S. Young

A spectropolarimetric atlas of Seyfert 1 galaxies

We present optical spectropolarimetry of the nuclei of 36 Seyfert 1 galaxies, obtained with the William Herschel and the Anglo-Australian Telescopes from 1996 to 1999. In 20 of these, the optical emission from the active nucleus is intrinsically polarized. We have measured a significant level of polarization in a further seven objects but these may be heavily contaminated by Galactic interstellar polarization. The intrinsically polarized Seyfert 1 galaxies exhibit a variety of characteristics, with the average polarization ranging from <0.5 to 5 per cent and with many showing variations in both the degree and position angle of polarization across the broad Hα emission line. We identify a small group of Seyfert 1 galaxies that exhibit polarization properties similar to those of Seyfert 2 galaxies in which polarized broad lines have been discovered. These objects represent direct observational evidence that a Seyfert 2-like far-field polar scattering region is also present in Seyfert 1 galaxies. Several other objects have features that can be explained in terms of equatorial scattering of line emission from a rotating disc. We propose that much of the diversity in the polarization properties of Seyfert galaxies can be understood in terms of a model involving both equatorial and polar scattering, the relative importance of the two geometries as sources of polarized light being determined principally by the inclination of the system axis to the line of sight.

The X-Ray, optical, and infrared counterpart to GRB 980703

We report on X-ray, optical, and infrared follow-up observations of GRB 980703. We detect a previously unknown X-ray source in the GRB error box; assuming a power-law decline, we find for its decay index α 1.3 × 1017 Hz. For this epoch we obtain an extinction of AV = 1.50 ± 0.11. From the X-ray data we estimate the optical extinction to be AV = 20.2+12.3-7.3, inconsistent with the former value. Our optical spectra confirm the redshift of z = 0.966. We compare the afterglow of GRB 980703 with that of GRB 970508 and find that the fraction of the energy in the magnetic field, B < 6 × 10-5, is much lower in the case of GRB 980703, as a consequence of the high frequency of the cooling break.

Equatorial scattering and the structure of the broad‐line region in Seyfert nuclei: evidence for a rotating disc

Original article can be found at: http://www3.interscience.wiley.com/ Copyright Royal Astronomical Society. DOI: 10.1111/j.1365-2966.2005.08895.x

An Edge-brightened Bicone in the Nuclear Regions of Cygnus A

Infrared-imaging observations that span the wavelength range of 0.8-2.35 μm have been obtained for the archetypal powerful radio galaxy Cygnus A using the Near-Infrared Camera and Multiobject Spectrograph on board the Hubble Space Telescope. At 2.25 μm, the images are dominated by the presence of a nuclear point source (FWHM < 021), whose flux is a factor of ~4 times less than the limits deduced from previous ground-based studies. The observations also reveal an edge-brightened biconical structure centered on the point source, which is strikingly similar to those observed around young stellar objects. The high polarization and orientation of the bicone relative to the radio axis lead us to conclude that it is an illuminated structure, while the edge brightening provides evidence that the bicone is defined as much by outflows in the nuclear regions as by the polar diagram of the illuminating quasar radiation field. A further implication of our observations is that not all of the anisotropy in the nuclear radiation field is caused by extinction on a scale less than 100 pc in the torus; some of the anisotropy must be generated by absorption and scattering in the dust lane on a 1 kpc scale.

Gemini Mid-IR Polarimetry of NGC 1068: Polarized Structures around the Nucleus

We present diffraction-limited, 10 μm imaging polarimetry data for the central regions of the archetypal Seyfert active galactic nucleus NGC 1068. The position angle of polarization is consistent with three dominant polarizing mechanisms. We identify three distinct regions of polarization: (1) north of the nucleus, arising from aligned dust in the narrow emission line region, (2) south, east, and west of the nucleus, consistent with dust being channeled toward the central engine, and (3) a central minimum of polarization consistent with a compact (≤22 pc) torus. These observations provide continuity between the geometrically and optically thick torus and the host galaxys nuclear environments. These images represent the first published mid-IR polarimetry from an 8 m-class telescope and illustrate the potential of such observations.

