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Optical Detection of the Hidden Nuclear Engine in NGC 4258
Belinda J. Wilkes 1
belinda@cfa.harvard
Gary D. Schmidt 2
gschmidt@as.arizona.edu
Paul S. Smith 2
psmith@as.arizona.edu
Smita Mathur 1
mathur@cfa.harvard.edu
Kim K. McLeod 1
kmcleod@cfa.harvard.edu
Received 4 Aug 1995; accepted 25 Sept 1995
1 Harvard�Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138
2 Steward Observatory, University of Arizona, Tucson, Arizona 85721

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ABSTRACT
The sub�parsec masing disk recently found to be orbiting a central mass
of � 3:6 \Theta 10 7 M fi in the Seyfert/LINER galaxy NGC 4258 (Miyoshi et al.
1995) provides the most compelling evidence to date for the existence of a
massive black hole in the nucleus of a galaxy. The disk is oriented nearly
edge�on and the X�ray spectrum is heavily absorbed. Therefore, in this galaxy,
the optical emission�line spectrum generally exhibited by an active galactic
nucleus is perhaps best sought using polarized light: probing for light scattered
off material surrounding the central source. New polarimetry of NGC 4258
has uncovered a compact polarized nucleus whose spectrum consists of a faint
blue continuum similar to those of unobscured quasars (F � / � \Gamma1:1 ), plus
broadened (�1000 km s \Gamma1 ) emission lines. The lines are strongly linearly
polarized (5 \Gamma 10%) at a position angle (85 o \Sigma2 o ) coincident with the plane of
the maser disk. This result provides substantiating evidence for a weakly active
central engine in NGC 4258 and for the existence of obscuring, orbiting tori
which impart many of the perceived distinctions between various types of active
galaxy.
Subject headings: polarization -- galaxies: individual: NGC 4258 -- galaxies:
active -- galaxies: nuclei

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1. Introduction
NGC 4258 (M106) is a giant SABbc galaxy (v = 472\Sigma4 km s \Gamma1 , Cecil, Wilson, &
Tully 1992) at a distance of � 7 Mpc (Tully 1988). Its morphology is complex, with inner
and outer spiral arms (Court'es et al. 1993), a dust lane and an optical/radio jet at a
position angle (� 150 o ; Cecil et al. 1992) close to that of the galaxy major axis. It has a
LINER/Seyfert 2 emission�line spectrum whose classification has been long debated. Early
reports of the presence of broad wings to Hff led to a Seyfert 1.9 classification (Stauffer
1982; Filipenko & Sargent 1985). These results have since been called into question by
St�uwe, Schulz, & H�uhnermann (1992), who argue that fitting the emission lines with
Lorentzian rather than Gaussian profiles avoids the need for a broad Hff component. The
strengths of the central (!5�) emission lines suggest the presence of a weak, central ionizing
continuum (Q� 10 52 s \Gamma1 , F � / � \Gamma1:2 ; St�uwe et al. 1992) significantly stronger than the
observed UV continuum (Ellis, Gondhalekar, & Efstathiou 1982).
The most compelling evidence for a past/present active nucleus derives from radio
measurements. High spatial resolution observations of the well�known, high�velocity H 2 O
maser emission have revealed a thin, edge�on disk (height/radius� 0:02, i = 83 o ) with
inner and outer radii of 0.13 and 0.25 pc, respectively (Miyoshi et al. 1995). The rotational
velocity is consistent with Keplerian motion and ranges from 700--1000 km s \Gamma1 across the
disk, implying a binding mass of � 1:4 \Theta 10 7 M fi . This result is highly suggestive of the
presence of a central black hole surrounded by a disk of material sufficiently thick to sustain
masing molecules. In addition, a compact radio source coincident with the nucleus has been
detected between 2 cm and 20 cm (Miyoshi et al. 1995; Hummel, Kraus, & Lesch 1989).
X�ray observations of NGC 4258 (Makishima et al. 1994) detected an unresolved (� 1 0 ),
strongly absorbed source coincident with the optical nucleus. The authors suggest that this
emission originates in a low�luminosity (� 4 \Theta 10 40 ergs s \Gamma1 ) active nucleus which is heavily

