Modelling accretion disk winds in quasars

Abstract

Outflows are ubiquitous in active galactic nuclei (AGN). They can take the form of either dramatic radio jets, which extend vast distances into the inter-galactic medium, or of much smaller scale winds - whose existence can be inferred only indirectly via their influence on the observed spectra.

There is good evidence to show that winds are likely to arise from the accretion disks thought to form the central engine of all AGN; they should therefore be observable in all such systems. The clearest observational signature comes in the form of the broad, blue-shifted absorption features seen in the spectra of broad absorption line quasars (BALQSOs) and the aim of this work is to investigate how the geometry and physical parameters of disk winds affect their absorption spectra.

We first discuss the changes made to an existing Monte Carlo radiative transfer code, python, in order to extend its capabilities to include modelling of AGN. These changes include the implementation of an approximate ionization scheme which takes account of arbitrary illuminating spectral energy distributions (SEDs), and the inclusion of heating and cooling effects likely to be important in the presence of high energy photons.

Next, we describe the second stage of the project which was to gain insight into the general properties of a wind exhibiting broad absorption line (BAL) features. We did this by performing radiative transfer and ionization calculations on a simple kinematic wind representation using python. We show that BAL features can be produced for plausible AGN parameters.

Finally, we present calculations carried out on a more complex wind geometry, generated from a hydrodynamic simulation of a line driven disk wind. This calculation does not produce BAL features at all, and in fact the wind is too highly ionised to permit efficient line driving. This result is significant because it illustrates the importance of detailed radiative transfer in hydrodynamic modelling, and suggests future work.

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