Inner Structure of AGN
(Hough, together with Visiting Professor Axon, and Visiting Fellows Robinson and Young)
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.
Recent 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.
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Polarization data for the BAL quasar PG1700+158:
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.
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Total flux image (color) 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. |
Previously, some results are:
- We have compiled the most extensive database of optical spectropolarimetric observations of Seyfert 1 galaxies that indicates a diversity of polarization characteristics, and variable polarization in some objects (Smith et al. 2002, MNRAS, 335, 773; Smith et al. 2004, MNRAS, 350, 140). A detailed model of Mrk 509 (Young et al. 1999, MNRAS, 303, 227; Young 2000, MNRAS, 312, 567), requires multiple polarization components including the 'Seyfert 2 scattering cone', dusty torus and a compact electron scattering region. We have also found polarimetric evidence for intrinsically anisotropic continuum emission in NGC 1068 (Young et al. 1997, ASPC, 121, 837).
- Detailed NIR imaging- and spectro-polarimetric studies of the Circinus galaxy (Ruiz et al. 2000, MNRAS, 316, 49) and Centaurus A (Alexander et al. 1999, MNRAS, 303, 17; Alexander et al. 2000, MNRAS, 313, 815) have set limits on the extinction to the active nucleus and have shown that the torus sizes derived from imaging polarimetry are consistent with those deduced by radiative transfer modelling of the SED. These results suggest that the tori in these objects are surprisingly small (<=10 pc), a conclusion that is substantiated by our HST imaging polarimetry.
- Spectropolarimetric observations of broad-line radio galaxies have resulted in the discovery of a high-speed scattering outflow in the radio-loud galaxy 4C 74.26 (Robinson et al. 1999, MNRAS, 305, 97) and have shown that the polarization properties of Arp 102B, 3C390.3 and 3C332 are consistent with a bi-polar outflow, rather than the widely accepted model in which their double-peaked H-alpha lines are emitted by a relativistically rotating disk (Corbett et al. 1997, ASPC, 121, 693; Corbett et al. 1998, MNRAS, 296, 721; Corbett et al. 2000, MNRAS, 319, 685). We have also successfully modelled the observed polarization of 3C234, within the context of the unified theory (Young et al. 1998, MNRAS, 294, 478).
- Spectroscopic monitoring and modelling shows that the broad H-alpha observed in BL Lac is consistent with this object harbouring a Seyfert-like nucleus (Corbett et al. 1996, MNRAS, 281, 737; Corbett et al. 2000, MNRAS, 311, 485).