THE ORIGIN OF MG-II ABSORPTION-LINE SYSTEMS OF QUASI-STELLAR OBJECTS

The origin of the Mg II absorption-line systems is analyzed from the point of view of the ionization mechanism prevailing in the region. The column densities derived from the observed equivalent widths of Mg II, Mg I, C IV, and C II lines are compared to theoretical values obtained from photoionization models. Two kinds of models are considered: (a) H II region ionized by a hot star, if the absorption lines are associated with star-forming regions, and (b) diluted gas ionized by the integrated background radiation of QSOs, if a halo origin is adopted. Twenty absorption systems have been analyzed. For most of them the H II region models that are consistent are those in which the absorption region is matter-bounded, ionized by an O4 star, with hydrogen density less than 10 cm-3 and subsolar abundances. On the other hand, halo models do not give a unique solution, since the systems could be roughly fitted by two kinds of models, depending on the input parameters (ionization parameter, optical depth at the Lyman limit, and chemical composition). In addition, the photoionization models cannot consistently fit the data if the line absorption systems are associated with clouds in the halo, unless a complementary energy source is present.