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Register a new userSpectroastrometry of rotating gas disks for the detection of supermassive black holes in galactic nuclei. I. Method and simulations
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A. Gnerucci
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This is the first in a series of papers in which we study the application of spectroastrometry in the context of gas kinematical studies aimed at measuring the mass of supermassive black holes. The spectroastrometrical method consists in measuring the photocenter of light emission in different wavelength or velocity channels. In particular we explore the potential of spectroastrometry of gas emission lines in galaxy nuclei to constrain the kinematics of rotating gas disks and to measure the mass of putative supermassive black holes. By means of detailed simulations and test cases, we show that the fundamental advantage of spectroastrometry is that it can provide information on the gravitational potential of a galaxy on scales significantly smaller (~ 1/10) than the limit imposed by the spatial resolution of the observations. We then describe a simple method to infer detailed kinematical informations from spectroastrometry in longslit spectra and to measure the mass of nuclear mass concentrations. Such method can be applied straightforwardly to integral field spectra, which do not have the complexities due to a partial spatial covering of the source in the case of longslit spectra.
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We present new CRIRES spectroscopic observations of BrGamma in the nuclear region of the Circinus galaxy, obtained with the aim of measuring the black hole (BH) mass with the spectroastrometric technique. The Circinus galaxy is an ideal benchmark for the spectroastrometric technique given its proximity and secure BH measurement obtained with the observation of its nuclear H2O maser disk. The kinematical data have been analyzed both with the classical method based on the analysis of the rotation curves and with the new method developed by us and based on spectroastrometry. The classical method indicates that the gas disk rotates in the gravitational potential of an extended stellar mass distribution and a spatially unresolved mass of (1.7 +- 0.2) 10^7 Msun, concentrated within r < 7 pc. The new method is capable of probing gas rotation at scales which are a factor ~3.5 smaller than those probed by the rotation curve analysis. The dynamical mass spatially unresolved with the spectroastrometric method is a factor ~2 smaller, 7.9 (+1.4 -1.1) 10^6 Msun indicating that spectroastrometry has been able to spatially resolve the nuclear mass distribution down to 2 pc scales. This unresolved mass is still a factor ~4.5 larger than the BH mass measurement obtained with the H2O maser emission indicating that it has not been possible to resolve the sphere of influence of the BH. Based on literature data, this spatially unresolved dynamical mass distribution is likely dominated by molecular gas and it has been tentatively identified with the circum-nuclear torus which prevents a direct view of the central BH in Circinus. This mass distribution, with a size of ~2pc, is similar in shape to that of the star cluster of the Milky Way suggesting that a molecular torus, forming stars at a high rate, might be the earlier evolutionary stage of the nuclear star clusters which are common in late type spirals.
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 ionized gas is then log(MBH (sin i)^2/Modot)=7.5 pm 0.1 which corresponds to MBH = 9.6(+2.5-1.8) times 10^7 Modot for an assumed disk inclination of i = 35deg. 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 MBH range to new domains which are currently not accessible: smaller BHs in the local universe and similar BHs in more distant galaxies.
(Abridged) We present a method that tracks the growth of supermassive black holes (BHs) and the feedback from AGN in cosmological simulations. Our model is a substantially modified version of the one by Springel et al. (2005). Because cosmological simulations lack both the resolution and the physics to model the multiphase interstellar medium, they tend to strongly underestimate the Bondi-Hoyle accretion rate. To allow low-mass BHs to grow, it is therefore necessary to increase the predicted Bondi-Hoyle rates in dense gas by large, ad-hoc factors. We explore the physical regimes where the use of such factors is reasonable, and through this introduce a new prescription for gas accretion. Feedback from AGN is modeled by coupling a fraction of the rest-mass energy of the accreted gas thermally into the surrounding medium. We describe the implementation as well as the limitations of the model and motivate all the changes relative to previous work. We investigate the robustness of the predictions for the cosmic star formation history, the redshift zero cosmic BH density, BH scaling relations, and galaxy specific star formation rates. We find that the freedom introduced by the need to increase the predicted accretion rates, the standard procedure in the literature, is the most significant source of uncertainty. Our simulations demonstrate that supermassive BHs are able to regulate their growth by releasing a fixed amount of energy for a given halo mass, independent of the assumed efficiency of AGN feedback, which sets the normalization of the BH scaling relations. Regardless of whether BH seeds are initially placed above or below the BH scaling relations they grow onto the same relations. AGN feedback efficiently suppresses star formation in high-mass galaxies.
Interaction of a binary supermassive black hole with stars in a galactic nucleus can result in changes to all the elements of the binarys orbit, including the angles that define its orientation. If the nucleus is rotating, the orientation changes can be large, causing large changes in the binarys orbital eccentricity as well. We present a general treatment of this problem based on the Fokker-Planck equation for f, defined as the probability distribution for the binarys orbital elements. First- and second-order diffusion coefficients are derived for the orbital elements of the binary using numerical scattering experiments, and analytic approximations are presented for some of these coefficients. Solutions of the Fokker-Planck equation are then derived under various assumptions about the initial rotational state of the nucleus and the binary hardening rate. We find that the evolution of the orbital elements can become qualitatively different when we introduce nuclear rotation: 1) the orientation of the binarys orbit evolves toward alignment with the plane of rotation of the nucleus; 2) binary orbital eccentricity decreases for aligned binaries and increases for counter-aligned ones. We find that the diffusive (random-walk) component of a binarys evolution is small in nuclei with non-negligible rotation, and we derive the time-evolution equations for the semimajor axis, eccentricity and inclination in that approximation. The aforementioned effects could influence gravitational wave production as well as the relative orientation of host galaxies and radio jets.
The spin angular momentum S of a supermassive black hole (SBH) precesses due to torques from orbiting stars, and the stellar orbits precess due to dragging of inertial frames by the spinning hole. We solve the coupled post-Newtonian equations describing the joint evolution of S and the stellar angular momenta Lj, j = 1...N in spherical, rotating nuclear star clusters. In the absence of gravitational interactions between the stars, two evolutionary modes are found: (1) nearly uniform precession of S about the total angular momentum vector of the system; (2) damped precession, leading, in less than one precessional period, to alignment of S with the angular momentum of the rotating cluster. Beyond a certain distance from the SBH, the time scale for angular momentum changes due to gravitational encounters between the stars is shorter than spin-orbit precession times. We present a model, based on the Ornstein-Uhlenbeck equation, for the stochastic evolution of star clusters due to gravitational encounters and use it to evaluate the evolution of S in nuclei where changes in the Lj are due to frame dragging close to the SBH and to encounters farther out. Long-term evolution in this case is well described as uniform precession of the SBH about the clusters rotational axis, with an increasingly important stochastic contribution when SBH masses are small. Spin precessional periods are predicted to be strongly dependent on nuclear properties, but typical values are 10-100 Myr for low-mass SBHs in dense nuclei, 100 Myr - 10 Gyr for intermediate mass SBHs, and > 10 Gyr for the most massive SBHs. We compare the evolution of SBH spins in stellar nuclei to the case of torquing by an inclined, gaseous accretion disk.
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