PASJ: Publ. Astron. Soc. Japan 52, 421-428 and Plate 40 (2000)
The Nuclear Region of the Seyfert 2 Galaxy NGC 3079
Satoko SAWADA-SATOH, Makoto INOUE, Katsunori M. SHIBATA, Seiji KAMENO, and Victor MIGENES*
National Astronomical Observatory, 2-21-1 Osama, Mitaka, Tokyo 181-8588
E-mail (SS): [email protected]
Naomasa NAKAI
Nobeyama Radio Observatory, Minamimaki, Minamisaku, Nagano 384-1305
and
Philip J. DIAMOND*
National Radio Astronomy Observatory, P.O. Box 0, Socorro, NM 87801, U.S.A.
(Received 1998 November 13; accepted 2000 February 3)
Abstract
We present a discussion on high-resolution studies of the nuclear region of NGC 3079 using observations
of an H2O maser and continuum emission as well as HI absorption with a global VLBI network including the
VLBA, the phased VLA and the Effelsberg 100 m telescope. Multi-frequency observations of the continuum
emission reveal the spectra of continuum components and suggest a core-jet morphology. The H2O maser
spots lie in a series of clusters distributed along a north-south direction, while we rule out any possibility
that the masering disk lies along the north-south direction. HI absorption features towards the continuum
components have a velocity gradient opposite to that observed in the galaxy as a whole. We identify the
nucleus and rotation axis of the central 10 pc region, and discuss the properties of a postulated rotating
torus in the nuclear region.
Key words: galaxies: individual (NGC 3079) — galaxies: nuclei — masers
0.6 km s 1 yr 1 in a highly variable maser feature around
1
VLSR — 1190 km s" (N. Nakai et al., private commuNGC 3079 is an edge-on disk galaxy with optical nication) has been detected. Baan and Haschick (1996)
and radio nuclear activity, and is classified as a LINER claim that some features around VLSR — 940, 950, 1010,
(Heckman 1980) or a Type-2 Seyfert (Ford et al. 1986). and 1030 km s" 1 may possibly have a small velocity drift
From the central region, bipolar jets or lobes of radio of 0.4 km s" 1 yr" 1 . VLBI observations by Trotter et al.
continuum (de Bruyn 1977; Seaquist et al. 1978; Duric (1998, hereafter T98) revealed that H2O maser emission
et al. 1983; Duric, Seaquist 1988), Ha (Ford et al. 1986; arose in compact clumps and was distributed over ~ 2 pc
Veilleux et al. 1994), and X-ray emission (Fabbiano et al. along the major axis of the galactic disk. The maser fea1992) emanate perpendicular to the galactic disk. The ture at 1190 km s" 1 is not associated with the clumps of
nucleus of the galaxy has been studied in some detail at emission, but lies 1 pc south of the cluster of the main
various wavelengths. Very luminous H2O maser emis- maser features. The continuum emission at 22 GHz was
sion is seen in the nucleus, the peak of the spectrum dominated by a compact source which is located 0.5 pc to
being blue-shifted by more than 100 km s" 1 from the the west of the brightest maser feature. There is no posystemic velocity of the galaxy (Vsys) (Henkel et al. sitional coincidence between H2O maser and continuum
1984; Haschick, Baan 1985). Monitoring observations emission.
of the H2O maser have been made periodically since its
From VLBI continuum observations, Irwin and
discovery. Nakai et al. (1995) have shown some time- Seaquist (1988, hereafter IS88) found three aligned convariability of the maser flux density, but no clear ve- tinuum components at 5 GHz in the nuclear region of
locity change for the main features covering the veloc- NGC 3079. Two were strong and a fainter component
ity range of 941-975 km s" 1 in VLSR with less than existed between them. IS88 interpreted these data as a
1.6 km s" 1 yr" 1 . Recently, a large velocity drift of 3.7 ± core-jet structure along the northwest-southeast direc* Present address: Departamento de Astronomia, Universidad tion. T98 detected another component at 5 GHz, which is
located ~ 4 pc southeast of the components of IS88 along
de Guanajuato, Guanajuato, CP 36000, Mexico.
f Present address: MERLIN/VLBI National Facility, Jodrell a line joining the three previously detected features. HI
Bank Observatory, Macclesfield, Cheshire, SK11 9DL, U.K.
and OH absorptions are also detected in the galaxy, with
1.
