ASTRONOMICAL JOURNAL, cilt.114, ss.1337-1344, 1997 (SCI İndekslerine Giren Dergi)
We report new HST and ground-based observations of a damped Ly alpha absorption system toward the QSO 0850+4400. The redshift of the absorption system is z = 0.163770+/-0.000054 and the neutral hydrogen column density of the absorption system is log N = 19.81+/-0.04 cm(-2). The absorption system is by far the lowest redshift confirmed damped Ly alpha absorption system yet identified, which provides an unprecedented opportunity to examine the nature, impact geometry, and kinematics of the absorbing galaxy in great detail. The observations indicate that the absorption system is remarkable in three respects: First. the absorption system is characterized by weak metal absorption lines and a low metal abundance, possibly less than 4% of the solar metal abundance. This cannot be explained as a consequence of obscuration by dust, because the neutral hydrogen column density of the absorption system is far too low for obscuration by dust to introduce any significant selection effects. Second, the absorption system is associated with a moderate-luminosity early-type S0 galaxy, although the absorption may actually arise in one of several very faint galaxies detected very close to the QSO line of sight. Third, the absorbing material moves counter to the rotating galaxy disk, which rules out the possibility that the absorption arises in a thin or thick co-rotating gaseous disk. These results run contrary to the expectation that low-redshift damped Ly alpha absorption systems generally arise in the gas-and metal-rich inner parts of late-type spiral galaxies. We suggest instead that mounting evidence indicates that low-redshift galaxies of a variety of morphological types may contain significant quantities of low metal abundance pas at large galactocentric distances. (C) 1997 American Astronomical Society.