Inner structure of AGN
(Hough)
This long-standing programme, using a combination of imaging and spectro-polarimetry at optical, near-infrared and mid-infrared wavelengths, together with detailed modelling, has made major contributions to our understanding of AGN in general and the Unified Theory in particular. Polarimetry can provide information on the geometry of structures that are unlikely to be resolved at optical wavelengths in the foreseeable future, and velocity-resolved spectropolarimetry gives the velocity of scatterers.
Highlights include:
The first observational evidence that rotating winds are launched vertically from the accretion disk of a quasar. Structures observed in polarized light across the broad H-alpha emission line of the quasar PG 1700 + 158 originate close to the accretion disk in an electron scattering wind, that has large rotational motions (~4000 km/s). The observations were made at the William Herschel Telescope.
Polarization data for the BAL quasar PG1700+158: a. position angle of polarization b. polarized flux spectrum c. percentage polarization d. total flux spectrum. Note the large change in PA across the broad H-alpha emission line, which reverses direction at the line peak. In polarized flux, the broad H-alpha emission line is red-shifted with respect to the wavelength of the peak in total. From Young et al. 2007, Nature, 450, 74.
10 micron imaging polarimetry of the archetypal Seyfert active galactic nucleus NGC 1068 has provided continuity in the structures between the geometrically and optically thick torus and the host galaxy's nuclear environments (Packham et al. 2007, ApJ, 661, 29). Observations taken with Michelle on Gemini-N show that dust is being channelled toward the central engine with a compact (≤22 pc) torus.
Total flux image (colour) with the polarization vectors for the central regions of NGC1068. The length of the vector is proportional to the degree of polarization, and the angle shows the PA of polarization. Each pixel is 0.1'', and the 10% polarization scale bar is shown in the upper right. North is up, and east is to the left.