Introduction
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422
S. Sawada-Satoh et al.
several velocity components around Vsys (Haschick, Baan
1985; Irwin, Seaquist 1991). Interferometric observations
have resolved the HI absorption towards the nucleus, and
suggest an outflow or rapid rotation of gas in the nuclear
region (Gallimore et al. 1994; Baan, Irwin 1995; Pedlar
et al. 1996). Thus, the nuclear region of NGC 3079 is
a complex object and its structure and kinematics can
be investigated using H2O maser emission, continuum
structures, and absorption features.
The velocity information provided by spectral lines
supplies a detailed picture of the kinematical structure
of the nuclear region. Parsec-scale regions in the nuclei of several galaxies have recently been studied using
VLBI observations of the H2O maser. Such observations
of NGC 4258 have revealed an edge-on thin molecular
disk with an inner radius of 0.13 pc and an outer radius
of 0.26 pc, showing Keplerian rotation around a massive black hole (Miyoshi et al. 1995). The H2O maser
emission in NGC 4258 consists of three groups: red- and
blue-shifted high-velocity features and systemic features
(Nakai et al. 1993). Monitoring programs of NGC 4258
showed that the systemic feature has a distinct velocity
drift, while no velocity drift is seen in the high-velocity
features (Haschick et al. 1994; Greenhill et al. 1995; Nakai
et al. 1995). The velocity drift of the systemic maser is
interpreted as centripetal acceleration due to a circularly
rotating disk (Miyoshi et al. 1995). H2O maser features
in the nucleus of NGC 1068 were fitted to an edge-on subKeplerian thin disk with an inner radius of 0.65 pc and
an outer radius of 1.1 pc (Greenhill 1998). In the case of
NGC 4945, H2O masers are distributed roughly linearly,
which suggests an edge-on disk within a radius of ~ 0.3 pc
(Greenhill et al. 1997). A radio galaxy NGC 1052, however, shows that the masers in that galaxy are unlikely
to be in orbit around a massive nuclear object (Claussen
et al. 1998). Another important result of these observations is that the disks in the nuclear regions do not
always have the same rotation axis as the parent galaxy
(e.g., Inoue 1998).
Continuum imaging of NGC 4258 showed an extended
structure offset from the center of the rotating disk along
the axis of rotation (Herrnstein et al. 1997). This is the
first example of the nucleus of a galaxy that shows the
relative position between the dynamical center and continuum features, since the strong maser emission makes it
possible to apply phase referencing technique to produce
images of the weak continuum. In conventional continuum VLBI images, a core-jet structure is often identified
by determing the spectra of individual components, the
core having a flat spectrum and the jet being steep (e.g.,
Pearson, Readhead 1981; Muxlow, Garrington 1991).
When a core-jet structure has been observed in the nuclear region of galaxies, the core is thought to be near to
the center of the nucleus.
We have conducted multi-frequency monitoring of
[Vol. 52,
NGC 3079 with VLBI in order to investigate its nuclear
system. In this paper, we report on high-resolution imaging from ourfirst-epochobservations of the nuclear region
of NGC 3079 using H2O maser and continuum emission
as well as HI absorption.
We adopt Vsys - 1116 km s" 1 (Irwin, Seaquist 1991)
and a distance of D = 15 Mpc for HQ = 75 km s" 1 Mpc" 1
to NGC 3079; hence, 1 mas corresponds to 0.073 pc in
the galaxy.
2.
Observations
VLBI observations towards the nucleus of NGC 3079
were carried out for 15 hours on 1996 October 20, at 1.4,
8.4, 15, and 22 GHz with a bandwidth of 8 MHz and
4 IF-channels at each frequency. The global VLBI network consisted of the Very Long Baseline Array (VLBA),
the phased VLA (Y) and the Effelsberg (EB) 100-m telescope. EB was used at 8.4, 15, and 22 GHz and the
VLA was used at 1.4 and 22 GHz. The VLBA was used
at all frequencies, except that at 1.4 GHz only antennas
at Brewster (BR), Fort Davis (FD), Hancock (HN), Kitt
Peak (KP), Los Alamos (LA), N. Liberty (NL), Owens
Valley (OV), Pie Town (PT) and the phased VLA were
used in order to detect HI absorption features. For a delay calibration, 4C 39.25 and 0917+624 were observed,
and 3C 345 was used as a flux and bandpass calibrator. The data were processed on the VLBA correlator at
NRAO; and after the correlation, data reduction including calibration and imaging proceeded using the Astronomical Image Processing System (AIPS) package.
