THE UNITED KINGDOM INFRARED TELESCOPE
ANNUAL REPORT
1995 AND 1996

2. Scientific Results during 1995 and 1996

2.2. Selected Scientific Results

2.2.3. IRPOL's Galactic and Extragalactic Commissioning Results

C. Packham and A.C. Chrysostomou (University of Hertfordshire)

During August and September of 1995 the University of Hertfordshire successfully commissioned a new polarimetry module (IRPOL2) and new polarimetry optics for IRCAM3 and CGS4. IRPOL2 now employs 90mm diameter half-wave retarders, with one for the JHK bands, one for the L band and one for the M band. New control software has significantly increased the reliability of the system.

IRCAM3 and CGS4 now include Wollaston prisms within the cryostats, which enable the o and e rays from an image or spectrum to be recorded simultaneously. This leads to an order of magnitude improvement in polarisation accuracy, compared to the more usual single-beam infrared polarimeters, because the measured polarisation is no longer sensitive to changes in atmospheric transmission and seeing which can occur between recording of images at the different waveplate positions.

In the following, we present two exciting results obtained whilst commissioning the system.

I. NUCLEAR TORUS IN NGC1068

The presence of a geometrically and optically thick dusty nuclear torus is central to the unification of type 1 and type 2 active galaxies. In this model all active galaxies have similar nuclear morphologies and it is the orientation of the nuclear torus along our line of sight which makes them appear to be different. A view along the poles of the torus allows the nuclear continuum and broad line region (BLR) to be observed directly (type 1 active galaxy), while the torus obscures these central regions for equatorial views (type 2 active galaxy).

Polarimetry provides a way to test this theory. The obscured nuclear regions in type 2 active galaxies would be seen in polarised light, since radiation from the nucleus, escaping along the poles of the torus, can be scattered into our line of sight. This scattered radiation would produce a biconical reflection nebula.

Although CO and HCN observations show that there are large concentrations of gas in the central regions of the type 2 active galaxy NGC1068, perhaps in the form of a torus, the torus itself had not been directly observed until the observations described below were obtained at UKIRT.

Figure 3 shows a polarised flux image in the H band (1.65 m) obtained with IRCAM3. Two regions of reflection nebulosity are observed separated by a dark band. Also clearly identified is a knot of polarised light about 4.5 arcseconds NE of the nucleus (first identified at optical wavelengths by Scarrott and Miller). The NE cone is seen at optical wavelengths whereas the SW cone is not. This is most easily explained by assuming the NE cone is directed towards us and is on the near side of the host galaxy, with the SW cone directed away from us, suffering significant extinction at optical wavelengths as it is viewed through the dusty disk of the host galaxy.

  
Figure 3: Polarised flux image of the nucleus of NGC1068 in the H-band; North is up and East to the left

Since the SW cone is pointing away from us it must also lie behind the torus and scattered radiation from this part of the cone will be blocked by it. Thus the torus can be observed as an absorption band against the SW cone when viewed in polarised flux. That the dark band is simply a region devoid of scatterers can be discounted as the dark band is not observed in the K band (2.2 m) polarised flux image. This is entirely consistent with the K band nuclear polarised flux being dominated by a dichroic view through the torus of the near-infrared emission region (Young et al. 1995).

Fitting ellipses to the torus absorption in polarised flux, we have derived a position angle on the sky for the torus of 32 3 degrees at an inclination to the line of sight of 42 4 degrees, and a torus diameter of at least 200pc. The derived inclination is entirely consistent with our models for the scattered nuclear radiation and also with modelling of the infrared emission of the torus (Efstathiou et al. 1995); the latter model also is consistent with the observed lower limit for the torus diameter. The derived position angle is consistent with the axis of the emission line cones and large scale radio structure.

II. POLARISATION FROM XCN AND CO-ICE

The cold dust particles in front of the heavily obscured source W33A are known to have frozen onto them a number of molecular species. Among these are two that produce strong absorption features in the 4.5-4.8 m wavelength band, carbon monoxide and an unidentified molecule currently given the name XCN.

W33A is known to be polarised in its near infrared continuum by the foreground dust. It is of interest to measure the polarisation at longer wavelengths, including the wavelengths at which absorption features occur. Therefore, spectropolarimetry between 4.5 and 4.8 m was obtained towards W33A during the commissioning of IRPOL2. Peaks in the amount of polarisation which correspond to the absorptions of XCN and CO-ice near 4.62 m and 4.67 m were clearly seen. This is the first time that excess polarisation has been detected in these features for any object.

  
Figure 4: Excess polarisation and optical depth across the XCN and CO-ice features

In Figure 4 the excess polarisation and optical depth across the features are shown. The maximum polarisations for the XCN and CO-ice features are shifted to slightly longer wavelengths relative to their peak optical depths. This result confirms that the observed polarisation excesses are produced by dichroic absorption (as opposed to scattering) of aligned grains, providing definitive proof that XCN and CO-ice mantled grains are aligned along the line of sight to W33A.

The Davis-Greenstein (DG) alignment mechanism (Davis-Greenstein 1951) often has been invoked to explain the alignment of grains in the interstellar medium. It relies on internal energy dissipation (paramagnetic relaxation), which leads to alignment of the grain axis of greatest moment of inertia with the ambient magnetic field. However, this mechanism will only work if there is a difference between the rotational temperature and the temperature of the grain material, and it predicts better alignment for smaller rather than larger grains. Observationally the reverse has been shown to be true (Whittet 1996).

Several processes have been proposed to enhance the alignment process. Purcell (1979) first suggested that molecular hydrogen is formed at specific sites on the grain surface, with subsequent ejections into the gas providing the necessary torque to spin-up the grain suprathermally. Jones and Spitzer (1967) proposed that ferromagnetic inclusions would allow weaker magnetic fields to align grains, and explain why larger grains are able to align more efficiently.

Processes such as those proposed by Jones and Spitzer are simply enhancements of the DG mechanism, and still require the gas and grain temperatures to be different. The problem is that, due to the volatile nature of CO-ice, the environment where CO-ice forms as a mantle on the surface of grains is likely to be of high density and very low temperature. With such physical conditions, it is expected that the gas and dust grains will be in thermal equilibrium, thus rendering the DG mechanism and its derivatives ineffective. At the same time, nearly all atomic hydrogen has been converted to its molecular form in these regions and therefore, the Purcell mechanism is unlikely to be effective.

We are left with considering other mechanisms to explain our observations, such as cosmic-ray interactions or Gold-type alignment mechanisms which we are only just beginning to investigate in some depth.

We gratefully acknowledge our collaborators in this work: D. Aitken (Hertford), A. Efstathiou (Imperial College London), J. Hough (Hertford), A. Lazarian (Texas), P. Roche (Oxford), D.C.B. Whittet (Lancashire), S. Young (Hertford).

References

Davis-Greenstein 1951, ApJ, 114, 206
Efstathiou et al. 1995, MNRAS, 277, 1134
Jones & Spitzer 1967, ApJ, 147, 943
Purcell 1979, ApJ, 231, 404
Whittet, D.C.B. 1996, in Polarimetry of the Interstellar Medium, eds. W.G. Roberge & D.C.B. Whittet (San Francisco: ASP Conf Series, Vol 97), p125
Young et al. 1995, MNRAS, 272, 513