Intrinsic AGN Absorption Lines

Any gaseous material along our line of sight to distant quasars or, more generally, active galactic nuclei (AGNs) will absorb light according to the amounts and range of ions present. Strong absorption lines are common in rest-frame UV spectra of AGNs due to a variety of resonant transitions, for example the HI Lyman series lines (most notably Lyα λ1216) and high-ionization doublets like CIV λλ1549,1551. The lines are called intrinsic if the absorbing gas is physically related to the AGN, e.g. if the absorber resides broadly within the radius of the AGN's surrounding " host " galaxy. Intrinsic absorption lines are thus valuable probes of the kine-matics, physical conditions and elemental abundances in the gas near AGNs. Studies of intrinsic absorbers have historically emphasized quasar broad absorption lines (BALs), which clearly identify energetic outflows from the central engines. Today we recognize a wider variety of intrinsic lines in a wider range of objects. For example, we now know that Seyfert 1 galaxies (the less luminous cousins of quasars) have intrinsic absorption. We also realize that intrinsic lines can form in a range of AGN environments — from the dynamic inner regions like the BALs, to the more quiescent outer host galaxies >10 kpc away. One complicating factor is that most AGNs also have absorption lines due to unrelated gas or galaxies far from the AGN (see quasars: absorption lines). Part of the effort, therefore, is to identify the intrinsic lines and locate their absorbing regions relative to the AGNs. Empirical line classifications are a good starting point for this work. AGN absorption lines are usually classified according to the widths of their profiles. These classes separate the clearly intrinsic broad lines from the many others of uncertain origin. The main empirical classes are 1) the BALs, 2) the narrow absorption lines (NALs), and 3) the intermediate " mini-BALs. " Broad absorption lines (Fig. 1) are blueshifted relative to the AGN emission lines, implying outflow velocities from near 0 km s −1 to as much as ∼60,000 km s −1 (∼0.2c). A representative line width is ∼10,000 km s −1 , although there is considerable diversity among BAL profiles. Velocity widths ∼ >3000 km s −1 and blueshifted velocity extrema ∼ >5000 km s −1 are usually considered minimum requirements for classification as a BAL. Some BALs have several distinct absorption troughs, while others are strictly " detached " from the …