At 1.4 GHz, self-calibration was performed using the
averaged continuum channels containing no absorption
line after correcting for the residual delay and rate. We
applied the result of a self-calibration in producing the
absorption images. Data reduction at 8.4 and 15 GHz
followed standard continuum VLBI imaging techniques
(e.g., Diamond 1995). At 22 GHz, a phase-referencing
technique was applied using the strongest maser feature
as a reference. Continuum channels were produced by
averaging line-free channels at a velocity range of 590925 km s" 1 after residual delay, rate, phase, and amplitude corrections. Because the observed velocity range
was 590-1008 km s 1 , redshifted maser features at more
than 1008 km s" 1 were not observed. The beam sizes and
the image noise for each frequency are given in table 1.
3.
Results
Figure 1 shows a radio-continuum image for each frequency. Our VLBI observations reveal several continuum components at 1.4 and 8.4 GHz, while only a single
Gaussian component with deconvolved size of less than
0.45 mas was detected at 15 and 22 GHz. Although we
applied a (u,v) Gaussian taper to the visibility data at 15
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Table 1. Beam dimension and rms noise of global VLB A images.
Antennas
[°]
Rms of images
[mJy beam" 1 ]
8
173
178
162
162
0.43
0.23
0.30
0.54
6.63
BR, FD, HN, KP, LA, NL, OV, PT, Y
All of VLBA, EB
All of VLBA, EB
All of VLBA, Y, EB
All of VLBA, Y, EB
Frequency
[GHz]
Major
[mas
Minor
[mas]
P.A.
14
84
15
22*
22f
14.7
1.3
0.66
0.41
0.41
12.8
0.91
0.57
0.30
0.30
'Continuum.
Spectral line.
f
1.4GHz
-40U
30
8.4GHz
20
10
0
-10
-20
MilliARC SEC
15GHz
-30
15
10
5
MilliARC SEC
22GHz
2
0 - 2
MilliARC SEC
- 4 - 6 - 8
MilliARC SEC
Fig. 1. Continuum images at 1.4, 8.4, 15, and 22 GHz of the nucleus of NGC 3079. Labels A, B, and C are the same
nomenclature of 5 GHz VLBI map of IS88. The HPBW is shown at the lower right, (a) CLEAN image at 1.4 GHz. The
contour levels are - 3 , 3, 4, 5, 6, 7 a. (b) CLEAN image at 8.4 GHz. The contour levels are - 3 , 3, 4, 5, 6, 8, 12, 17, 22,
27, 32, 37 u. (c) CLEAN image at 15 GHz. The contour levels are - 3 , 3, 13, 23, 33, 43, 53, 63 a. (d) CLEAN image at
22 GHz. The contour levels are - 5 , 5, 10, 15, 20, 25, 30, 35 a.
and 22 GHz, and obtained images had broadened synthe- our continuum image revealed three areas of emission:
sized beam sizes of 3 x 3 mas, and neither image showed one compact component (< 0.062 pc) in the northwest,
any clear evidence of other components. At 8.4 GHz, one compact component with an extended structure in
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[Vol. 52,
S. Sawada-Satoh et al.
424
Table 2.
Frequency
[GHz]
(1)
Component
1.4
Parameters of continuum components.
(2)
Si,
[mJy]
(3)
Major
[mas]
(4)
Minor
[mas]
(5)
B
A+C
5.7+1.7
5.9+1.7
25.8+5.7
20.6+4.2
14.1+3.8
20.0+4.1
B
A
C
14.2+0.7
1.9+0.6
2.1+0.9
1.52+0.05
1.4+0.3
2.6+0.9
15
B
24.4+0.6
22
B
12.1+0.7
8.4
P. A.
[°]
(6)
R
[mas]
(7)
9
[°]
(8)
3+21
46+90
25.6+5.3
134+12
1.50+0.05
1.3+0.3
1.3+0.5
58+90
33+80
159+18
25.5+0.2
15.6+0.8
127+1
125+3
0.71+0.01
0.63+0.01
104.8+4.8
0.47+0.01
0.33+0.01
93.1+2.9
Note. Col.(l) measured frequency; Col.(2) numerical name of continuum component; Col. (3) flux density of each component;
Cols. (4)-(6) parameters of Gaussian model—major and minor axes of each component (FWHM) and the position angle of
the major axis, respectively; Col. (7) spatial separation of each component from the component B; Col. (8) position angle of
each component with respect to the component B.
