THE PECULIAR BALMER LINE-PROFILES OF OQ-208

We present spectrophotometric observations of the broad line radio gal axy OQ 208 (= Mrk 668 = 14040 + 286) obtained between 1985 and 1991. W e show that the Balmer line fluxes and profile shapes undergo remarkab le changes. The ratio of intensities between the broad and narrow comp onents of Hbeta increased monotonically from approximately 15 in 1985 to approximately 40 in 1991. The peak of the broad components of Hbeta and Halpha were known to be strongly displaced to the red. We have di scovered a correlation between the amplitude of the broad peak displac ement and the luminosity of Hbeta, in the sense that the displacement is larger when the line luminosity is higher. The line profiles and th e correlation are analyzed in light of several competing models for th e broad line region (BLR). We conclude that the observations are not c ompatible with either a binary BLR model or one involving ballistic ac celeration of the line-emitting gas. Radiative acceleration of a syste m of outflowing clouds readily explains the correlation between line s hift and luminosity as well as the peculiar line profiles. Self-absorp tion in the Balmer lines or dust absorption in the nonilluminated face of the clouds is necessary to explain the asymmetry associated with t he red peak. Thus it seems that most or all of the Balmer emission ori ginates from the inward face of the clouds. Theoretical line profiles computed for clouds accelerated by radiation pressure in various geome trical configurations suggest that the observed Hbeta 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 th in shell contained in a cone of half-opening angle approximately 12-de grees seen along its axis. Another composite model for the BLR, in whi ch the line is emitted partly by (1) a relativistic disk seen nearly p ole-on and (2) infalling matter is not favored-but not excluded-on the basis of the present data.