggesting that they are part of the same system. Given the rarity of the blobs and our discovery of a close pair, we speculate that blobs occupy the highest density regions and thus may be precursors of today's rich cluster galaxies. |
