The nuclear Disk of NGC 4261: HST Images and WHT Spectra

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Session 70 -- AGNs and Radio Galaxies II
Oral presentation, Thursday, 10:30-12:00, Dwinelle 145 Room

[70.05] The nuclear Disk of NGC 4261: HST Images and WHT Spectra

L.Ferrarese, H.C.Ford (JHU, STScI), W.Jaffe, F. van den Bosch (Leiden Observatory), R.W.O'Connell (University of Virginia)

The properties of the nuclear region of the elliptical galaxy NGC4261 have been studied using high resolution HST Planetary Camera images and La Palma WHT spectra. The images of NGC 4261 where obtained with the F555W filter during a survey of a complete sample of elliptical galaxies in the Virgo cluster (Jaffe et al. 1993) and reveal a sharply defined elliptical disk centered on the photometrical centre of the galaxy and on the faint (m$_V$ = 23.6) unresolved optical nucleus. Assuming that the disk is intrinsically circular, the plane of the disk is inclined 64$^{\circ}$ to the plane of the sky. The apparent size of the major axis of the disk, 1".71, corresponds to $\sim$ 120 pc at the distance of the Virgo cluster (14.7 Mpc, Jacoby et al. 1990). Our analysis shows that the disk is physically thin ($\leq$ 10 pc) and optically thick ($\tau \sim$ 0.8).\\ The ground-based spectra show strong H$\alpha$, [SII] and [NII] emission lines with narrow cores and unusually broad bases. The broad emission lines are symmetrical and have a mean velocity equal to the galaxy systemic velocity, which suggests that the broad wings may be caused by the rotation of ionized gas in the inner parts of the the disk. For central masses of $10^7\rightarrow 10^8 M_\odot$ the high velocity material which causes the broad wings would be orbiting at radii of $0.03\rightarrow 0.3$ pc from the center. Furthermore, the hypothesis that NGC4261 harbours a massive compact central object is suggested by the fact that the galaxy has an active nucleus which produces a bright bi-direction radio jet and double lobed radio source (Birkinshaw and Davies, 1985; Jaffe and McNamara, 1993). The rotation curve derived from the narrow [NII] $\lambda$ 6584 emission is successfully reproduced by adding a central point mass of $4 \times 10^7$ M$_{\odot}$ to the potential of the exponential stellar distribution derived from the luminosity profile for a constant M/L$_B$=5 and to the (insignificant) potential of the disk with M = 10$^6$ M$_\odot$. We conclude that the rotation curve for NGC 4261 is consistent with the presence of a central black hole with a mass between $10^{7.4}$ M$_{\odot}$ and $10^{7.8}$ M$_{\odot}$.

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