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obscured along our line of sight (equivalent NH � 10 23 cm \Gamma2 ).
The presence of a disk coupled with strong obscuration of the nuclear regions makes
NGC 4258 a prime candidate in which to search for hidden broad emission lines, visible
in polarized light alone, as are often seen in otherwise narrow emission�line objects (Miller
1994). In this paper we report the detection of a polarized blue continuum and emission
lines from the central regions of NGC 4258 which portray an active nucleus hidden from our
direct view. This galaxy provides a direct link between the obscuring torus often inferred in
an active galactic nucleus (AGN) and a disk of material orbiting a black hole of high mass.
2. Imaging and Spectropolarimetry of NGC 4258
Observations were made on several occasions between 1994 November 26 and 1995
June 20 using the CCD Spectropolarimeter described by (Schmidt, Stockman, & Smith
1992) at the 2.3 m telescope of Steward Observatory and the 1.9 m Perkins telescope of
Lowell Observatory/Ohio State Univ. Spectropolarimetry utilized a slit of 3� or 4� width
centered on the core of the optical image, and oriented either NS or EW. For the imaging
observations, a plane mirror was substituted for the grating and a large entrance aperture
provided polarization maps in broad wavelength bands over the central 50�\Theta50� region of
the galaxy with 0: 00 5 square pixels.
A polarization map made in the V photometric band (4900 \Gamma 6000 � A) is shown as
Figure 1. Here, vector length is proportional to the local surface brightness of polarized
light, while orientation indicates the electric vector position angle. Significant polarization
(p = 0:1 \Gamma 1%) due to selective extinction by aligned dust grains in the interstellar medium
of NGC 4258 exists throughout the map. The degree of polarization of the nucleus itself is
small, p � 0:25%, but because of its brightness is the dominant source of polarized flux in
the map. The PA measured in both V and R (not shown) is essentially EW -- identical to

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the 86 o PA of the maser disk on the sky (Miyoshi et al. 1995). The polarized nucleus is
highly concentrated in both bands, with no detectable NS extension above the seeing profile
(FWHM = 1: 00 5) and a possible small EW extension of !
� 3�.
Spectropolarimetric results for a 3�\Theta7� slit centered on the nucleus are depicted in
Figure 2. These are our highest�resolution observations, covering the interval ��4580 \Gamma 7110
at a resolution of 400 km s \Gamma1 FWHM. The total flux spectrum (top) shows well�known
LINER/Seyfert features: narrow (�500 km s \Gamma1 ) emission lines of [OIII] ��4959, 5007, Hff,
[NII] ��6548, 6583 and [SII] ��6716, 6731. In Stokes flux q'\ThetaF � (equivalent to polarized
flux, bottom), these lines appear both broader (�1000 km s \Gamma1 ) and in different relative
strengths. In addition, Hfi and [OI] ��6300, 6363 may be weakly present in polarized flux.
Measurements of line flux, width, and polarization are provided in Table 1. Uncertainties in
the fluxes and widths are estimated to be ! 20% except where noted by a ``:''. The PAs of
all lines are consistent, and the co�added value for all features, 85 o \Sigma2 o , matches the PA of
the plane of the maser disk. The total Hff flux measured in our 3�\Theta7� extraction aperture,
4.5\Theta10 \Gamma14 ergs cm \Gamma2 s \Gamma1 , is somewhat lower than the 7.41\Theta10 \Gamma14 ergs cm \Gamma2 s \Gamma1 found for a
4� aperture by Stauffer (1982).
The degree of polarization in the continuum rises smoothly to the blue from a value
of p = 0:18% at 7000 � A to p = 0:29% at 4800 � A. The measured Stokes flux at 5500 � A,
3:2 \Sigma 0:3 \Theta 10 \Gamma17 erg cm \Gamma2 s \Gamma1 � A \Gamma1 , is equivalent to a visual magnitude of 20.2. A
power law provides an adequate fit to the spectral dependence over our observed range,
��4150 \Gamma 8000, yielding q'\ThetaF � / � \Gamma1:1\Sigma0:2 , similar to the nonthermal spectra of many
unobscured AGN. The PA differs slightly from the emission�line value and possibly between
the two orthogonal slit positions. Both discrepancies are probably the result of contributions
by differently�polarized, off�nuclear regions of the galaxy which are contained within the
extraction apertures.

