The location and intrinsic luminosity of the hidden nucleus of NGC 1068

Adv. Space Res. Vol. 23, No. 516, pp.899-904, 1999 0 1999 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177199 $20.00 + 0.00 PII: SO273-1177(99)00262-8

Pergamon www.elsevier.nvlocat

THE LOCATION AND INTRINSIC NUCLEUS OF NGC 1068 M. Kishimoto

l Department

LUMINOSITY

OF THE HIDDEN

’

of Astronomy,

Faculty

of Science,

Kyoto

University,

Sakyo-ku,

Kyoto

606-8502,

Japan

ABSTRACT Two key problems on the ‘hidden’ nucleus of NGC 1068 are discussed, based on the archival Hubble Space Telescope images. First, we discuss the accurate location of the nucleus. We have found that the most probable location is only - O.“l (- 7 pc) south of the UV brightest cloud. Second, we consider the intrinsic luminosity of the hidden nucleus. We show that its lower limit is as large as - 2 x 1O45 erg/set, suggesting that the luminosity is almost at the Eddington limit. 01999 COSPAR. Published by Elsevier Science Ltd.

INTRODUCTION The nucleus of the Seyfert 2 galaxy NGC 1068 is now firmly believed to be hidden from direct view. We discuss here two important problems on this hidden nucleus. First we discuss its location on the Hubble Space Telescope (HST) images. The location which is accurate to O.“l or better is now being required for the investigations with other high-resolution images, such as VLBI radio maps. The second problem is the intrinsic luminosity of the hidden nucleus, which is one of the most fundamental quantities for the physics of the nucleus. Below we briefly describe our results on these two problems using the HST Faint Object Camera (FOC) polarization image and Wide Field Planetary Camera 2 (WFPC2) Hcu image.

WHERE

IS THE NUCLEUS

?

We derive the nuclear location from the map of the position angles (PAS) of polarization from FOC/UV imaging polarimetry (F253M filter), which we believe is the most direct, way so far to determine the nuclear location on the HST images. The nucleus should be located at the center of the centrosymmetric pattern of the PA distribution, which is expected to be observed if the radiation from the nucleus is being scattered by the surrounding gas. Capetti et al. (1995) have determined the nucleus location in this way. However, clear deviations from the centrosymmetric pattern are seen in some portions of their map and they did not state if these deviations are real, or discuss the observational error of the PA at each position in the image. If the deviations are real, the nuclear position determined by them would not be very accurate. In order to obtain a more reliable nuclear location, we have extended their work to examine the PA distribution and then to re-determine the nucleus location within a convincing error circle. First we have done an extensive estimation of the observational errors of the FOC polarimetry. Second we have considered the effect of the focus problem of the data, since the HST had an extraordinarily poor focus at the time of this polarimetry 899

M. Kishimoto

900

-0.2 i

- 1.5

-

1 .o

-0.5 Offset

0.0 west

0.5

-0.2

1.0

(a)

Black vectors

image (FOC/F253M)

indicate

with

(6) Enlarged

white dot-dash

0.1

0.2

(orcsec)

contour

The nuclear locations

view of the central represents determined

observed PAS drawn as white two vectors for each position

the PAS centrosymmetric

as a white plus sign with an error circle of 99% confidence regions.

0.0 west

(b)

Figure 1: (u) UV continuum (PA +z opt).

-0.1 Offset

(orcsec)

to the most probable level. The regions

N 0.“5 of (a), but underlying

the UV continuum by Capetti

image.

which is drawn

by white lines are the excluded

is the [0111] image (FOC/F50lN),

The names

et al. (1995) and Braatz

nuclear location,

enclosed

of the clouds

are from Evans

while the et al. (1991).

et ul. (1993) are also indicated.

observation. Our conclusion is very simple - the PA distribution is completely consistent with a centrosymmetric pattern within the estimated accuracy of the FOC polarimetry, if we exclude. the region contaminated bz~ the bud focus e$ect (Figure la). Fortunately, we have another archival image of NGC 1068 taken with a fine focus by the FOC through the same filter (F253M). We have taken the ratio of the total flux image from the polarimetry to this image with a fine focus, and excluded the region with this ratio much larger than unity. Such regions are considered to be affected by the significant leak from the surrounding brighter region, due to the bad focus. The excluded regions include the ones around the central brightest clouds, The deviations from the centrosymmetric pattern in the map of Capetti et al. were seen especially in these regions. Furthermore, primarily due to this masking-out procedure, the most probable location of the nucleus has moved by 0.“2 to the north, compared with the point determined by Capetti et al. (see Figure lb). The location is very close to the brightest clouds in the UV and in the [OIII] image. This new location will greatly affect the study of the physical conditions and kinematics in this innermost region.

