Physical conditions and the locations of the absorption-line systems in the QSO 3C 286 and the BL Lacertae object PKS 0735 + 178

Abstract

The redshifted 21-cm and Mg II absorption lines observed in the QSO 3C 286 and the BL Lacertae object 0735+178, respectively, are analyzed to determine the location of the absorbing gas. We first describe evidence which supports the presence of neutral hydrogen intrinsic to QSOs, and then examine excitation conditions influencing the formation of 21-cm absorption lines. Due to the high spin temperature T/subs/ resulting from the bright 21-cm continuum, the 21-cm absorption line observed in 3C 286 can arise close to the QSO only if L$alpha$ radiation determines the excitation of the hyperfine levels. The 21-cm line could also form in an intervening galaxy. The absence of redshifted Mg II absorption in the spectrum of 3C 286 places and upper limit of 3 x 10$sup -3$/T/ subs/ on the Mg$sup +$/H I. ratio in the column of absorbing H I. This is compatible with a solar abundance ratio of Mg/H; but if the observed upper limit to Mg II absorption were reduced by a factor 2, this would require Mg$sup +$/H I<10$sup -2$ (Mg/H)/sub sun/. An analysis of the Mg II absorption lines seen in 0735+178 leads to two possibilities: (a) a large column density of Mg$sup +$ ionsmore » and low velocity dispersion; or (b) a low column density and large velocity dispersion. Whereas (b) is kinematically inconsistent with an intervening galaxy, (a) is not. We propose that 21-cm absorption features be searched for in the 30-cm continuum of 0735+178. If no absorption is seen, and (a) applies, then the spin temperature must be much higher than that observed in absorbing clouds in our Galaxy, thus making an intervening galaxy implausible. (AIP)« less