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3. A Hidden Active Nucleus seen in Scattered Light
The close agreement between the PAs of the apparent major axis of the maser disk
and the emission�line and continuum polarization suggests that all three are related to
a common geometry. Synchrotron emission is ruled out by the polarized emission lines,
while the blue continuum and high line polarization (p Hff � 9%) argues strongly against
extinction by aligned dust grains. Because the disk is oriented nearly edge�on (i = 83 o ,
Miyoshi et al. 1995), a consistent explanation is that light emitted in the nucleus is able to
escape the disk through axial holes and is subsequently scattered off particles situated above
and/or below the disk. For near right�angle scattering, the polarization of the reflected
light can be strong, with an electric vector perpendicular to the axis of the disk, or parallel
to its apparent image on the sky. The phenomenon is identical to that which permits the
detection of hidden broad emission�line regions in the nuclei of many Seyfert 2 galaxies
(Antonucci & Miller 1985; Miller 1994).
In the optically�thin limit, the surface brightness of light scattered off material of
characteristic dimension l situated at a distance r from a source of luminosity L � and
composed of particles with number density N and scattering cross�section oe � can be
written 3
B � �
N oe � lL �
16� 2 r 2
:
Since the polarimetric images appear marginally resolved, a reasonable value for l and r
might be 1� (35 pc). The Stokes flux measured at 5500 � A of 3:2 \Theta 10 \Gamma17 erg cm \Gamma2 s \Gamma1 � A \Gamma1
implies a polarized surface brightness of p \Theta B 5500 � 2 \Theta 10 \Gamma17 erg cm \Gamma2 s \Gamma1 � A \Gamma1 arcsec \Gamma2 if
we assume two identical 1� diameter scattering clouds. The intrinsic degree of polarization
3 An error in the derivation of this formula by Dutil et al. (1995) led them to underestimate
the surface brightness by a factor of 10 9 and conclude that scattering is not viable.

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of the scattered light is unknown, but must exceed 9% -- the net polarization of Hff -- and
be less than 100%. We take p = 20% to be representative. This leads to an estimate
for the luminosity of the nuclear continuum source of L 5500 � 6 \Theta 10 16 =N oe ergs s \Gamma1 � A \Gamma1 .
Scattering by electrons for example in HII regions surrounding the nucleus has the feature
that the cross�section is wavelength�independent (oe e = 6:65 \Theta 10 \Gamma25 cm 2 ), but places the
added restriction that the resulting recombination radiation not exceed what is measured.
For the assumed material, the observed (total) Hff flux of 4.5\Theta10 \Gamma14 ergs cm \Gamma2 s \Gamma1 requires
N e
!
� 30 cm \Gamma3 , leading to a nuclear luminosity of L 5500
?
� 3 \Theta 10 39 ergs s \Gamma1 � A \Gamma1 (�L 5500 �
10 10 L fi ). Material located closer to the nucleus would require a lower luminosity continuum
source, or a lower particle density, to achieve the same scattered surface brightness.
Astrophysical dust grains are found in a variety of sizes and compositions, but in general
have scattering cross�sections per unit mass far greater than that for electrons. For example,
a ``typical'' interstellar grain from the mixture of Mathis, Rumpl, & Nordsieck (1977) has
a radius of �0.05 �m and scattering cross�section in the visible of �1 \Theta 10 \Gamma12 cm 2 (White
1979). If these populate the above clouds with a typical gas�to�dust ratio of ae H =ae D � 100,
the scattered surface brightness off grains would exceed that from electrons by nearly two
orders of magnitude. The required nuclear illuminating source is commensurately fainter,
�L 5500 � 10 8 L fi . It should be noted that the blackbody temperature for grains �35 pc
from the central source is less than 20 K, well below their sublimation temperature.
Using the scattering estimates above as a guide and assuming negligible extinction
toward the scattering material, a luminosity L 5500 � 10 37 \Gamma 10 39 ergs s \Gamma1 � A \Gamma1 coupled with
the absorption�corrected 2 \Gamma 10 keV flux of 6.4\Theta10 \Gamma12 ergs cm \Gamma2 s \Gamma1 (Makishima et al.
1994) implies an intrinsic optical�to�X�ray spectral index for the nucleus of 1:0 ! ff ox ! 1:8,
spanning the normal range for AGN. The bolometric nuclear luminosity determined from
the polarized flux is �10 42\Gamma44 erg s \Gamma1 , comparable to that deduced from the emission lines
(St�uwe et al. 1992) and consistent with the luminosity expected for an AGN with central