the southeast and a weaker extended component with
a substructure between the two main features. Because
the alignment of the continuum structure and the spatial separations among these components are consistent
with the 5 GHz VLBI results of IS88, we hereafter use
the same nomenclature for these components as used in
IS88: component A being the southeast component, B
the northwest, and C the fainter feature between A and
B. Component C shows three peaks within its extended
structure; the most western peak is the strongest. The
spatial separation between components A and B is estimated to be 25.7±0.2 mas (1.9 pc), which is 3.0 mas
(0.22 pc) larger than the result obtained by T98. At
1.4 GHz, only two components are seen, because the resolution is too low to resolve components A and C. We did
not detect component D seen by T98 at any frequency,
although the noise in our images is only slightly worse
than that obtained by T98. The Gaussian-fitted parameters of the continuum components or each frequency are
listed in table 2.
Figure 2 shows the continuous spectra of component
B and the combined component, A+C, at all frequencies
between 1.4 and 22 GHz, as determined from our observations. The flux densities of component A+C at 8.4, 15,
and 22 GHz were estimated by convolution with a beam
size of 14.7 mas x 12.8 mas, as used at 1.4 GHz. The two
components detected at 1.4 GHz have almost the same
flux density, while the combined intensity of components
A and C is fainter than that of component B at 8.4 GHz.
As can be seen in figure 2, the single component detected
at 15 and 22 GHz is easily identified as component B.
The combined spectrum of components A and C is steep
between frequencies at 1.4 and 15 GHz, indicating a spectral index of a = -0.36 ± 0.35 (5 oc va) between 1.4 and
8.4 GHz and a < -0.47 between 1.4 and 15 GHz. For
component B, wefinda spectral index of a = +0.51±0.17
between 1.4 and 8.4 GHz, a = 0.83 ± 0.21 between 8.4
and 15 GHz, and a = -1.8 + 0.2 between 15 and 22 GHz.
A cross-power spectrum for the H2O maser emission
from the nucleus of NGC 3079 is shown in figure 3a
(Plate 40). The velocity range of the detected H2O maser
emission is 926-1008 km s" 1 in VLSR- The relative positions of the H2O masers and the continuum component B are shown in figure 3b (Plate 40). Mainly, the
maser spots are divided into two clusters. The main cluster contains maser spots with a velocity range of 9261008 km s" 1 , which are distributed in a compact cluster within a radius of 0.05 pc. This cluster includes the
strongest maser at 956 km s"1, which is located 6.7 mas
(0.42 pc) west of the continuum component B. A second cluster is located 7 mas (0.5 pc) north of the main
cluster. The velocity of all masers within this northern
cluster is > 997 km s~1. The spatial separation between
the maser clusters and the continuum component which
we observed agrees well with that of the previous observation in 1995 by T98. The relative positions of component B from the strongest maser feature at 956 km s" 1
are listed in table 3.
We show the HI absorption spectrum and the absorption images infigure4. We fitted a Gaussian profile to the
spectrum and identified three Hi absorption features at
peak velocities of 1015+4, 1127+6, and 1230+30 km s" 1
in VLSR- A different velocity structure from that of galactic rotation is seen in the nuclear region. In table 4,
we give the velocity integrated absorption (J rdv) and
the column density of HI for each of the absorption features, assuming a spin temperature of 100 K. The hard
X-ray observation towards the nucleus in NGC 3079 with
ASCA implies a HI column density of < 1022 atoms cm"2
(Fukazawa, private communication), which agrees well
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Seyfert 2 NGC 3079
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Table 3.
Relative position of the strongest
maser from the continuum component B.
Component A+C '
Component B
Epoch
(3)
6.7±0.1
81
6.67±0.05
81.7±0.3
(1)
'10
1995 January (T98)
1996 October
e
R
[mas]
(2)
,*
[°]
X
Note. Col. (1) epoch of observation ; Col. (2) spatial separation of the strongest maser at velocity of 956 km s" 1 from the
component B ; Col. (3) position angle of the strongest maser
at velocity of 956 km s" 1 with respect to the component B.
