Gemini Mid-IR Polarimetry of NGC1068: Polarized Structures Around the
  Nucleus
Gemini Mid-IR Polarimetry of NGC1068: Polarized Structures
Around the Nucleus
C. Packham1, S. Young2, S. Fisher3, K. Volk3, R. Mason3, J. H. Hough2 P. F. Roche4 M.
Elitzur5, J. Radomski6, and E. Perlman7
packham@astro.ufl.edu
ABSTRACT
We present diffraction limited, 10µm imaging polarimetry data for the cen-
tral regions of the archetypal Seyfert AGN, NGC1068. The position angle of
polarization is consistent with three dominant polarizing mechanisms. We iden-
tify three distinct regions of polarization: (a) north of the nucleus, arising from
aligned dust in the NLR, (b) south, east and west of the nucleus, consistent with
dust being channeled toward the central engine and (c) a central minimum of
polarization consistent with a compact (≤22pc) torus. These observations pro-
vide continuity between the geometrically and optically thick torus and the host
galaxy’s nuclear environments. These images represent the first published mid-
IR polarimetry from an 8-m class telescope and illustrate the potential of such
observations.
Subject headings: galaxies: nuclei — galaxies: Seyfert — galaxies: structure
galaxies: individual(NGC 1068) — infrared: galaxies — polarization: galaxies
1University of Florida, Department of Astronomy, 211 Bryant Space Science Center, P.O. Box 112055,
Gainesville, FL, 32611-2055, USA
2Center for Astrophysics Research, University of Hertfordshire, Hatfield, AL10 9AB, UK
3Gemini Observatory, Northern Operations Center, 670 N. A’ohuku Place, Hilo, Hawaii,96720, USA
4University of Oxford, Department of Astrophysics, Keble Road, Oxford, OX1 3RH, UK
5University of Kentucky, Department of Physics and Astronomy, 600 Rose Street, Lexington, KY, 40506,
6Gemini Observatory, Southern Operations Center, c/o AURA, INC., Casilla 603, La Serena, Chile
7Physics and Space Sciences Department, Florida Institute of Technology, 150 West University Boulevard,
FL, USA
http://arxiv.org/abs/0704.1250v1
– 2 –
1. Introduction
The unified theory of Seyfert (Sy) type active galactic nuclei (AGN) holds that all
types of Sy AGN are essentially the same object, viewed from different lines of sight (LOS).
Surrounding the central engine is a geometrically and optically thick, dusty, molecular torus,
obscuring the broad emission line region from some LOS. In this scheme, the Sy classification
depends solely on the LOS and exact torus properties. Such theories received a major boost
through the detection of scattered, and hence polarized, broad emission lines in the spectrum
of NGC1068 (Antonucci & Miller 1985), revealing an obscured Sy 1 central engine in the
previously classified Sy 2 AGN, entirely consistent with unified theories.
Whilst fundamental to unified theories, the torus remains difficult to image directly
at optical/IR wavelengths, with perhaps the most direct observation of the torus made
by speckle interferometry in the near-IR (Weigelt et al. 2004). Strong evidence for signifi-
cant amounts of obscuring material in the central 100pc-scale nuclear regions of NGC1068,
possibly in the form of a torus, is provided by observations of CO and HCN emission
(Planesas et al. 1991; Jackson et al. 1993; Schinnerer et al. 2000), and recent Chandra X-
ray observations (e.g. Ogle et al. 2003). Mid-IR spectroscopy reveals a moderately deep
(τ9.7 ≈ 0.4) silicate absorption feature at the nucleus (Roche et al. 1984; Siebenmorgen et al.
2004), whose strength is approximately constant up to ∼1′′ south of the brightest mid-IR
point (Mason et al. 2006; Rhee & Larkin 2006). Applying the Nenkova et al. (2002) clumpy
torus model, Mason et al. (2006) suggested the torus is compact (≤ 15pc), in good agreement
with mid-IR interferometric observations (Jaffe et al. 2004). Further evidence for a compact
torus was found through AO fed H2 1-0S(1) observations (Davies et al. 2006), finding a 15pc
clump of H2 extending from the nucleus at the same PA as the line of masers. The observed
extent of the torus, or nuclear obscuring material in NGC1068, is partly dependent on the
wavelength and/or observational technique. Young et al. (1996) used imaging polarimetry
to observe the silhouette of obscuring material against the southern ionization cone, which
they attributed to the torus, with a derived diameter of ∼200pc in the H-band.
