We used the near infrared spectrometer ISAAC at the ESO Very Large Telescope to map the velocity field of Centaurus A (NGC 5128) at several position angles and locations in the central 20 ′′ of the galaxy. The high spatial resolution (∼ 0. ′′ 5) velocity fields 1 Based on observations collected at the European Southern Observatory, Paranal, Chile (ESO Program 63.P-0271A) 2 Affiliated to Space Science Department of ESA -2from both ionized and molecular gas (Paβ, [Fe ii], Brγ and H 2 ) are not compromised by either excitation effects or obscuration. We identify three distinct kinematical systems: (i) a rotating "nuclear disk" of ionized gas, confined to the inner 2 ′′ , the counterpart of the Paα feature previously revealed by HST/NICMOS imaging; (ii) a ring-like system with a ∼ 6 ′′ inner radius detected only in H 2 likely the counterpart of the 100 pc-scale structure detected in CO by other authors; (iii) a normal extended component of gas rotating in the galactic potential. The nuclear disk is in keplerian rotation around a central mass concentration, dark (M/L > 20 M ⊙ /L K⊙ ) and point-like at the spatial resolution of the data (R < 0. ′′ 25 ∼ 4 pc). We interpret this mass concentration as a supermassive black hole. Its dynamical mass based on the line velocities and disk incli-The ring-like system is probably characterized by non-circular motions; a "figure-of-8" pattern observed in the H 2 position-velocity diagram might provide kinematical evidence for the presence of a nuclear bar.

We report new HST WFPC2 and NICMOS observations of the center of the nearest radio galaxy Centaurus A (NGC 5128) and discuss their implications for our understanding of the active nucleus and jet. We detect the active nucleus in the near-IR (K and H) and, for the first time, in the optical (I and V), deriving the spectral energy distribution of the nucleus from the radio to X-rays. The optical and part of the near-IR emission can be explained by the extrapolation of the X-ray power law reddened by A_V~14mag, a value consistent with other independent estimates. The 20pc-scale nuclear disk discovered by Schreier et al. (1998) is detected in the [FeII] 1.64mic line and presents a morphology similar to that observed in Pa alpha with a [FeII]/Pa alpha ratio typical of low ionization Seyfert galaxies and LINERs. NICMOS 3 Pa alpha observations in a 50"x50" circumnuclear region suggest enhanced star formation (~0.3Msun/yr) at the edges of the putative bar seen with ISO, perhaps due to shocks driven into the gas. The light profile, reconstructed from V, H and K observations, shows that Centaurus A has a core profile with a resolved break at ~4" and suggests a black--hole mass of ~10^9 Msun. A linear blue structure aligned with the radio/X-ray jet may indicate a channel of relatively low reddening in which dust has been swept away by the jet.Comment: 19 pages, 13 figures, Astrophysical Journal, in press. High quality figures available at http://www.arcetri.astro.it/~marconi/colpic.htm

We present gaseous and stellar kinematics for 14 gas-rich early-type galaxies. Half of the sample exhibit irregular gaseous velocity proÐles ; gas/star counter-rotation is visible in Ðve galaxies. We also Ðnd Ðve counter-rotating stellar cores, while Ðve more galaxies display inner components kinematically decoupled from the main stellar body. We interpret our results as an indication that the ionized gas is of external origin, is generally not in equilibrium, and may have been acquired recently. The merging or accretion events that brought the gas into the galaxy have also a †ected the stellar kinematics.

We report on HST and Palomar optical images of the field of GRB 990123, obtained on 8 and 9 February 1999. We find that the optical transient (OT) associated with GRB 990123 is located on an irregular galaxy, with magnitude V = 24.20 ± 0.15. The strong metal absorption lines seen in the spectrum of the OT, along with the low probability of a chance superposition, lead us to 12 Alfred P. Sloan Research Fellow 13 Hubble Fellow -2conclude that this galaxy is the host of the GRB. The OT is projected within the ∼ 1 ′′ visible stellar field of the host, nearer the edge than the center. We cannot, on this basis, rule out the galactic nucleus as the site of the GRB, since the unusual morphology of the host may be the result of an ongoing galactic merger, but our demonstration that this host galaxy has extremely blue optical to infrared colors more strongly supports an association between GRBs and star formation. We find that the OT magnitude on 1999 Feb 9.05, V = 25.45 ± 0.15, is about 1.5 mag fainter than expected from extrapolation of the decay rate found in earlier observations. A detailed analysis of the OT light curve suggests that its fading has gone through three distinct phases: an early rapid decline (f ν ∝ t −1.6 for t < 0.1 days), a slower intermediate decline power-law decay (f ν ∝ t −1.1 for 0.1 < t < 2 days), and then a more rapid decay (at least as steep as f ν ∝ t −1.8 for t > 2 days). The break to steeper slope at late times may provide evidence that the optical emission from this GRB was highly beamed.

We present new HST Space Telescope Imaging Spectrograph observations of the nearby radio galaxy NGC 5128 (Centaurus A). The bright emission line with longest wavelength accessible from HST, [S III]λ9533 Å, was used to study the kinematics of the ionized gas in the nuclear region with a 0. 1 spatial resolution. The STIS data were analized in conjunction with the ground-based near-infrared Very Large Telescope ISAAC spectra used by Marconi et al. (2001, ApJ, 549, 915) to infer the presence of a supermassive black hole and measure its mass. The two sets of data have spatial resolutions differing by almost a factor of five but provide independent and consistent measures of the BH mass, which are in agreement with our previous estimate based on the ISAAC data alone. The gas kinematical analysis provides a mass of M BH = (1.1 ± 0.1) × 10 8 M for an assumed disk inclination of i = 25 deg or M BH = (6.5 ± 0.7) × 10 7 M for i = 35 deg, the largest i value allowed by the data. We performed a detailed analysis of the effects on M BH of the intrinsic surface brightness distribution of the emission line, a crucial ingredient in the gas kinematical analysis. We estimate that the associated systematic errors are no larger than 0.08 in log M BH , comparable with statistical errors and indicating that the method is robust. However, the intrinsic surface brightness distribution has a large impact on the value of the gas velocity dispersion. A mismatch between the observed and model velocity dispersion is not necessarily an indication of non-circular motions or kinematically hot gas, but is as easily due to an inaccurate computation arising from too course a model grid, or the adoption of an intrinsic brightness distribution which is too smooth. The observed velocity dispersion in our spectra can be matched with a circularly rotating disk and also the observed line profiles and the higher order moments in the Hermite expansion of the line profiles, h 3 and h 4 , are consistent with emission from such a disk. To our knowledge, Centaurus A is the first external galaxy for which reliable BH mass measurements from gas and stellar dynamics are available and, as in the case of the Galactic Center, the M BH gas kinematical estimate is in good agreement with that from stellar dynamics. The BH mass in Centaurus A is in excellent agreement with the correlation with infrared luminosity and mass of the host spheroid but is a factor ∼2−4 above the one with the stellar velocity dispersion. But this disagreement is not large if one takes into account the intrinsic scatter of the M BH − σ e correlation. Finally, the high HST spatial resolution allows us to constrain the size of any cluster of dark objects alternative to a BH to r • < 0. 035 ( 0.6 pc). Thus Centaurus A ranks among the best cases for supermassive Black Holes in galactic nuclei.