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Adv. Space Res. Vol. 23, No. 5/6, pp. 1007-1010, 1999 Q 1999 COSPAR. Published by Elsevier Science Ltd. All rights reserved

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MOLECULAR GAS IN THE CENTRAL REGIONS OF NEARBY AC- TIVE GALAXIES

L.J. Tacconi,’ E. Schinnerer, ’ R. Genzel,’ A. E&art’

’ Max-Planck-Institut fiir Extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany

ABSTRACT

We are carrying out a program of 0.6”-6.8” resolution 12C0 J=2-+1 observations with unprecedented high sensitivity in a sample of nearby AGN with the IRAM interferometer. Here we give some general

results of the program and highlight results from the Seyfert 1 galaxy NGC 7469. This source shows tightly wound molecular spiral arms, a bar-like gas distribution running from the arms to the central regions, and a barely resolved, rapidly rotating central component.

INTRODUCTION 01999 COSPAR. Published by Elsevier Science Ltd.

The recent capability of millimeter interferometers to achieve sub-arcsecond resolution has provided the exciting possibility of studying the gas dynamics and distributions of.nearby galactic nuclei on scales of a few hundred parsecs or less. Molecular gas at these distances is likely the main fuel for active nuclei. It may also contribute to the obscuration of the nucleus in Seyfert 2 galaxies. In order to study the molecular gas distributions, dynamics, and fueling mechanisms of these central regions we are mapping a sample of nearby Seyfert galaxies at high resolution in the J=2--+1 transition of 12C0 with the IRAM interferometer on the Plateau de Bure, France’. In this paper we present some general results and highlight the observations of the Seyfert 1 galaxy NGC 7469. Full descriptions of the observations, results and analysis will ge given in several papers which are I~OW in progress.

MOLECULAR ISM IN SEYFERT GALAXIES

We have observed the 12C0 J=2-+1 emission in five nearby Seyfert galaxies using the 5-element IRAM interferometer in its AB configuration during the winters of 1995-1997. Depending on the source declination angular resolutions of the CO maps ranged from 0.6”-1.0” with uniform weighting. Amplitudes, phases and bandpasses were calibrated using interleaved observations of strong point sources. We mapped and cleaned the calibrated data in the usual way, and smoothed the data to velocity resolutions of 10-20 km s-i to improve the S/N.

In the Seyfert galaxies that we have observed to date the molecular gas is distributed in ring/spiral arm structures both at scales of -1 kpc and -100 pc. In two of the sources, NGC 1068 (see Figure

‘IRAM is supported by INSU/CNRS (Fk ante), MPG (Germany) and IGN (Spain).

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L. J. Tacconi er al.

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Fig. 1. The total CO J=l+O (left) and J=2+1 (right) integrated flux maps of NGC 1068 made with the IRAM interferometer. The synthesized beams shown in the bottom left corner of each panel are 2.5” and 0.7” respectively. In both panels the lowest contour is at 3a with levels increasing in steps of 3~. The asterisk indicates the location of the K-band continuum peak. For the l-+0 map contours run from 1-29 Jy beam-’ km s-i in steps of 1 Jy beam -i km s-l. For the 2-+1 map contours run from run from 2-36 Jy beam -i km s-l in steps of 2 Jy beam-’ km s-l.

1) and NGC 7469 (Figure 2 and following section) the kpc “rings” are associated with recent massive star formation contributing significantly to the bolometric luminosity. In NGC 1068 the 100 pc scale gas structure is anti-correlated with the NLR ionization cone observed by Macchetto et al. (1994) and Capetti et al. (1997), and is likely contributing to the large scale collimation of the narrow line region. In all of the observed Seyfert the 100 pc scale molecular gas stuctures likely represent the locations of resonances where gas piles up as it is channeled in to fuel the active nucleus. The molecular gas kinematics of the central regions are dominated by circular rotation of a simple disk (e.g Figure 2b), but there is also evidence for non-circular motions, such as elliptical streaming along a bar potential. The observed molecular gas structures are similar to those found in normal galaxies (e.g. Kenney et al. 1992). There seems to be no obvious difference in the circumnuclear gas distributions of galaxies with or without an AGN. There is also no difference in the central gas distributions and kinematics between the Seyfert 1 and Seyfert 2 systems which we have studied.

