ar X iv : a st ro - p h / 93 01 00 1 v 1 7 J an 1 99 3 The peculiar Balmer line profiles of OQ 208 1

We present spectrophotometric observations of the Broad Line Radio Galaxy OQ 208 (≡ Mrk 668 ≡ 14040+286) obtained between 1985 and 1991. We show that the Balmer line fluxes and profile shapes undergo remarkable changes. The ratio of intensities between the broad and narrow components of Hβ increased monotonically from ∼ 15 in 1985 to ∼ 40 in 1991. The peak of the broad components of Hβ and Hα were known to be strongly displaced to the red. We have discovered a correlation between the amplitude of the broad peak displacement and the luminosity of Hβ, in the sense that the displacement is larger when the line luminosity is higher. The line profiles and the correlation are analyzed in light of several competing models for the Broad Line Region (BLR). We conclude that the observations are not compatible with either a binary BLR model or one involving ballistic acceleration of the line emitting gas. Radiative acceleration of a system of outflowing clouds readily explains the correlation between line shift and luminosity as well as the peculiar line profiles. Self-absorption in the Balmer lines or dust absorption in the non illuminated face of the clouds is necessary to explain the asymmetry associated with the red peak. Thus it seems that most or all of the Balmer emission originates from the inward face of the clouds. Theoretical line profiles computed for clouds accelerated by radiation pressure in various geometrical configurations suggest that the observed Hβ profile is best fit assuming the contribution of an ensemble which might be spherical or confined in a thick disk in addition to a component emitted in a thin shell contained in a cone of half opening angle ∼ 12 • seen along its axis. Another composite model for the BLR, in which the line is emitted partly by (1) a relativistic disk seen nearly pole on and (2) infalling matter is not favored – but not excluded – on the basis of the present data.