Theoretical models to explain the variable 21 cm absorption spectrum in AO 0235+164

The variability of the highly redshifted 21 cm absorption spectrum in AO 0235+164 presents us with two essential facts. First, rapid changes occur in the line depths of foreground clouds with redshift z approximately equal to 0.524. Second, the background radio source is among the most violently variable objects known. Thus it is reasonable to suppose that changes in the lines stem from activity in the background BL Lac object. The question is how. Two alternatives are considered here. The intrinsic scenario assumes that activity in 0235 somehow alters the 21 cm opacity of absorbing HI within its neighborhood. Since the redshift of 0235 is z>0.85, one is confronted with the task of explaining how objects with redshifts differing by ΔZapproximately0.3 can be physically associated. The extrinsic scenario assumes that activity in 0235 results in time-dependent changes of the brightness centroid of the 932 MHz radio source. When viewed through a non-uniform distribution of absorbing clouds the shifting light path brings about changes in line depth, without changes in opacity. In this case one assumes that 0235 is cosmologically distant from the foreground HI. (Auth.)