Highly polarized structures in the near-nuclear regions of Cygnus A: intrinsic anisotropy within the cones

We present near-infrared imaging polarimetry observations of the nucleus of Cygnus A (z= 0.0567), taken with the NICMOS camera of the Hubble Space Telescope (HST) at a wavelength of 2.0 μm. These maps reveal a highly collimated region of polarized emission straddling the nucleus and extending to a radius of 1.2 arcsec. Remarkably, this feature coincides with one, but only one, limb of the edge-brightened bicone structure seen in the total intensity image. The high degree (Pk∼25 per cent) and orientation of the extended polarization feature are consistent with a scattering origin. Most plausibly, the detection of polarization along only one limb of the bicone is a consequence of intrinsic anisotropy of the near-infrared continuum within the radiation cones, with the direction of maximum intensity of the near-infrared radiation field significantly displaced from the direction of the radio axis. The unresolved nuclear core source is also highly polarized (Pk>28 per cent), with a position angle close to perpendicular to the radio axis. Given that this high degree of nuclear polarization can only be explained in terms of dichroic extinction if the dichroic mechanism is unusually efficient in Cygnus A, it is more likely that the nuclear polarization is caused by the scattering of nuclear light in an unresolved scattering region close to the active galactic nucleus. In this case, the flux of the core source in the K band is dominated by scattered rather than transmitted quasar light, and previous extinction estimates based on K-band photometry of the core substantially underestimate the true nuclear extinction.

Hubble Space Telescope Infrared Imaging Polarimetry of Centaurus A: Implications for the Unified Scheme and the Existence of a Misdirected BL Lacertae Nucleus*

We report results from the Hubble Space Telescope (HST)/near-infrared camera and multiobject spectrometer (NICMOS) 2 μm imaging polarimetry of the central region of Centaurus A. In the vicinity of the nucleus we observe a complex polarization structure, which we explain by a combination of scattering of nuclear light and dichroic polarization associated with the dust lane. The scattered nuclear radiation is found in an angular region that extends over 70°, and thus it does not originate from a highly collimated beam but is associated with more omnidirectional nuclear illumination. These observations also show the presence of an unresolved, highly polarized (P = 11.1%) nuclear source whose polarization angle θ = 1482 is perpendicular to the jet axis. We set an upper limit of 004 (~0.8 pc) to its extent. The observed nuclear polarization is naturally accounted for if we are observing scattered light from an otherwise obscured nucleus provided that both the scattering region and the occulting torus are extremely compact, with an outer radius of less than ~1 pc. Alternatively, we might be directly seeing the infrared counterpart of the radio core, similar to those found in other low-luminosity radio galaxies observed with HST. We discuss these results in the framework of the FR I/BL Lac unifying model.

A near‐infrared polarized bipolar cone in the Circinus galaxy

We present near–infrared broad–band polarization images of the nuclear regions of the Circinus galaxy in the J, H and K bands. For the first time the south–eastern reflection cone is detected in polarized light, which is obscured at optical wavelengths behind the galactic disk. This biconical structure is clearly observed in J and H band polarized flux whilst in the K band a more compact structure is detected. Total flux J–K and H–K colour maps reveal a complex colour gradient toward the south–east direction (where the Circinus galactic disk is nearer to us). We find enhanced extinction in an arc shaped structure, at about 200pc from the nucleus, probably part of the starformation ring. We model the polarized flux images with the scattering and torus model of Young et al. , with the same basic input parameters as used by Alexander et al. in the spectropolarimetry modelling of Circinus. The best fit to the polarized flux is achieved with a torus radius of �16pc, and a visual extinction AV , through the torus, to the near–infrared emission regions of >66 mags.

HST infrared imaging polarimetry of Centaurus A: implications for the unified scheme and the existence of a mis-directed BL Lac nucleus

We report results from the Hubble Space Telescope (HST)/near-infrared camera and multiobject spectrometer (NICMOS) 2 μm imaging polarimetry of the central region of Centaurus A. In the vicinity of the nucleus we observe a complex polarization structure, which we explain by a combination of scattering of nuclear light and dichroic polarization associated with the dust lane. The scattered nuclear radiation is found in an angular region that extends over 70°, and thus it does not originate from a highly collimated beam but is associated with more omnidirectional nuclear illumination. These observations also show the presence of an unresolved, highly polarized (P = 11.1%) nuclear source whose polarization angle θ = 1482 is perpendicular to the jet axis. We set an upper limit of 004 (~0.8 pc) to its extent. The observed nuclear polarization is naturally accounted for if we are observing scattered light from an otherwise obscured nucleus provided that both the scattering region and the occulting torus are extremely compact, with an outer radius of less than ~1 pc. Alternatively, we might be directly seeing the infrared counterpart of the radio core, similar to those found in other low-luminosity radio galaxies observed with HST. We discuss these results in the framework of the FR I/BL Lac unifying model.

Estimations of the magnetic field strength in the torus of IC 5063 using near-infrared polarimetry

An optically and geometrically thick torus obscures the central engine of Active Galactic Nuclei (AGN) from some lines of sight. From a magnetohydrodynamical framework, the torus can be considered to be a particular region of clouds surrounding the central engine where the clouds are dusty and optically thick. In this framework, the magnetic eld plays an important role in the creation, morphology and evolution of the torus. If the dust grains within the clouds are assumed to be aligned by paramagnetic alignment, then the ratio of the intrinsic polarisation and visual extinction, P(%)/Av, is a function of the magnetic