Keywords

There is growing evidence of star formation in the vicinity of supermassive black holes (SMBHs) in galactic nuclei. A viable scenario for this process assumes infall of a massive gas cloud toward the SMBH and subsequent formation of a dense accretion disk, which gives birth to the young stars. Numerical hydrodynamical models indicate that this star formation process is rather fast and precedes full circularization of the accretion flow, i.e., the new stars are born on elliptic orbits. By means of direct numerical N-body modeling, we show in this paper that the nonzero eccentricity of the stellar disks around the SMBH leads to an onset of various types of the Kozai–Lidov oscillations of a non-negligible subset of individual orbits in the disk, showing a remarkable robustness of this classical mechanism. Among others, we demonstrate that under certain circumstances, the presence of an additional spherical cluster (which is generally known to damp Kozai–Lidov oscillations) may trigger such oscillations as a result of affecting the internal flow of the angular momentum through the disk. We conclude that the Kozai–Lidov oscillations are capable of substantially modifying the initial structure of the disk (its thickness and distribution of eccentricities, in particular).

We report the detection of the Galactic nuclear disk in line-of-sight kinematics of stars, measured with infrared spectroscopy from the Apache Point Galactic Evolution Experiment. This stellar component of the nuclear disk has an extent and rotation velocity V ∼ $120\;\mathrm{km}\;{{\rm{s}}}^{-1}$ comparable to the gas disk in the central molecular zone. The current data suggest that this disk is kinematically cool and has a small vertical extent of the order of 50 pc. The stellar kinematics suggest a truncation radius/steep decline of the stellar disk at a galactocentric radius R ∼ 150 pc and provide tentative evidence for an overdensity at the position of the ring found in the molecular gas disk.

In a previous work, we have shown that the formation of Fermi bubbles can be due to the interaction between winds launched from the hot accretion flow in Sgr A* and the interstellar medium (ISM). In that work, we focus only on the morphology. In this paper we continue our study by calculating the gamma-ray radiation. Some cosmic-ray protons (CRp) and electrons (CRe) must be contained in the winds, which are likely formed by physical processes such as magnetic reconnection. We have performed MHD simulations to study the spatial distribution of CRp, considering the advection and diffusion of CRp in the presence of magnetic field. We find that a permeated zone is formed just outside of the contact discontinuity between winds and the ISM, where the collisions between CRp and thermal nuclei mainly occur. The decay of neutral pions generated in the collisions, combined with the inverse Compton scattering of background soft photons by the secondary leptons generated in the collisions and primary CRe, can well explain the observed gamma-ray spectral energy distribution. Other features such as the uniform surface brightness along the latitude and the boundary width of the bubbles are also explained. The advantage of this "accretion wind" model is that the adopted wind properties come from the detailed small-scale MHD numerical simulation of accretion flows and the value of mass accretion rate has independent observational evidences. The success of the model suggests that we may seriously consider the possibility that cavities and bubbles observed in other contexts such as galaxy clusters may be formed by winds rather than jets.

S-stars, discovered in the vicinity of the massive black hole (MBH) in the Galactic center (GC), are anticipated to provide unique dynamical constraints on the MBH spin and metric, in addition to the mass. In this paper, we develop a fast full general relativistic method to simultaneously constrain the MBH mass, spin, and spin direction by considering both the orbital motion of a star close to the GC MBH and the propagation of photons from the star to a distant observer. Based on the current observations and dynamical model predictions, we assume six example stars with different semimajor axes (${a}_{\mathrm{orb}}$) and eccentricities (${e}_{\mathrm{orb}}$) and numerically calculate their projected trajectories in the sky plane and redshift curves. Two of those stars are set to have orbital configurations similar to that of S0-2/S2 and S0-102. We find that the spin-induced effects on the projected trajectory and redshift curve of a given star, including the leading term by the Lense–Thirring precession and the frame dragging, and the high-order precession due to the quadruple moment, depend on both the absolute value and the direction of the spin. The maximum values of the spin-induced position displacement and the redshift differences of the star over a full orbit may differ by a factor of several to more than one order of magnitude for two cases with significantly different spin directions. The dependence patterns of the position displacements and redshift differences on the spin direction are different, and thus the position and the redshift data are complementary for constraining the MBH spin and its direction. Adopting the Markov Chain Monte Carlo fitting technique, we illustrate that the spin of the GC MBH is likely to be well constrained by using the motion of S0-2/S2 over a period of ∼45 years if the spin is close to one and if the astrometric and spectroscopic precisions can be as high as $({\sigma }_{{\rm{p}}},{\sigma }_{Z})\sim (10\;\mu \mathrm{as},1\;\mathrm{km}\;{{\rm{s}}}^{-1})$, which is expected to be realized by future facilities like GRAVITY on the Very Large Telescope Interferometer, the thirty meter telescope, and the European extremely large telescope. If ${\sigma }_{{\rm{p}}}$ and ${\sigma }_{Z}$ can be further improved by a factor of several, the MBH spin can be well constrained by monitoring S0-2/S2 over a period of ∼15 years. In the mean time, the distance from the sun to the GC and the MBH mass can also be constrained to an unprecedented accuracy (0.01%–0.1%). If there exists a star with a semimajor axis that is a few times smaller, and eccentricity larger, than those of S0-2/S2, the MBH spin and its direction can be constrained with high accuracy over a period of $\lesssim 10$ years by future facilities, even if the spin is only moderately large. Our results suggest that long-term monitoring of the motions of stars in the vicinity of the GC MBH by the next generation facilities is likely to provide a dynamical test, for the first time, to the spin and metric of the GC MBH.

