Keck high resolution spectroscopy of PKS 0123+257: Intrinsic absorption in a radio-loud quasar

We present results from Keck I high resolution spectroscopy of the radio-loud quasar PKS 0123 + 257 (z(e) = 2.364, V = 17.5). In this object we detect Ly alpha, N V lambda lambda 1238,1242, Si IV lambda lambda 1393,1402, and C IV lambda lambda 1548,1550 in an absorption system at a redshift of 2.369. The Ly alpha line has a square-bottomed profile suggesting a high column density of gas, yet the line does not reach zero intensity. The resolved C IV doublet ratio also clearly demonstrates that the absorbing clouds at this redshift do not fully occult the background light source along our line-of-sight. The absorption Lines are positioned near the centers of the broad emission-lines and the coverage fraction of the strongest absorption lines varies inversely proportionally with the strength of the corresponding emission lines. This implies that although the absorption-line region may obscure the continuum source, it does not completely occult the broad emission-line region. This effect suggests that the lines are formed close to the QSO central region. A model is proposed in which the apparent coverage fraction derived for the weaker absorption lines may vary with the column density of the lines. Broad absorption-lines (which are known to be intrinsic) are found nearly exclusively in radio-quiet objects. Intrinsic narrow absorption lines have previously been found in radio-quiet QSOs; it is therefore significant that an intrinsic absorption system has been verified in a radio-loud quasar. (C) 1997 American Astronomical Society.