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mass � 10 7 M fi (Wandel & Mushotzky 1986). Figure 3 compares the total and polarized
flux spectral energy distributions of NGC 4258 with that of a typical radio�quiet quasar
(Elvis et al. 1994).
According to the line ratio criteria of Veilleux & Osterbrock (1987), the polarized flux
spectrum of NGC 4258 would be classified among narrow�line AGNs, the single exception
being the relative weakness of [SII] ��6716, 6731. The line width of �1000 km s \Gamma1 is
somewhat broader than most Seyfert 2 nuclei, but might be associated with motion in a
disk whose outer regions are seen in the light of H 2 O masers.
4. Conclusions
Much has been made in recent years about the effects of orientation on the discovery
and classification of active galaxies. Antonucci (1993) stresses polarimetric evidence that
many AGN contain opaque tori and that consideration of the effects of such structures
can be taken to ``unify'' what have traditionally been considered distinct classes of objects.
NGC 4258 has provided a key element to that argument by revealing a direct link between
the orientations of an inferred obscuring torus and a maser disk which orbits a compact
object of very large mass. The fact that this has been deduced from an object which
has been termed a LINER implies not only that the unification picture can be applied to
AGN over several decades in luminosity (cf. Hines et al. 1995), but also that all AGN are
powered, in part, by accretion of material onto a central massive object.
GDS thanks the staff of Lowell Observatory for their hospitality and telescope time
during a short sabbatical leave. BJW thanks Oxford University Astrophysics Department
for their hospitality during the completion of this work. We would also like to thank the
referee, Dean Hines, for his careful comments which have added to the clarity of the paper.

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Financial support was provided by grants NSF AST 91--14087 (GDS), NASA NAG 5--1630
(PSS), NASA NAG W--3134 (KKM, BJW), and NAG W5--2201, NAG W--4490 (SM).

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Table 1. Measurements of Polarized Emission Lines and the Continuum
Line � obs Stokes Flux a FWHM p PA
( � A) (\Theta10 \Gamma15 ) (km s \Gamma1 ) (%) ( ffi )
Hfi 4871.1 0.92: 1100 ... ...
[OIII] �4959 4966.1 0.85 800 1.1 \Sigma0.4 84\Sigma 9
[OIII] �5007 5014.6 3.10 1300 3.0 \Sigma0.3 83\Sigma 3
[OI] �6300 6310.6 0.46: 310 4.6 \Sigma1.8 77\Sigma11
[OI] �6363 6374.3 0.27: 610: ... ...
[NII] �6548 6557.9 0.75: 1030: 3.4 \Sigma1.0 85\Sigma 8
Hff 6576.8 4.27 980 9.4 \Sigma0.5 88\Sigma 2
[NII] �6583 6593.3 2.31 870 4.5 \Sigma0.7 87\Sigma 5
[SII] ��6716 + 6731 6742.1 0.59 ... 1.0 \Sigma0.5 77\Sigma14
Continuum (slit EW) 5100 \Gamma 6500 0.23\Sigma0.02 83\Sigma 3
Continuum (slit NS) 5100 \Gamma 6500 0.23\Sigma0.01 79\Sigma 2
a q'\ThetaF in ergs cm \Gamma2 s \Gamma1 for a coordinate system rotated to the systemic position angle
(PA) of polarization (here 85 ffi ). Stokes flux is equivalent to polarized flux but avoids the
bias and peculiar error distribution associated with p =
p
q 2 + u 2 . Estimates for individual
lines in the Hff/[NII] complex required deblending.

