Broad Absorption Line Quasars and the Radio‐Loud/Radio‐Quiet Dichotomy

Abstract

ABSTRACTThe observation that the extremely broad, blueshifted absorption troughs which characterize broad absorption line quasars (BALQs) occur exclusively in radio‐quiet quasars (RQQs) suggests that this class of active galactic nuclei (AGN) may offer important clues to the radio‐loud/radio‐quiet (RL/RQ) dichotomy in quasars. Interestingly, there is also substantial observational evidence for similar, but lower velocity, intrinsic absorption outflows in some Seyfert galaxies and radio‐loud quasars (RLQs) as well. Theoretically, however, it is difficult to interpret this broad range of mass ejection phenomena in the context of the standard model for BALQs. Thus, a new model is considered here in which the thermal gas producing the blueshifted absorption troughs is associated with a poorly collimated outflow of weakly radio‐emitting plasma—in essence, a weak jet. This model provides an appropriate framework not only for assessing the possible connection between the BAL phenomenon in RQQs and related intrinsic absorption outflows in stronger radio sources and in less luminous sources, both of which are known to possess jetlike radio structure, but also for understanding the RL/RQ dichotomy in light of recent observations which indicate that at least some RQQs possess central engines that are capable of producing weak versions of the powerful radio jets characteristic of RLQs. In the context of a weak jet model for BALQs, it is shown that observational constraints on the physical properties of the radio‐emitting plasma are consistent with other theoretical arguments suggesting that the differences amongst RL and RQ sources can be attributed to jets with intrinsically different physical properties. Similarly, theoretical constraints on the physical properties of absorbing clouds embedded in weak jets are shown to be consistent with the properties directly inferred from the observed BAL troughs. Most importantly, however, it is argued that a weak jet model provides a successful explanation for the anticorrelation between the terminal velocity of the absorption outflow and the radio power of the quasar.