As part of her PhD thesis Schinnerer (1999) is modeling the observed gas distributions and kinematics in the Seyfert 2 galaxy, NGC 1068 and the Seyfert 1 galaxy, NGC 3227, using a system of tilted rings to simulate a warped disk (e.g. Quillen et al. 1992). In both cases, the observed molecular gas ring-like structures and the kinematic features are well fit by a model where a CO “disk” warps out of the plane of the galaxy. From molecular gas kinematics, at least at scales of 100 pc the molecular gas need not be distributed in a thick torus, but rather a thinner warped disk. We are also currently investigating whether models involving gas traveling on elliptical orbits in bar-like potentials can also fit the observed CO distributions and kinematics, since, especially in NGC 1068, there is known to be a strong NIR bar (Thronson et al. 1989; Thatte et al. 1997). We plan to apply both types of models to all of the galaxies observed thus far.

Molecular Gas in AGNs

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Fig. 2. Left: The total CO J=2-+1 integrated flux map in NGC 7469 represented as contours and a greyscale. Contours run from l-25 Jy beam-’ km s-l in steps of 1OO.l Jy beam-’ km s-l, where 1 Jy beam-’ km s-l represents the 30 cutoff that was used in calculating the integrated flux. The 0.7” synthesized beam is shown in the lower left corner of the figure. Right: Position-velocity map taken along the kinematic major axis (P.A. = 128”) of NGC 7469. Contours run from 8 to 240 mJy beam-’ in steps of 16 mJy beam-‘.

NGC 7469: A VERY GAS-RICH SEYFERT 1

NGC 7469 is a relatively nearby (D=66 Mpc, 1”=320 pc for Ho=75 km s-l Mpc-‘) galaxy harboring a classical Seyfert 1 nucleus plus a region of prodigious star formation located in a ring or tightly wound spiral arms at a radius of ~1.5”. This starburst region is clearly delineated in the 6 cm VLA map of Wilson et al. (1991), and in the 0.4” resolution near-infrared K-band image of Genzel et al. (1995). Observations of the 2.3pm CO bandhead feature by Genzel et al. indicates that most of the K-band flux from the starburst region is stellar. Emission from this starburst ring accounts for 20.5 Lb01 of the entire galaxy (Genzel et al. 1995). At the 0.7” resolution of the CO J=2-1 map we resolve the molecular gas distribution for the first time into in a rapidly rotating, central ring-like source, molecular spiral arms at a radius of -500 pc, and a bright molecular bar leading from the spiral arms to the central source (Figure 2; Tacconi et al. 1999a). This bar structure is unique to the molecular gas, and is not prominent in NIR K-band images (e.g. Genzel et al. 1995). nor in the 8 GHz radio continuum map of Condon et al. (1991). The barely resolved central source is likely distributed in a 100 pc scale disk/ring structure, similar to what we have observed in the more nearby Seyfert galaxies (see Figure 1). CO observations at resolutions of 10.3” will be necessary to observe the detailed structures in AGN at the distance of NGC 7469.

In Figure 2b we show a position-velocity cut taken along the kinematic major axis of the galaxy (PA = 128”). To illustrate the main kinematic features of this multi-component gas distribution. The velocity field shows that the gas motions are dominated by circular rotation. The rota,tion curve rises

1010 L. J. Tacconi et al.

very steeply within the central arcsec, and then has a secondary rise in the region of the molecular spiral arms. Both fits to the CO and K-band isophotes as well as kinematic modeling of the CO data cube suggest that the gas lies in a plane with an inclination angle of 30 - -45”. The derived kinematic parameters imply a dynamical mass of -1.5~10~’ Mo in the central 3.5” (1.2 kpc). This is roughly twice the mass in stars over the same region, and, together with the radial dependence of the rotation curve implies, therefore, that the mass of the gas is comparable to the mass in stars. The interstellar gas plays a dominant role in the mass distribution and kinematics of the circumnuclear region of this Seyfert galaxy.

ACKNOWLEDGMENTS

We would like to thank the IRAM staff in Grenoble and on the Plateau de Bure, especially D. Downes and R. Neri, for making the observations and for their help with the data reduction and calibration. We also thank Anne Kinney and the other organizers for a wonderful conference.

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