11
1
10
Frequency [GHz]
Fig. 2. Comparison of the continuous spectrum for each
component in the nuclear region of NGC 3079. The
flux densities of component A+C were obtained by
convolution with a beam size of 14.7 mas x 12.8 mas,
as used at 1.4 GHz. The error bars represent ±1<T.
At 1.4 GHz, we could not resolve component C from
A. Considering spectral continuity between 1.4 GHz
and 8.4 GHz, the single component seen at 15 GHz
and 22 GHz can be identified as the component B
in 1.4 GHz and 8.4 GHz.
Table 4. Column density of HI absorption.
Peak velocity
[km s-1]
[km s"1]
J rdv
Column density*
[1021 atoms cm"2
1015 ± 4
1127 ± 6
1230 ± 30
59 ± 10
107 ± 21
43 ± 22
10.6 ± 1.8
19.3 ± 3.8
7.7 ± 4.0
*A spin temperature of 100 K is assumed.
If the maser gas is associated with the rotation disk, it
should be gravitationally bound to the central mass. Our
VLBI monitoring observations at 22 GHz have shown
with our results.
that the position of A relative to the maser feature moved
with an apparent subluminal velocity, while the position
4. Discussion
of B did not show such a rapid change. We will report on
In the case of NGC 4258, the dynamical center is eas- our monitoring programs in a subsequent paper. Thus,
ily defined, since a rotating disk is identified by both this suggests that B and A are the nucleus and the jet,
Keplerian rotation of high-velocity masers and the ve- respectively.
This is correct, it is easy to explain the significant diflocity shift of the systemic masers (Miyoshi et al. 1995).
In the case of NGC 3079, the location of the center is ferences in the spectra of component A+C and compoless obvious from the observations of masers. Hence, we nent B in figure 2. A nuclear system with the convex
now discuss the location of the nucleus while including spectrum exhibited by B and the steep-spectrum associated with A+C is similar to that seen in well-known corethe results of the continuum and the HI observations.
Here, we consider the location of the nucleus by means jet morphology (e.g., Pearson, Readhead 1981; Muxlow,
of the relative motion among the continuum components Garrington 1991). The non-detection of A+C at 15 and
and the maser gas. We find a significant change in the 22 GHz is ascribed to its steep-spectrum seen between
spatial separation between A and B at 8.4 GHz from flux measurements at 1.4 and 8.4 GHz. In addition, B
22.7±0.3 mas in 1992 (T98) to 25.5±0.2 mas in 1996 (our is compact (< 0.4 mas ~ 0.03 pc) at 8.4 and 22 GHz,
work), which corresponds to a subluminal motion with while the image at 8.4 GHz of both A and C shows exan apparent velocity of 0.16±0.02 c. T98 showed that tended structures. The compactness of B and the exthe proper motion of the two components at 5 GHz from tended structures A+C also support their interpretation
1986 to 1992 was 0.06 c, and the spatial separation at as a core-jet structure.
The shape of the spectrum of A(+C) is different from
22 GHz in 1995 was 24.1±0.3 mas. The spatial separation
is plotted in figure 5. The separation at 22 GHz fits well that determined in T98, which showed a convex specwith the motion at 8.4 GHz. However, we could not trum with a peak frequency of between 8.4 and 22 GHz.
detect any significant change in the spatial separation The difference between the two spectra can be underbetween B at 22 GHz and the strongest maser feature stood as being the result of time variation. Table 5 lists
from 1995 (T98) to 1996 (our work), as shown in table 3. the epoch of observations and flux density for each com-
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426
S. Sawada-Satoh et al.
30 20 10 0 -10 -20 -30
MilliARC SEC
400
[Vol. 52,
800
1200
VLSR [km s 1 ]
1600
Fig. 4. (a) Gray scale images of the HI opacity towards the nucleus, overlayed by a contour map of continuum emission at
1.4 GHz. Contours are drawn at 3, 4.5, 6, 7.5, and 9 a ; 1 a is 0.43 mjy beam" 1 . Three Hi absorption features were
detected towards the continuum components at (1) 1230 km s - 1 , (2) 1127 km s" 1 near Vsys, and (3) 1015 km s" 1 . Large
opacity (T > 0.5) absorption of 1230 km s" 1 and 1015 km s" 1 can be only seen in B and A, respectively, while the
absorption feature at 1127 km s" 1 is seen in both components, (b) HI absorption spectra at the nucleus of NGC 3079
averaging data of FD, KP, LA, PT, and Y. Spectral resolution is 13.2 km s" 1 per channel. The arrow shows the Vsys.
