VLBI IMAGING OF WATER MASER EMISSION

FROM A NUCLEAR DISK IN NGC 1068

L. J. GREENHILL

Harvard-Smithsonian Center for Astrophysics

60 Garden St, M.S. 42, Cambridge, MA 02138

AND

C. R. GWINN

University of California

Department of Physics, Broida Hall, Santa Barbara, CA 93106

1. Introduction

In active galactic nuclei, water masers serve as point-like tracers of the dy-

namics and physical conditions within regions of dense molecular gas. Typ-

ically, the maser spectra consist of many individual Doppler componentseach a few km s�1 wide, which subtend much less than a milliarcsecond

(mas) on the sky. At 1 cm wavelength, maser emission is not subject to

dust extinction along the line of sight, which is particularly important. In

general, masers are well suited to study with Very Long Baseline Inter-

ferometry (VLBI) because although interferometer sensitivity is low, the

brightness temperature (i.e., surface brightness) of maser emission is very

high, usually > 1010 K. Typical VLBI arrays provide angular and velocity

resolutions of < 0:5 mas (0.04 pc at 15 Mpc) and < 1 km s�1, respectively.

Masers require 1) relatively quiescent conditions for the formation and

survival of molecular gas, 2) a coherent velocity �eld, to support ampli�-

cation, and 3) a pump source, such as shock or X-ray heating. In addition,

maser emission from elongated volumes is anisotropic and the observer must

be situated within the beam solid angle. For example, emission from an ide-

alized thin disk (i.e., vertical height� radius) is beamed into a narrow solidangle centered on the disk plane.

The best studied nuclear water maser lies in NGC4258. VLBI maps

are strongly suggestive of an exceptionally thin annulus that is between

Astrophysics and Space Science 248: 261{267, 1997.c 1997 Kluwer Academic Publishers. Printed in Belgium.

262 L. J. GREENHILL AND C. R. GWINN

0.4

0.2

0.0

1600140012001000800600

Figure 1. Spectrum of the H2O maser in NGC1068, obtained with the 100-m antennaat E�elsberg, Germany. The horizontal axis re ects heliocentric velocity, in kms�1, forwhich the optical astronomical de�nition of velocity is assumed. The vertical axis is uxdensity calibrated in Jy. The dashed line indicates the systemic velocity of 1148 � 5kms�1, estimated from HI emission, and adopted elsewhere in this volume (see Brinkset al.).

0.13 and 0.25 pc from the AGN central engine (Miyoshi et al. 1995) and

that that is perpendicular to the axis of the radio jet associated with theAGN (Herrnstein et al. 1997). The maser displays emission close to the

systemic velocity of the galaxy from material on the near side of the disk

that is moving perpendicular to the line of sight (hereafter the \systemic

emission"). More signi�cantly, high-velocity emission that is red and blue-

shifted by �(800� 1100) km s�1 arises on the disk diameter perpendicular

to the line of sight, where the motion is parallel to the line of sight. Here

the masers sample a Keplerian rotation curve (v / r�

1

2 ) with deviation of

< 1%. This arrangement of masers in the disk is also strongly supported

by the observed accelerations (e.g., Greenhill et al. 1995, Nakai et al. 1995)

and proper motions (Herrnstein 1997) of the masers.

Water maser emission in NGC1068 extends about �300 km s�1 from

the systemic velocity (Figure 1). It is associated with radio component S1,

which lies at the base of the roughly north-south radio jet (Gallimore etal. 1996). Gallimore et al. (1996) �rst resolved a position o�set between

the red and blue-shifted high-velocity emission, corresponding to a fraction

of the beam of the VLA in A-con�guration. VLBI observations resolved

the velocity structure of the red-shifted and systemic emission (Greenhill

et al. 1996) and showed that the maser source is elongated over about 1

pc at a position angle of �45� (i.e., not perpendicular to the radio jet).