The close proximity (1′′ ≡ 72pc) and high brightness of NGC1068 makes it an ideal target
for polarimetry, a traditionally photon-starved application. Near-IR studies of NGC1068 by
Packham et al. (1997), Lumsden et al. (1999) and Simpson et al. (2002) clearly detected the
bi-conical ionization structure in scattered light. In the nuclear regions, there is a trend to
a position angle (PA) of polarization being perpendicular to the radio jet with increasing
wavelength. Modeling of the nuclear regions requires both an extended area of scattering
particles as well as dichroic absorption of nuclear emission, possibly by dust in, or associated
with, the torus (Young et al. (1995), Watanabe et al. (2003)).
Bailey et al. (1988) found that the PA of polarization rotates by ∼70◦ between 4 and
– 3 –
5µm. The 10µm spectropolarimetry of Aitken et al. (1984) showed a similar PA of polariza-
tion to that at 5µm, and a constant degree of polarization through the silicate absorption
feature. These results are entirely consistent with the predicted 90◦ change from dichroic
absorption to dichroic emission from aligned dust grains. That the PA change was only ∼70◦
is attributable to dilution of the dichroic emission component by polarized flux in the ex-
tended scattering cones, most likely from dichroic emission from dust in the narrow emission
line region (NLR) (Bailey et al. 1988).
To investigate the contributions of the various polarizing mechanisms and structures
in the nucleus of NGC1068, Lumsden et al. (1999) performed imaging polarimetry using
a broad-band 8-13 µm filter. These data represented the first and only published mid-IR
imaging polarimetry of an AGN, but their interpretation was complicated by the ∼0.7′′
resolution of the data. To take advantage of the improved spatial resolution attainable from
an 8 m-class telescope, we obtained new mid-IR imaging polarimetry of NGC1068 during
commissioning of this mode at the Gemini North telescope.
2. Observations
We obtained imaging polarimetry of NGC1068 during commissioning of the polarimetry
unit of Michelle (Glasse et al. 1997) on UT 2005 December 19 on the Gemini North 8.1m
telescope. These observations were primarily aimed at measuring the degree and position
angle (PA) of polarization with NGC 1068 as a test object, and hence used a limited on-source
time of 148 seconds. Michelle uses a Raytheon 320 x 240 pixel Si:As IBC array, providing
a plate scale of 0.1′′ per pixel in imaging mode. Images were obtained in the 9.7µm (δλ
= 1.0µm, 50% cut-on/off) filter only, using the standard chop-nod technique to remove
time-variable sky background, telescope thermal emission and so-called “1/f” detector noise.
The chop throw was 15′′ and the telescope was nodded every ∼90 s. The chop throw was
fixed at 0◦ (N-S). Conditions were photometric and the observations were diffraction limited
(∼0.30′′FWHM).
Michelle employs a warm, rotatable half wave retarder (or half wave plate, HWP) to
modulate the polarization signal, located upstream of the entrance window of the dewar. A
cold wire grid polarizer is used as the polarimetric analyzer, located in a collimated beam.
Images were taken at four HWP PAs in the following sequence: 0◦, 45◦, 45◦, 0◦, 22.5◦, 67.5◦,
67.5◦, 22.5◦ in the first nod position, and the sequence repeated in the second nod position.
In this manner, the Stokes parameters can be computed as close in time as possible, reducing
the effects of variations in sky transmission and emission. This sequence, however, requires
many motions of the HWP, and is therefore under evaluation with a view to reducing the
– 4 –
number of HWP motions to increase observing efficiency. Data were reduced using the
Gemini IRAF package in conjunction with Starlink POLPACK software (Berry & Gledhill
2003). The difference for each chop pair in a given nod position and HWP PA was calculated,
and then differenced with the second nod position at the same HWP PA. Images were aligned
through shifting by fractional pixel values to account for slight image drift between frames,
and then the Stokes parameters I, Q and U computed using POLPACK. A total of 20 nod
positions were recorded, and residual array/electronic noise was removed through use of a
median-filter noise mask. The data were reduced through creation of four individual I, Q
and U maps and also through coadding all frames at their respective HWP PA first and
then producing a single I, Q and U map. The S/N in the latter method is slightly higher in
the individual Q and U maps, presumably due to a ’smoothing’ of the Q and U during the
co-addition; these are the data used in this paper.
The efficiency and zero angle calibration of the polarimeter were measured through
observations of two polarized sources and comparison with measurements published by
Smith et al. (2000). The instrumental polarization was estimated to be ≤0.3% through
observations of two stars that fulfilled the criteria of (a) high proper motion (hence nearby),
(b) high galactic latitude (to minimize the presence of Galactic dust) and (c) an intermediate
spectral type star (to minimize intrinsic stellar nebulosity).
