Studies of galaxies giving rise to QSO absorption systems and observations of the high-redshift universe

I present a study of the galaxies that give rise to Lyman-alpha (Lyalpha) and triply ionized carbon (CIV) absorption lines observed in the spectra of background quasi-stellar objects (QSOs), as well as on studies of the high-redshift universe. By comparing the redshifts of galaxies and Lyalpha absorption systems along common lines of sight, I confirmed the existence of an anti-correlation between Lyalpha absorption equivalent width and galaxy impact parameter. Further analysis showed that tenuous gas is likely to be distributed around galaxies in spherical halos rather than in flattened disks with the gaseous extent scaling with galaxy B-band and K-band luminosities. I found that extended gaseous halos are a common and generic feature of galaxies over a wide range of luminosity and morphological type and Lyalpha absorption systems traced a significant and representative portion of the galaxy population. Applying the scaling relation between galaxy gaseous radius and galaxy B-band luminosity to predict the incidence of Lyalpha absorption systems originating in extended gaseous envelopes of galaxies, I found that luminous galaxies can explain about 50% of Lyalpha absorption systems with absorption equivalent width W > 0.3 A.; By comparing the redshifts of galaxies and CIV absorption systems along common lines of sight, I found that extended gaseous halos of galaxies have been metal contaminated out to large galactocentric radii, ≈100 h-1 kpc. The covering factor of ionized gas in galactic halos was estimated to be 0.93 with a 1 sigma lower bound of 0.83, which may strongly constrain the possibilities that CIV absorption systems arised in accreting satellite galaxies or in filaments of gravitationally collapsed structures.; To study the high-redshift universe, I analyzed very deep slitless spectroscopy observations acquired by the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. These observations are especially suited for identifying very distant galaxies due to high sensitivity and less contaminating background light. A galaxy at z = 6.68 and five emission-line objects at ⟨z⟩ = 4.69 were identified in this field, providing constraints on star formation in the early universe.