Previous abstract Next abstract

Session 119 - QSOs and BL Lacs.
Oral session, Thursday, January 18
1st Floor, La Villita Assembly Building

[119.03] The Nature of Quasar Emission Line Regions

M. S. Brotherton (Univ. of Texas, Austin)

The spectra of active galactic nuclei (AGNs) display prominent emission lines arising from gas photoionized by a nonstellar continuum source. Forbidden lines are narrow (Doppler widths of hundreds of km/s) while semi-forbidden and permitted lines are broad (Doppler widths of thousands of km/s). Narrow lines vary over years or more, whereas broad lines vary over days and months (depending on luminosity). These considerations identified the narrow line region (NLR) as a low-density region up to hundreds of parsecs from the central engine with velocities characteristic of motion within a galactic potential, and the broad line region (BLR) as a higher density region within the central parsec with velocities characteristic of motion within the potential of a supermassive blackhole.

While it has proven difficult to determine directly the physical mechanisms of AGNs' central engines, the broad lines indicate the kinematics and physical conditions of the local environment through which AGN fuel and exhaust must pass. The narrow lines characterize interactions with the host galaxies. The emission lines also trace chemical evolution at large look-back times.

With collaborators, I undertook statistical investigations of the broad emission lines in several samples of intermediate redshift QSOs, and discovered strong relationships among the line ratios, widths, asymmetries, and redshifts of Ly\alpha, the \lambda1400 feature, C IV \lambda1549, C III] \lambda1909, and other weaker UV lines characterizing a range of excitation conditions. These relationships plus photoionization modeling with CLOUDY led to the next order approximation in describing the BLR: two quasi-independent regions with different properties, a very-broad-line region (VBLR) and an intermediate-line region (ILR). New IR spectra of H\beta and [O III] \lambda5007 in the previously investigated QSOs confirm the hypothesis that the ILR is associated with NLR. The physical implications of this new way of looking at the AGN environment are discussed.

Program listing for Thursday