3. Results
Figure 1 shows the total flux image (color-scale and contours) with the polarization
vectors overlaid. The polarization vectors are plotted where the S/N is ≥54 in total flux,
and contours are linearly spaced in intensity, starting at a S/N of 27. Figure 2 shows the
polarized intensity map, produced by multiplying the degree of polarization by the total
intensity image. As in Figure 1, only where the S/N in the total intensity image is ≥54
are data plotted. The resultant polarization vectors are contained within an approximate
N-S oriented ellipse, major/minor axes 1.7′′/1.2′′ respectively. The integrated degree of
polarization within that ellipse is 2.48±0.57% at a PA of 26.7±15.3◦. The errors in the
degree and PA of polarization are estimated through independent measurements of the four
individual polarization maps and computing the standard deviation. It should be noted that
the exposure time in those four individual maps was very short, where systematic effects could
dominate, and hence the quoted errors may be an overestimation. The degree of polarization
is higher than the 1.30% measured by Lumsden et al. (1999) in a 2′′ aperture, consistent with
an increased observed polarization as often arises with improved spatial resolution. The PA of
polarization is significantly different from Lumsden’s measurement of 49◦ in a 2′′ aperture.
– 5 –
However, our data shows a PA rotation of 94◦ from the K
band data of Packham et al.
(1997) and Lumsden et al. (1999), entirely as expected if the dominant polarizing mechanism
changes from dichroic absorption to emission between the two wavebands, as described in
§1. We speculate the Lumsden et al. (1999) ∼0.68′′ results suffered significantly greater
contamination in their beam, possibly from surrounding extended polarization, as compared
to our ∼0.30′′ results. Additionally, the wider bandwidth of Lumsden et al. (1999) would
have been more affected by the different and competing polarizing components.
The degree and PA of polarization suggests contributions from three components. The
first extends ∼1′′ north of the mid-IR peak and is coincident with the inner regions of the
radio jet, with a PA of polarization approximately N-S. The second region extends south,
east and west of the nucleus, with a PA of polarization of ∼35◦. Finally, the degree of
polarization drops to a minimum very close to the mid-IR total flux peak, which we believe
is most likely to arise from an unresolved polarization contribution with a PA of polarization
approximately orthogonal to that of the more extended emission, leading to a reduction in the
measured polarization. The polarized intensity image reveals polarized emission extending
north of the mid-IR peak, and two areas of enhanced flux east and west of the mid-IR peak,
and a minimum close to the mid-IR total flux peak. Table 1 summarizes the locations and
polarization components.
4. Discussion
Polarization at mid-IR wavelengths most likely arises from either dichroic absorption or
emission, both due to dust grains with a preferred alignment. The integrated PA of polar-
ization in these data confirms and enhances the interpretation of the near-90◦ PA flip from
near- to mid-IR wavelengths, with the factor ∼2.5 increase in spatial resolution providing a
more accurate and consistent result. Galliano et al. (2003, 2005) suggested, based on spatial
coincidence, the [OIII] clouds in the ionization cone are the dominant mid-IR sources away
from the compact torus. The polarized flux image shows a similar spatial correspondence
with the [OIII], and the PA of polarization north of the nucleus is consistent with the in-
terpretation of Bailey et al. (1988) of dichroic emission in the NLR, possibly through dust
alignment via jet streaming or a helical magnetic field associated with the jet. We discount
directly observed synchrotron radiation from the radio jet accounting for the polarization,
as an extrapolation of the radio emission to the mid-IR provides too little flux. Hence, this
data confirms the extended mid-IR polarized emission north of the nucleus is dominantly
from dust in the ionization cone.
South, east and west of the nucleus, as the PA of polarization is perpendicular to that
– 6 –
in the near-IR where the polarization is thought to be produced by dichroic absorption, we
suggest the dominant polarizing mechanism is dichroic emission by grains aligned to the same
field direction as the absorbing grains, in agreement with other authors (i.e. Bailey et al.
(1988)). Due to the Barnett effect (Lazarian (2003), and references therein), grains align with
their short axes parallel to the local magnetic field, and the PA of polarization is parallel
to the direction of the magnetic field for dichroic absorption and orthogonal for dichroic
emission. The location of the polarized emission areas and PA of polarization is suggestive
of warm aligned dust grains being channeled from the host galaxy toward the torus. Indeed,
the PA of the polarized flux is coincident with the H2 material that Davies et al. (2006)
associated with molecular material in a compact torus.
