Time Variable Associated Absorption in the QSO UM 675

Previous abstract Next abstract

Session 11 -- Luminous AGN and QSOs
Display presentation, Monday, 30, 1994, 9:20-6:30

[11.07] Time Variable Associated Absorption in the QSO UM 675

F.W. Hamann, T.A. Barlow, E.A. Beaver, E.M. Burbidge, R.D. Cohen, V.T. Junkkarinen and R.W. Lyons (UCSD)

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.

Monday program listing