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This manuscript was prepared with the AAS L A T E X macros v4.0.

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Figure Captions
Figure 1: Polarization and surface brightness map of the central 47�\Theta47� (1.6\Theta1.6 kpc) of
NGC 4258 obtained in the photometric V �band. Contours represent 20% increments in
total surface brightness, while the surface brightness and position angle of polarized light is
shown on a 1� (35 pc) grid by short line segments. Note the centrally concentrated peak
in polarized flux with an EW position angle, identical to the plane of the H 2 O maser disk.
Several contour intervals in the central brightness peak have been omitted for clarity.
Figure 2: Nuclear spectra of NGC 4258 in total flux (top) and Stokes flux (bottom; see also
Table 1). In Stokes flux, a prominent emission�line spectrum with FWHM �1000 km s \Gamma1 is
superposed on a continuum of approximate shape F � / � \Gamma1:1 .
Figure 3: The observed (open boxes), polarized and inferred nuclear continuum levels of
NGC 4258 compared with a typical radio�quiet quasar (RQQ) spectral energy distribution
and the ionizing continuum from St�uwe et al. (1992, dashed line). The optical and near
infrared data, which seems to be dominated by the galactic bulge, were obtained on the
SAO 1.22 m and Steward Observatory 1.55 m telescopes in June 1995. Other references
are: UV: Ellis, Gondhalekar, & Efstathiou (1982); X�ray: Makishima et al. (1994); radio:
Hummel et al. (1989); Turner & Ho (1994); and IR: Rieke & Lebofsky (1978); Dyck,
Becklin, & Capps (1978).

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Fig. 1.--- Polarization and surface brightness map of the central 47�\Theta47� (1.6\Theta1.6 kpc)
of NGC 4258 obtained in the photometric V �band. Contours represent 20% increments in
total surface brightness, while the surface brightness and position angle of polarized light is
shown on a 1� (35 pc) grid by short line segments. Note the centrally concentrated peak
in polarized flux with an EW position angle, identical to the plane of the H 2 O maser disk.
Several contour intervals in the central brightness peak have been omitted for clarity.

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Fig. 2.--- Nuclear spectra of NGC 4258 in total flux (top) and Stokes flux (bottom; see also
Table 1). In Stokes flux, a prominent emission�line spectrum with FWHM �1000 km s \Gamma1 is
superposed on a continuum of approximate shape F � / � \Gamma1:1 .

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Fig. 3.--- The observed (open boxes), polarized and inferred nuclear continuum levels of
NGC 4258 compared with a typical radio�quiet quasar (RQQ) spectral energy distribution
and the ionizing continuum from St�uwe et al. (1992, dashed line). The optical and near
infrared data, which seems to be dominated by the galactic bulge, were obtained on the
SAO 1.22 m and Steward Observatory 1.55 m telescopes in June 1995. Other references
are: UV: Ellis, Gondhalekar, & Efstathiou (1982); X�ray: Makishima et al. (1994); radio:
Hummel et al. (1989); Turner & Ho (1994); and IR: Rieke & Lebofsky (1978); Dyck, Becklin,
& Capps (1978).