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We measure the black hole mass in the nearby active galaxy Centaurus A (NGC 5128) using a new method based on spectroastrometry of a rotating gas disk. The spectroastrometric approach consists in measuring the photocenter position of emission lines for different velocity channels. In a previous paper we focused on the basic methodology and the advantages of the spectroastrometric approach with a detailed set of simulations demonstrating the possibilities for black hole mass measurements going below the conventional spatial resolution. In this paper we apply the spectroastrometric method to multiple longslit and integral field near infrared spectroscopic observations of Centaurus A. We find that the application of the spectroastrometric method provides results perfectly consistent with the more complex classical method based on rotation curves: the measured BH mass is nearly independent of the observational setup and spatial resolution and the spectroastrometric method allows the gas dynamics to be probed down to spatial scales of  ~0.02″, i.e. 1/10 of the spatial resolution and  ~1/50 of BH sphere of influence radius. The best estimate for the BH mass based on kinematics of the ionised gas is then log (MBHsin i2/M⊙) ≃ 7.5 ± 0.1 which corresponds to MBH= 9.6-1.8+2.5× 107M⊙for an assumed disk inclination of i= 35°. The complementarity of this method with the classic rotation curve method will allow us to put constraints on the disk inclination which cannot be otherwise derived from spectroastrometry. With the application to Centaurus A, we have shown that spectroastrometry opens up the possibility of probing spatial scales smaller than the spatial resolution, extending the measured MBHrange to new domains which are currently not accessible: smaller BHs in the local universe and similar BHs in more distant galaxies.

Galaxy dynamical masses are important physical quantities to constrain galaxy evolutionary models, especially at high redshifts. However, at z≳ 2 the limited signal to noise ratio and spatial resolution of the data usually do not allow spatially resolved kinematical modeling and very often only virial masses can be estimated from line widths. But even such estimates require a good knowledge of galaxy size, which may be smaller than the spatial resolution. Spectroastrometry is a technique which combines spatial and spectral resolution to probe spatial scales significantly smaller than the spatial resolution of the observations. Here we apply it to the case of high-zgalaxies and present a method based on spectroastrometry to estimate dynamical masses of high zgalaxies, which overcomes the problem of size determination with poor spatial resolution. We construct and calibrate a “spectroastrometric” virial mass estimator, modifying the “classical” virial mass formula. We apply our method to the [O III] or Hαemission line detected in z~ 2−3 galaxies from AMAZE, LSD and SINS samples and we compare the spectroastrometric estimator with dynamical mass values resulting from full spatially resolved kinematical modeling. The spectroastrometric estimator is found to be a good approximation of dynamical masses, presenting a linear relation with a residual dispersion of only 0.15 dex. This is a big improvement compared to the “classical” virial mass estimator which has a non linear relation and much larger dispersion (0.47 dex) compared to dynamical masses. By applying our calibrated estimator to 16 galaxies from the AMAZE and LSD samples, we obtain masses in the  ~107−1010M⊙range extending the mass range attainable with dynamical modeling.

We present a SINFONI integral-field kinematical study of 33 galaxies at z~ 3 from the AMAZE and LSD projects, which are aimed at studying metallicity and dynamics of high-redshift galaxies. The number of galaxies analyzed in this paper constitutes a significant improvement over existing data in the literature, and this is the first time that a dynamical analysis is obtained for a relatively large sample of galaxies at z~ 3. Eleven galaxies show ordered rotational motions (~30% of the sample). In these cases we estimate dynamical masses by modeling the gas kinematics with rotating disks and exponential mass distributions. We find dynamical masses in the range 2 × 109M⊙− 2 × 1011M⊙with a mean value of  ~ 2 × 1010M⊙. By comparing observed gas velocity dispersion with what is expected from models, we find that most rotating objects are dynamically “hot”, with intrinsic velocity dispersions of  ~ 90 km s-1. The median value of the ratio between the maximum disk rotational velocity and the intrinsic velocity dispersion for the rotating objects is 1.6, much lower than observed in local galaxies value (~10) and slightly lower than the z~ 2 value (2–4). Finally we use the maximum rotational velocity from our modeling to build a baryonic Tully-Fisher relation at z~ 3. Our measurements indicate that z~ 3 galaxies have lower stellar masses (by a factor of ten on average) compared to local galaxies with the same dynamical mass. However, the large observed scatter suggests that the Tully-Fisher relation is not yet “in place” at these early cosmic ages, possibly owing to the young age of galaxies. A smaller dispersion of the Tully-Fisher relation is obtained by taking the velocity dispersion into account with the use of the S0.5indicator, suggesting that turbulent motions might play an important dynamical role.