ponent determined by IS88, T98, and this paper. At
5 GHz, observations in 1986 (IS88) and in 1992 (T98)
showed a large decrease in the flux density of A, while
B varied by a much smaller amount during the same period. At 8.4 GHz, observations in 1992 (T98) and ours
in 1996 show that both components have weakened over
the subsequent four years.
Notably, the flux density of A decreased by 87% over
that period. At 22 GHz, A has weakened considerably
over a period of 22 months, even disappearing below our
detection limit, while no significant variation is seen in
B. The turnover frequency of component A may have
moved downwards in frequency between 1992 and 1996.
Component D detected by T98 with a flux density of
14.3±0.5 mjy at 5 GHz in 1992 is likely to be a jet component which was ejected from the nucleus prior to A.
The non-detection of D in our observations can, in the
light of the strong variations noted above, be ascribed to
a flux decrease. As we have discussed, a significant time
variation is observed in the continuum components, and
the discrepancy of the spectra between T98 and this paper is explained quite naturally in terms of a time variation. Simultaneous multi-frequency observation is the
only method whereby accurate component spectra can
be determined.
If NGC 3079 has a thin edge-on maser disk similar to
those observed in NGC 4258 and NGC 1068, the nucleus
should be located on the line of maser alignment. T98
proposed that the masering disk lies along a north-south
line almost parallel to the galactic disk. However, we rule
out this possibility because the position of the proposed
nuclear continuum component B is offset from this line.
© Astronomical Society of Japan • Provided by the NASA Astrophysics Data System
Seyfert 2 NGC 3079
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Table 5. Flux densities of continuum components at epoch of observations.
Frequency
Epoch
A
[mJy]
B
[mJy]
Beam size
[mas x mas]
Reference
1986.4
1992.7
1992.7
1996.8
1995.0
1996.8
12±5
6±0.5
15±2
1.9±0.6
6±1
<0.49
12±7
16.8±0.4
37±2
14.2±0.7
16±1
14.5±2.6
2.24x0.74
8.4x2.5
1.8x0.6
1.3x0.91
1.15x0.98
0.41x0.30
IS88
[GHz]
22.
T98
T98
our work
T98
our work
nent B and the drifting maser component at 1190 km s 1 ,
with a resulting position angle of ~ 30°. The direction is
almost parallel to the direction of jet component A from
nucleus B (P.A. ~ 30°-50°). On the other hand, since
the main H2O maser features exhibit no, or a slight velocity drift, it is likely that they are relatively far from
the rotation axis, and it is therefore hard to detect their
velocity drift.
From these results, one may speculate as to the nuclear
structure of NGC 3079 (figure 6). Hi gas and maser gas
reside in orbital motion around B. If we assume a nuclear
torus around B, the torus must be viewed almost edgeon, and the near side of the torus covers the continuum
1986
1988
1990
1992
1994
1996
1998
components. Its thickness is then at least a few parTime [year]
sees. The velocity gradient of Hi absorption on 100 pcscale dominates the galactic rotation (Pedlar et al. 1996)
Fig. 5. Spatial separation between A and B versus time.
and is opposite direction to the velocity gradient of HI
It shows a significant change of the separation, corabsorption from our observations. It would indicate ~
responding to in a subluminal motion. Plots in 1986,
10 pc-scale HI gas in the nuclear region.
1992, and 1995 are from IS88 and T98. respectively.
The apparent velocity at 5 GHz from 1986 to 1992
The position angle of the rotation axis of the torus is
and at 8.4 GHz from 1992 to 1996, are estimated to
~
—30°, which differs by ~ 110° from the rotation axis
~ 0.06 c (T98) and 0.16 c, respectively. The sepaof
whole
galactic region of NGC 3079. The absorption of
ration at 22 GHz in 1995 agrees well with that at
8.4 GHz.
1127 km s x, seen both on A and B, presumably comes
from galactic neutral hydrogen gas surrounding the outer
region of the rotating torus. It should be noted that this
=
1
Only one maser feature (VLSR 1190 km s ) shows absorption feature has a velocity close to Vsys.
