INTERPRETING DOUBLE-PEAKED EMISSION LINE PROFILE VARIABILITY IN AGNS USING MODELS FOR ACCRETION DISK STRUCTURES

Open Access
Author:
Flohic, Helene Marie
Graduate Program:
Astronomy and Astrophysics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 22, 2008
Committee Members:
  • Michael Eracleous, Committee Chair
  • Stephane Coutu, Committee Member
  • Mercedes Richards, Committee Member
  • Steinn Sigurdsson, Committee Member
  • Richard Alan Wade, Committee Member
  • Donald P Schneider, Committee Member
Keywords:
  • AGNs
  • accretion disk
Abstract:
A subset of Active Galactic Nuclei (AGNs) have broad, double-peaked Balmer emission lines, regarded as the best kinematic evidence for the production of broad lines by the accretion disk. It has been suggested that double-peaked emitters have a weak or non-existent disk-wind, hence offering a direct view of the accretion disk. The variability of the double-peaked line profile, which is uncorrelated with variations of the continuum flux, then traces changes in the structure of the accretion disk. In this thesis, we use the double-peaked line profile variability to probe the accretion disk structure, and we demonstrate that the double-peaked emitters are connected to the whole population of AGNs, allowing us to generalize those results. A set of 30 double-peaked emitters has been monitored for nearly a decade in order to observe long-term profile variability. We present the line profile variability for 10 of these objects and find only short-lived perturbations to the line profile. We find a potential anti-correlation between the amplitude of the line profile variability and the mass of the central supermassive black hole. This anti-correlation could be the result of an observational bias since the most massive objects were not monitored for a complete dynamical timescale. We suggest strategies to confirm the presence of this anti-correlation. The short-timescale variability of the double-peaked line profiles cannot be explained by global perturbations in the accretion disk, which can only explain the long-term variability of some objects. We develop a stochastically perturbed accretion disk model and compare the simulated line profile variations with the variability observed for the two best monitored AGNs: Arp 102B and 3C 390.3. We are able to constrain some properties of the perturbations. For Arp 102B in particular, the perturbations must be located primarily in the outer part of the line-emitting region and the transition radius for the perturbations is consistent with the radius of marginal self-gravity of the accretion disk. This supports the hypothesis of clump production in the accretion disk due to self-gravity. It has been suggested that the double-peaked line profile can be significantly altered by radiative transfer effects through a disk-wind, producing a single-peaked emission line. We confirm this theory and explore the range of parameters that best reproduce the observed single-peaked line profiles. The resulting single-peaked line profiles are primarily sensitive to the velocity field and optical depth of the disk wind and to the size of the line-emitting region of the accretion disk. For most single-peaked lines to be as cuspy as observed, a large outer radius of the line-emitting region is required, much larger than what is obtained from fits to double-peaked line profiles. This suggests that the optically thick wind can scatter more ionizing photons towards the disk, increasing the size of the line emitting region of single-peaked emitters. Finally, we add perturbations in the disk-wind and the accretion disk to test whether the presence of the wind changes the character of the line profile variability. We find that a rotating filament in the disk-wind can produce a variability pattern uncharacteristic of a rotating perturbation, that is often observed in double-peaked emitters.