HOW LUMINOUS

IS THE HIDDEN

NUCLEUS

?

Now the HST UV imaging polarimetry has shown that each clump is highly polarized due to scattering. This UV image also shows that the nuclear gas is very clumpy. The important fact is that the clumps which are rather far from the nucleus (- 100 pc away) have still considerable amounts of

The Hidden Nucleus of NGC 1068

901

polarized flux, which are the lower limit of the amounts of the scattered light. The hidden nucleus must be illuminating these distant clouds strongly enough so that the clouds have such a significant brightness in scattered light. Therefore the observed polarized flux provides a direct lower limit of the luminosity of the hidden nucleus. The following is the more detailed description of our method. Basically, we can measure the nuclear flux illuminating a cloud from three quantities; (1) the amount of the scattered light, (2) the distance from the nucleus to the cloud, (3) the amount of the scatterers in the cloud. Let us denote the nuclear flux as Ff”’ which we would observe if we were to see the nucleus without any obscuration. The observed projected distance rP is the lower limit of the actual distance between the nucleus and the cloud. With given polarized flux PF,, (I?,, is the total flux and P is its fractional polarization), the lower limit of the nuclear flux (F,“““)min is given by the combination of the observed values as (F,“u’),in = PFU .

(gyy’.

where N, and V are the electron density and volume of the cloud, respectively, and 0~ is Thomson scattering cross section. We assume here that the scatterers are electrons (Antonucci and Miller 1985; Miller, Goodrich, and Mathews 1991). We evaluate ni:, using the observed Ha flux FH” from the relation D2FH” = yH”(T)N,2V,

A’, = No. <(7’) where

NO = D

Y Ha is the Ha emission

coefficient (defined here as Ha recombination coefficient multiplied by Ha photon energy per steradian), T is the temperature, and D is the distance t#o NGC 1068. With the latter equation, we mean that we separate the T dependent portion as t(T) SC)that No becomes the electron density derived with any particular temperature assumed, which we took as lO*K. Due to this T dependence in N,, (Fy)min also depends on T. If we write the nuclear flux derived for T = 104K as (Fy’)o, we have

(F,“““)min *C(T) = (F,?‘“)o =

1679/7H”(104K) T;

PF,

DAfP

3g.T

As T increases, y Ha decreases so that E increases. To know the true lower limit of Pi”“, we need an upper limit of T. This upper limit can be derived from the fact that the nebular emission FFb (free-free and free-bound plus two-photon emission, which always accompany the recombination-line emission) has to be less than the flux left, i.e., the unpolarized flux (1 - P)F,. The nebular emission is given by _ +fb(T) F&I FTb Fleb = (FFb)o. G&‘-1, yHa(T) ’ Or where yeeb is the nebular emission coefficient and (Fyb) 0 is the nebular 104K. Then the T dependent portion E,(T) has a maximum as c,,(T) 5 E?

z+

(I-

emission derived with T =

P)E

(FY”“~)o

If we increase T, yrb decreases but slower than y”” does, so E, increases with T. Hence ET provides an upper limit of T. Therefore, with (F$““)o and ey given (which can be all constructed from the observed values of P, F,, FHa, r,, and V), we can derive (FyC)min.

902

M. Kishimoto

k -1 5

-1

0

-0.5 Offset

0.0 west

0.5

1 .o

(arcsec)

(a)

Figure 2: (a) Cloud positions and sizes on the UV continuurn image in log scale. (b) The same as (a) but on the,Ha image in log scale (HST/WFPC2/F658N).

micron

Angstrom

this

keV

work

Px.

‘.. , scattered

1 .o

1.5

moX [Maximum

2.0

25

Temperature

3.0

a

3.5

10

12

14

16

18

log Y (Hz)

lndlcator] @I

Figure 3: (CL)Plot of (Fy)o

versus 6~.

(Fy)c

is the nuclear UV flux derived with T = 104K assumed. crax

determines the maximum temperature allowed in each cloud. The dotted contours show the lower limit of the nuclear UV flux (Fy”UC),in. For a given (Fy)c,

(Ft”c)min becomes smaller for larger c?.

(b) The observed flux of NGC 1068

compared with the lower limit obtained in this paper, indicated by a square. The solid and dotted optical-UV-X-ray spectrum is from Pier et al. (1994), while the dot-dash line is the same spectrum but scaled to fit our lower limit at the UV. Triangles are from Bailey et al. (1988), o p en circles from Rieke and Low (1975) and Lebofsky et al. (1978), and pluses from Telesco et al. (1984). The crosses and filled circles are from Gallimore et al. (1996) for their ‘C’ and ‘Sl’ sources, respectively.

