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We discuss dramatic changes in the $z_a \approx z_e$ absorption system of the $z_e = 2.15$ QSO UM 675 (Q0150$-$203). The C IV $\lambda$1550 and N V $\lambda$1240 doublets at $z_a = 2.1344$ strengthened by a factor of $\sim$3 between the observations of Sargent, Boksenberg and Steidel (1988, ApJS, 68, 539; measured November 1981) and our earliest measurements (November and December 1990). During this time, C IV in the $z_a = 2.0083$ system may also have strengthened. The variability of other lines in these systems is unknown. Continued monitoring is in progress.
We consider several models of the $z_a \approx z_e$ absorption environment, and conclude that the absorbing clouds are close to the QSO and photoionized by the QSO continuum. The variability timescale ($\lap$2.9 yrs rest) requires gas densities $\gap$4000 cm$^{-3}$ to allow changes in the ionization balance. This minimum density, and the high ionization needed to produce the Ne VIII $\lambda$774 and O VI $\lambda$1035 absorptions reported previously (E. M. Burbidge et al. , 1993, BAAS, 24, 1135), requires clouds $\lap$200 pc from the QSO. The full range of absorption line ionizations (including C III $\lambda$977 and N III $\lambda$989) implies that the clouds are segregated, spanning a factor of $\gap$10 in distance or $\gap$100 in density. Across these regions the H I fraction varies from $\sim$10$^{-3}$ to $\sim$10$^{-6}$. The total hydrogen column ranges from a few times $10^{18}$ cm$^{-2}$ in the low ionization gas to $\sim$10$^{20}$ cm$^{-2}$ where the Ne VIII lines form. The Lyman continuum is expected to be optically thin throughout, consistent with the measured absence of a Lyman edge. The metal abundances are roughly solar or above. Implications of these results are discussed.
This work is supported by NASA grant NAG 5-1630.