Velocity gradients and linear structure were suggestive of a rotating edge-

on structure. However, the blue-shifted emission was not observed, and

NGC1068 H2O MASER 263

-15

-10

-5

0

5

Nor

th-S

outh

Offs

et (

mas

)

15 10 5 0

East-West Offset (mas)

Figure 2. (Left) Distribution of H2O maser emission in NGC1068, associated with radiocomponent S1. The gray scale re ects line-of-sight velocity. The blue-shifted emission isdarkest. The cross marks the emission with a velocity of 1126 kms�1, the systemicvelocity estimated from the VLBI data. Note opposing velocity gradients among thered-shifted emission, where velocity increases with distance from the systemic emission,up to a few mas, beyond which it decreases with distance. The path of the radio jet is notknown on scales smaller than about 10 pc, but at that scale it is roughly north-south.At a distance of 15 Mpc, 0.7 pc subtends 10 mas. The astrometric positions of radiocomponent S1 (Gallimore, personal communication) and map origin are consistent witheach other; the latter cannot be well established from the VLBI data.

the predicted structural symmetries could not be con�rmed. In light of the

orientation of the jet, Greenhill et al. (1996) proposed that the masers trace

the part of the northern limbs of an axially thick molecular torus that are

directly illuminated by the central engine, where the axis of the torus is

roughly north-south and along the jet.

A second VLBI observation, which covers the 600 km s�1 range of the

maser and is intended to show the position of the blue-shifted emission, has

been completed. It clearly shows that the maser source is linear, extending

from the red-shifted to the blue-shifted extreme (Figure 2), and may be

264 L. J. GREENHILL AND C. R. GWINN

1400

1200

1000

800

Hel

ioce

ntric

Rad

io V

eloc

ity (

km s

-1)

10 0 -10

Impact Parameter (mas)Figure 3. Position-velocity diagram. The impact parameter is measured with respect tothe systemic emission. The approximate symmetry in position and velocity with respectto the systemic emission and the declining rotation curve at large radii are characteristicof a rotating edge-on disk. The preliminary model shown represents a disk of inner andouter radii 0.65 and 1.1 pc, respectively, rotation velocity of 330 kms�1 at the inner edge,and estimated systemic velocity of � 1126 kms�1 (indicated by the dashed line). Scatterin the data may indicate turbulent velocities of up to a few tens of km s�1.

described approximately by a thin edge-on disk with rotation velocities of

up to 330 km s�1. In preliminary analyses the inner and outer disk radii

are estimated to be 0.65 and 1.1 pc, respectively, and the disk inclination isgreater than about 80�. The enclosed mass is about 1:5� 107 M� however,

the rotation curve falls more gradually than v / r�

1

2 or is \sub-Keplerian"

(Figure 3). Startlingly, a full quarter of the approximate inner edge of the

disk is alight with maser emission, in contrast to the case of NGC4258,

where only the sector that lies in front of the central engine and jet is

bright (Figure 4). The more recent of the two VLBI observations will per-

mit construction of a rather robust disk model because of the dynamical

simplicity of a thin disk, as opposed to a fattened torus. Nonetheless, the

NGC1068 H2O MASER 265

Vr o t( r )

r

Vr o t( r )

Figure 4. (top) { Schematic deprojection of the maser disk dynamics. The visible maseremission arises roughly along the diameter perpendicular to the line of sight and along onequadrant of the inner edge. The observer is pictured below the disk, in the plane of thepage. A declining rotation curve is illustrated, whose line-of-sight projection is responsiblefor the maser velocity structure. (bottom) The projection of the orbital velocity over anarrow range of radii, r, (i.e. for emission along the disk's inner edge) may be responsiblefor the linear trend in maser velocity vs. impact parameter, b, when b < r.

earlier VLBI observations contributed good approximations of radius and

binding mass.

In the context of the better understood system in NGC4258, and of

other observations of the NGC1068 AGN, the thin disk model proposed

here for NGC1068 invites further study along particular lines.

1. Now that both red and blue-shifted emission have been mapped, the

orientation of the rotation axis of the disk is better established. It is

also unexpected. The position angle of the disk rotation axis, � 45�,

266 L. J. GREENHILL AND C. R. GWINN

di�ers from that of the radio jet axis, seen on scales of > 10 pc, by

30�40�. However, it is interesting that 1) the position angles of the disk

axis and the nuclear CO bar di�er by only � 20�, and 2) the position

angles of the velocity gradients in CO (Helfer & Blitz 1995; Helfer, this

volume) and HCN (Tacconi et al. 1994; Tacconi, this volume) emission,

observed on 100 pc scales, are almost perpendicular to the disk rotation

axis.