Dichroic absorption in an unresolved optically thick central region could account for
the minimum in polarization close to the peak of mid-IR flux, with a PA of polarization
approximately orthogonal to the more extended dichroic emission to the east, west and
south. Alternatively the mid-IR flux in the innermost regions could arise from a strong
mid-IR, intrinsically unpolarized, source. However, there is tentative evidence of the central
regions showing a twist in the PA of polarization, tending toward a similar PA found in the
dichroic absorption at near-IR wavelengths (e.g. Packham et al. (1997)), which supports the
dichroic absorption interpretation. In both possibilities, a compact (≤0.3′′ (≤22pc) diameter)
torus could account for this result. If correct, the polarization minimum indicates the true
position of the central engine, which is not coincident with the mid-IR total flux peak, but
displaced by ∼0.2′′ to the west.
CO (Schinnerer et al. 2000) and optical HST (Catchpole & Boksenberg 1997) obser-
vations are interpreted as evidence of a warped molecular disk on 100pc scales, partially
obscuring the nuclear regions of the host galaxy and ionization cone pointing away from
Earth. Indeed, Schinnerer et al. (2000) speculate this material, rather than a compact torus,
is responsible for obscuring the AGN. Elitzur & Shlosman (2006) suggest the 100pc molec-
ular structure is the extension of the pc scale disk of masers (Greenhill & Gwinn (1997),
Gallimore et al. (2001), Galliano et al. (2003)). We suggest that our data provide continuity
between the geometrically thick torus (height/radius ∼1) to the flatter, larger galactic disk
(height/radius ∼0.15).
The western polarized feature is considerably larger than the compact (≤ few pc) torus
suggested by several authors (e.g. Jaffe et al. 2004; Mason et al. 2006; Packham et al. 2005;
Radomski et al. 2006) on the basis of mid-IR imaging and modeling, but much smaller than
the suggested torus detected by Young et al. (1996). However, the feature is detected in
polarized flux, a technique that increases contrast by removing the dominant, unpolarized,
emission. Distinct from total flux, polarimetric observations are therefore potentially much
– 7 –
more sensitive to emission from the putative faint, outer regions of the torus where the
interaction with the inner regions of the host galaxy must occur. We suggest that a way
to reconcile the evidence for a compact torus with these observations and others, such as
extended silicate absorption (Roche et al. (2006), Roche et al. (2007)) and 100 pc-scale CO
discs, is that the compact, geometrically and optically thick torus is often surrounded by a
larger, more diffuse structure, associated with the dusty central regions of the host galaxy.
Where the torus ends and the host galaxy dust structure starts may be more of a question of
semantics rather than a true physical boundary. These observations examine the interaction
between the host galaxy and possible entrainment into the outer torus regions. Further
multiple-wavelength polarimetric observations of both NGC1068 and other AGN are required
to test this hypothesis.
We are grateful to the Gemini, UKIRT and ATC science and engineering staff for their
outstanding work on Michelle and the Gemini telescope, and wish to note especially Chris
Carter. Based on observations obtained at the Gemini Observatory, which is operated by
the Association of Universities for Research in Astronomy, Inc., under a cooperative agree-
ment with the NSF on behalf of the Gemini partnership: the National Science Foundation
(United States), the Particle Physics and Astronomy Research Council (United Kingdom),
the National Research Council (Canada), CONICYT (Chile), the Australian Research Coun-
cil (Australia), CNPq (Brazil) and CONICET (Argentina).
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This preprint was prepared with the AAS LATEX macros v5.2.
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Table 1: Polarization component summary
Locale, Aperture, λ Degree of Polarization PA of Polarization Emitting Component
Nucleus, 1.7′′x1.2′′ 2.48±0.57% 26.7±15.3◦ Several
9.7µm
North region ∼2% ∼8◦ Dichroic emission from dust
9.7µm aligned through jet interaction
East, West, South ∼3.5% ∼35◦ Dichroic emission from
regions, 9.7µm galactic dust or torus outer edge
Innermost region ≤1% - Dichroic absorption or
9.7µm unpolarized source
Nucleus, 2′′ 1.3±0.05% 49±3◦ Several, including dichroic
10µm emission
Nucleus, 2′′ 4.11±0.46% 120.6±2.38◦ Several, including dichroic
2.2µm absorption
a9.7µm data from this paper
b10µm data from Lumsden et al. (1999)
c2.2µm data from Packham et al. (1997)
– 11 –
Fig. 1.— 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.
– 12 –
Starlink GAIA::Skycat
Total Intensity
all_coadded_pi_thresh20.sdf
160.0 121.5 
packham Jan 17, 2007 at 13:07:23
Fig. 2.— Polarized flux image of the central regions of NGC1068. The “X” shows the
position of the peak total flux. Each pixel is 0.1′′.
	Introduction
	Observations
	Results
	Discussion
