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Astron. Astrophys. 354, L45-L48 (2000)

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1. Introduction

Perhaps the most striking feature of the FRI radio galaxy NGC 4261 (3C 270) is its approximately 240 pc radius circumnuclear dust disk revealed by HST observations (Jaffe et al. 1993). Further optical observations of the kinematics of emission lines in the inner regions of this disk by Ferrarese et al. (1996) have since provided evidence for a central black hole of mass of [FORMULA][FORMULA]. In the radio band, H I absorption has been detected toward the core of NGC 4261 using the VLA (Jaffe & McNamara 1994). It was argued by these authors that this absorption is due to atomic hydrogen in the inner part of the HST disk. Such a disk interpretation is consistent with high resolution VLBI or MERLIN H I absorption observations in a number of other AGN. For example VLBA H I observations of another FRI galaxy, Hydra A, are consistent with a 20 pc flattened disk structure (Taylor 1996). In this Letter we report on VLBI observations of NGC 4261 performed using the high-sensitivity antennas of the European VLBI Network (EVN). The aims were to confirm that the H I is indeed associated with the HST dust disk and to better constrain the disk geometry and physical properties.

Detailed studies of the dynamics and chemistry of circumnuclear disks such as that found in v are important for several reasons. Such disks almost certainly provide the fuel which powers AGN, but the accretion process is poorly understood. In addition such flattened circumnuclear structures are required by orientation-based unified schemes. While the inner edges of these occulting structures must be on BLR scales (0.1 - 1 pc) their outer radii are poorly defined and may extend to hundreds of parsecs. Evidence for circumnuclear gas on a variety of scales in different physical states has been accumulating. Examples include HST imaging of parsec scale ionised gas in M87 (Ford et al. 1994), 100 - 1000 pc scale molecular CO in Centaurus A (Rydbeck et al. 1993) and HCN in the Seyfert 2 NGC 1068 (Tacconi et al. 1994). Amongst all the objects observed NGC 4261 is unique in showing optical dust, H I absorption and VLBI scale free-free absorption (Jones & Wehrle 1997), allowing us to study the disk on a variety of scales.

The HST optical imaging of NGC 4261 provides strong constraints on the disk geometry at 100 pc scales. The ratio of the apparent major and minor axes (Ferrarese et al. 1996, Jaffe et al. 1996) implies, if the disk is circular, that its normal is inclined [FORMULA] to the line of sight. Modeling of the dust obscuration shows that it is the East side of the disk which is closest to us. Such modeling also shows that the dust disk is thin, with a thickness [FORMULA] pc at its outer edge (Jaffe et al. 1996). The normal to the disk is found in projection to be roughly oriented along the radio axis, making an angle of [FORMULA] to the kiloparsec scale radio jets. At both 1.6 and 8.4 GHz VLBI observations show a two-sided jet in the same position angle as the kiloparsec jets (Jones & Wehrle 1997). The Eastern jet, which is slightly weaker, is assumed to be the counterjet, given that the Eastern edge of the HST dust disk is tilted toward us and the radio jets are roughly aligned along the disk axis. Consistent with this orientation, Jones & Wehrle (1997) argue that a narrow gap in the 8.4 GHz radio emission toward the Eastern jet is from free-free absorption via occultation by an inner ionised accretion disk of radius 0.2 pc. In the remainder of the paper we discuss our H I VLBI observations and the additional constraints on disk geometry and physical properties they provide. Throughout this paper we assume a distance of [FORMULA] Mpc (Nolthenius 1993), so 1 mas corresponds to 0.14 pc.

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© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000