In many cases, it is thought that the rotating disk is
a large velocity drift, while the others (VLSR = 9261
perpendicular
to the direction of the jet from the nucleus
1008 km s" ) appear almost stationary, or show a little
(e.g.,
NGC
4258;
Herrnstein et al. 1997). However, the
velocity drift (Nakai et al. 1995; Baan, Haschick 1996;
putative
position
angle
of the rotation axis of the torus
N. Nakai, private communication). A similar increasin
NGC
3079
is
different
from the position angle of the
ing drift is observed for the maser features close to the
core-jet
structure
by
~
30°.
In this scenario, the rotasystemic velocity in NGC 4258, and is interpreted as retion
plane
of
the
torus
is
not
perpendicular to the jet.
sulting from centripetal acceleration of maser gas in the
Another
smaller
disk
which
is
perpendicular to the jet
rotation disk as it moves across our line of sight to the
direction may exist inside the torus, closer to the nurotation axis. If the velocity drift in NGC 3079 is due to
cleus. It has recently been shown that broad emissioncentripetal acceleration, the maser spot with the highest
line regions (BLRs) are not always coplanar with respect
l
velocity drift, the feature at 1190 km s~ should be loto accretion disks in Type-1 Seyfert galaxies (Nishiura
cated on the near side of the rotation system, and its proet al. 1998). Typical radii of BLRs are ~ 0.01 pc (e.g.,
jected distance from the rotation axis should be nearly Peterson 1993), which is a smaller scale than the torus
zero. The rotation axis should then run through compo-
Astronomical Society of Japan • Provided by the NASA Astrophysics Data System
428
S. Sawada-Satoh et al.
Water mascr
Rotatioii axis
VLSR = 920 - 1030 km
P.A.
Water maser
30°
/B
VLSR = 997 - 1070 km s"1
/
0.5pc
A
Water maser
= 1190 km
Fig. 6. Possible model of the parsec-scale nuclear structure of NGC 3079. Component B is the nucleus and
the dynamic center of the rotating disk or torus with
H2O maser and neutral hydrogen. All H2O masers
are at the near side of the rotation system. The
maser at 1190 km s" 1 is located near to the midpoint of A and B (T98), nearly on the plane which
contains the rotation axis and observer.
of NGC 3079. Therefore, it is possible that the rotation
plane of the ~ 10 pc-scale torus is not perpendicular to
the jet.
Thus, we interpret the nuclear region of NGC 3079 as
rotation around the nucleus. However, this interpretation
comes from only a single epoch observation. We have
performed VLBA monitoring to search for the proper
motion of the maser spots and time variation of the flux
density of the continuum components in order to support
our hypothesis.
We would like to express our thanks to A. S. Trotter
and Y. Fukazawa for informing us of their results. We
also thank an anonymous referee for careful reading and
making valuable comments. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement
by Associated Universities, Inc. This research was supported by a Grant-in-Aid for JSPS Fellows by the Ministry of Education, Science, Sports and Culture.
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Astronomical Society of Japan • Provided by the NASA Astrophysics Data System
H
CM
Plate 40
2-
930
940
950
960
970
990
980
1000
VLSR [km s"1]
1
1
1
1
8
(b)
I
6
•
926-934
•
935-942
•
943-950
o
951-958
•
959-966
9
967-974
975-982
4
O
LJJ
(J
DC
—
983-990
:, 991-998
—
•
Mill
< 2
—
999-1007
•
0
CD
-2 —
B
0
1
1
1
-2
-4
MilliARC SEC
-6
-8
Fig. 3. (a) Cross-power spectrum of H2O maser emission with a velocity range of 926-1008 km s 1 . The spectral resolution
is 0.42 km s" 1 per channel. The color indicates the velocity shift of the profile, (b) The distribution of the H2O maser
spots and continuum component at 22 GHz, is phase-referenced to the strongest maser feature at 956 km s^ 1 . The
color full circles reflect the velocity shift of H2O maser emission in (a), as shown in the upper-right box. The positions
of H2O masers and the continuum component B are significantly separated (~ 0.42 pc).
S. SAWADA-SATOH et al. (See Vol. 52,
424)
© Astronomical Society of Japan • Provided by the NASA Astrophysics Data System