903

The Hidden Nucleus of NGC 1068

We have extracted the UV flux and UV polarized flux, as well as the Ho flux of the clouds shown in Figure 2. Cloud centers are chosen as local intensity maxima of the UV image, and cloud sizes are determined as FWHMs of the UV intensity profiles around the intensity maxima. In Figure 3a, we have plotted (Fy)e and EF for each cloud, where T,““~is evaluated at the UV (F253M; - 2500A). The contours show the value of (Fy)min. Reading the largest lower limit indicated by the cloud No.19 and 20, we conclude that the nuclear flux is brighter than - 2 Jy at - 2500A if it is observed without any obscuration. The main uncertainty comes from the estimation of cloud volumes. We have estimated the volumes assuming spherical clouds, but the clouds could be more extended along the line of sight. This uncertainty in V would be the limit of our method, though the dependency on V is not so strong. Since (F~“c)a is proportional to l/n, (Fy)min depends on V in the same way. Note that the possible existence of foreground or background diffuse clouds along the line of sight would not affect our result significantly. The contribution of these clouds to the polarized flux would be small! because of their larger distance from the nucleus and the lower polarization of their scattered light. We compare the derived flux with the observed flux at several other wavelength bands in Figure 36. Using the observed shape of the spectrum from the optical to X-ray, suggested by Pier et al. (1994), the lower limit of the intrinsic luminosity of the hidden nucleus is calculated to be - 2 x 10‘15erg/set. We assumed here D = 14.4 Mpc (Tully 1988, He = 75km/sec/Mpc). The derived luminosity has the distance dependence of c( D3i2, since (FyC)min c( D- Ii2 The kinematics from the water maser observations suggest that the mass of the central black hole is 1 - 2 x 107A!& (Greenhill et al. 1996; Greenhill and Gwinn 1997). The corresponding Eddington luminosity is 1 - 3 x 103” erg/set, so the luminosity of the hidden nucleus is almost at the Eddington limit. We note that Bland-Hawthorn et al. (1997) have also obtained a similar result from a very different ground, using the result of the mid-infrared observation. Our result is based on the clouds 19 and 20 (or 17), which seem rather clouds have large polarized flux in spite of their large distance from possibility that these clouds are very extended along the line of sight. become smaller than the inferred range of the Eddington luminosity volumes of the clouds over a factor of - 4, which seems to be unlikely clouds.

“peculiar” in Figure 3~. These the nucleus. There could be a The nuclear luminosity would if we were underestimating the from the direct images of these

We have also found that the consideration of volume filling factors will not change our result. details will be published in Kishimoto (1999a,b).

More

ACKNOWLEDGMENTS The author thanks

COSPAR

for the financial support.

REFERENCES Antonucci, R., and J. S. Miller, Astrophys. J., 297, 621 (1985). Bailey, J., D. J. Axon, J. H. Hough, M. J. Ward, I. McLean, and S. R. Heathcote, Illon. Not. R. astr. Sot., 234, 899 (1988). Bland-Hawthorn, J., S. L. Lumsden, G. M. Voit, G. N. Cecil, and J. C. Weisheit, Astrophys. and Space Science, 248, 177 (1997). Braatz, J. A., A. S. Wilson, D. Y. Gezari, F. Varosi, and C. A. Beichman, Astrophys. J., 409, L5 (1993). Capetti, A., F. Macchetto, D. J. Axon, W. B. Sparks, and A. Boksenberg, Astrophys. J., 452, L87 (1995).

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Evans, I. N., H. C. Ford, A. L. Kinney, R. R. J. Antonucci, L. Armus, and S. Caganoff, Astrophys J., 369, L27 (1991). Gallimore, J. F., S. A. Baum, C. P. O’Dea, and A. Pedlar, Astrophys. J., 458, 136 (1996). Greenhill, L. J., C. R. Gwinn, R. Antonucci, and R. Barvainis, Astrophys. J., 472, L21 (1996). Greenhill, L. J., and C. R. Gwinn, Astrophys. and Space Science, 248, 261 (1997). Kishimoto, M., Astrophys. J., in press (1999a). Kishimoto, M., in preparation (199913). Lebofsky, M. J., G. H. Rieke, and J. C. Kemp, Astrophys. J., 222, 95 (1978). Miller, J. S., R. W. Goodrich, and W. G. Mathews, Astrophys. J., 378, 47 (1991). Pier, E. A., R. Antonucci, T. Hurt, G. Kriss, and J. Krolik, Astrophys. J., 428, 124 (1994). Rieke, G. H., and F. J. Low, Astrophys. J., 199,L13 (1975). Telesco, C. M., E. E. Becklin, and C. G. Wynn-Williams, Astrophys. J., 282, 427 (1984).