2. The sub-Keplerian rotation curve of the disk may be evidence of non-

gravitational forces acting on the disk (e.g., radiation) or of a non-point

source potential (e.g., a nearby stellar cluster), which may also lead tononcircular orbits. The estimated systemic velocity agrees with that

estimated from CO observations by Helfer & Blitz (1995) and is � 20

km s�1 less than that obtained from H I observations (Brinks et al. this

volume).

3. Although the luminosity of the NGC1068 central engine (Pier et al.

1994) is estimated to be a factor of 100{1000 times the luminosity

of NGC4258 (Makishima et al. 1994, Wilkes et al. 1995), the disk in

NGC1068 is relatively thin. However, the radius at which dust subli-

mates is close to the suggested inner radius of the maser disk (Greenhill

et al. 1996), and Pier & Krolik (1992) suggest that radiation pressure

should fatten the disk close to this radius.

4. Gallimore, Baum, & O'Dea (these proceedings) report the detection of

optically-thin free-free emission at 3.6 cm wavelength from hot gas in

component S1. The angular extent of this emission is close to that ofthe inner diameter of the maser disk. It is possible that the maser disk

has a hard inner limit within which lies the hot gas, although the posi-

tion angles of the disk and hot gas on the sky are somewhat di�erent,

and their relative positions have not been carefully measured on angu-

lar scales comparable to their angular extents. The two epochs of VLBI

observation at 1.3 cm wavelength have failed to detect compact high

brightness temperature emission that presumably would be closely as-

sociated with the central engine, in contrast to NGC4258 (Herrnstein

et al. 1997), which would assist in the relative astrometry. It is possible

that the compact 1.3 cm emission is absorbed or scattered.

5. If the inferred free-free source lies within the inner radius of the disk,

then the maser emission may amplify it, which would explain why so

much of the near side of the disk appears to be aglow with maser

emission. In this context, the relative orientation of the maser diskand the free-free source may be responsible for the dearth of of blue-

shifted maser emission along the inner edge of the disk. (Gallimore

et al. (1996) show that much of the emission between 1000 and 1200

km s�1 lies toward radio jet component C.)

NGC1068 H2O MASER 267

6. The centripetal acceleration at the inner edge of the disk, for an in-

ferred orbital velocity of about 330 km s�1 is about 0.16 km s�1 yr�1,

substantially less than the width of an individual spectral feature.

Claims of even larger accelerations among the red-shifted high-velocity

emission (Baan & Haschick 1997) are very di�cult to understand in

the context of this model and merit a critical review. Large acceler-

ations among any of the maser features in NGC1068 would provide

important new constraints on disk models.

References

Baan, W. A., & Haschick, A. D. 1997, ApJ 473, 269Gallimore, J. F., Baum, S. A., O'Dea, C. P., Brinks, E., & Pedlar, A. 1996, ApJ, 462,

740Greenhill, L. J., Henkel, C., Becker, R., Wilson, T. L., & Wouterloot, J. G. A. 1995a,

A&A, 304, 21Greenhill, L. J., Gwinn, C. R., Antonucci, R., & Barvainis, R. E. 1996, ApJ, 472, L21Helfer, T. T., & Blitz, L. 1995, ApJ, 450, 90Herrnstein, J. R. 1997, PhD Dissertation, Harvard.Herrnstein, J. R., Moran, J. M., Greenhill, L. J., Diamond, P. J., Miyoshi, M., Nakai, N.,

& Inoue, M., 1997, ApJ, 475, L17Makishima, K., et al. 1994, PASJ, 46, L77Miyoshi, M., Moran, J. M., Herrnstein, J. R., Greenhill, L. J., Nakai, N., Diamond, P.

J., & Inoue, M. 1995, Nat, 373, 127Nakai, N., Inoue, M., Miyazawa, K., Miyoshi, M., & Hall, P. 1995, PASJ, 47, 771Pier, E. A., & Krolik, J. H. 1992, ApJ, 401, 99Pier, E. A., Antonucci, R. R. J., Hurt, T., Kriss, G., & Krolik, J. 1994, ApJ, 428, 124Tacconi, L. J., Genzel, R., Blietz, M., Cameron, M., Harris, A. J., & Madden, S. 1994,

ApJ, 426, L77Wilkes, B. J., Schmidt, G. D., Smith, P. S., Mathur, S., & K. K. McLeod 1995, ApJ, 455,

L13