At radio wavelengths, scattering in the interstellar medium distorts the appearance of astronomical sources. Averaged over a scattering ensemble, the result is a blurred image of the source. However, Narayan & Goodman and Goodman & Narayan showed that for an incomplete average, scattering introduces refractive substructure in the image of a point source that is both persistent and wideband. We show that this substructure is quenched but not smoothed by an extended source. As a result, when the scatter-broadening is comparable to or exceeds the unscattered source size, the scattering can introduce spurious compact features into images. In addition, we derive efficient strategies to numerically compute realistic scattered images, and we present characteristic examples from simulations. Our results show that refractive substructure is an important consideration for ongoing missions at the highest angular resolutions, and we discuss specific implications for RadioAstron and the Event Horizon Telescope.

Nuclear stellar cluster (NSCs) are known to exist around massive black holes (MBHs) in galactic nuclei. Two formation scenarios were suggested for their origin: (1) buildup of NSCs from consecutive infall of stellar clusters and (2) continuous in situ star formation. Though the cluster infall scenario has been extensively studied, the in situ formation scenario has been hardly explored. Here we use Fokker–Planck (FP) calculations to study the effects of star formation on the buildup of NSCs and its implications for their long-term evolution and their resulting structure. We use the FP equation to describe the evolution of stellar populations and add appropriate source terms to account for the effects of newly formed stars. We show that continuous star formation even 1–2 pc away from the MBH can lead to the buildup of an NSC with properties similar to those of the Milky Way NSC. We find that the structure of the old stellar population in the NSC with in situ star formation could be very similar to the steady-state Bahcall–Wolf cuspy structure. However, its younger populations do not yet achieve a steady state. In particular, formed/evolved NSCs with in situ star formation contain differential age-segregated stellar populations that are not yet fully mixed. Younger stellar populations formed in the outer regions of the NSC have a cuspy structure toward the NSC outskirts, while showing a core-like distribution inward, with younger populations having larger core sizes. In principal, such a structure can give rise to an apparent core-like radial distribution of younger stars, as observed in the Galactic center.

We report new observations of the Galactic Center source G2 from the W. M. Keck Observatory. G2 is a dusty red object associated with gas that shows tidal interactions as it nears its closest approach with the Galaxy's central black hole. Our observations, conducted as G2 passed through periapse, were designed to test the proposal that G2 is a 3 Earth mass gas cloud. Such a cloud should be tidally disrupted during periapse passage. The data were obtained using the Keck II laser guide star adaptive optics system (LGSAO) and the facility near-infrared camera (NIRC2) through the K' [2.1 μm] and L' [3.8 μm] broadband filters. Several results emerge from these observations: (1) G2 has survived its closest approach to the black hole as a compact, unresolved source at L', (2) G2's L' brightness measurements are consistent with those over the last decade, (3) G2's motion continues to be consistent with a Keplerian model. These results rule out G2 as a pure gas cloud and imply that G2 has a central star. This star has a luminosity of ∼30 L_☉ and is surrounded by a large (∼2.6 AU) optically thick dust shell. The differences between the L' and Br-γ observations can be understood with a model in which L' and Br-γ emission arises primarily from internal and external heating, respectively. We suggest that G2 is a binary star merger product and will ultimately appear similar to the B-stars that are tightly clustered around the black hole (the so-called S-star cluster).

We have detected substructure within the smooth scattering disk of the celebrated Galactic center radio source Sagittarius A* (Sgr A*). We observed this structure at 1.3 cm wavelength with the Very Long Baseline Array together with the Green Bank Telescope, on baselines of up to 3000 km, long enough to completely resolve the average scattering disk. Such structure is predicted theoretically as a consequence of refraction by large-scale plasma fluctuations in the interstellar medium. Along with the much-studied θ_d∝λ² scaling of angular broadening θ_d with observing wavelength λ, our observations indicate that the spectrum of interstellar turbulence is shallow with an inner scale larger than 300 km. The substructure is consistent with an intrinsic size of about 1 mas at 1.3 cm wavelength, as inferred from deconvolution of the average scattering. Further observations of the substructure can set stronger constraints on the properties of scattering material and on the intrinsic size of Sgr A*. These constraints will guide our understanding of the effects of scatter broadening and the emission physics near the black hole in images with the Event Horizon Telescope at millimeter wavelengths.

A pair of giant gamma-ray Bubbles has been revealed by Fermi-LAT. In this paper we investigate their formation mechanism. Observations have indicated that the activity of the supermassive black hole located at the Galactic center, Sgr A*, was much stronger than at the present time. Specifically, one possibility is that while Sgr A* was also in the hot accretion regime, the accretion rate should be 10³–10⁴ times higher during the past ∼10⁷ yr. On the other hand, recent magnetohydrodynamic numerical simulations of hot accretion flows have unambiguously shown the existence and obtained the properties of strong winds. Based on this knowledge, by performing three-dimensional hydrodynamical simulations, we show in this paper that the Fermi Bubbles could be inflated by winds launched from the "past" hot accretion flow in Sgr A*. In our model, the active phase of Sgr A* is required to last for about 10 million years and it was quenched no more than 0.2 million years ago. The central molecular zone (CMZ) is included and it collimates the wind orientation toward the Galactic poles. Viscosity suppresses the Rayleigh–Taylor and Kelvin–Helmholtz instabilities and results in the smoothness of the Bubbles edge. The main observational features of the Bubbles can be well explained. Specifically, the ROSAT X-ray features are interpreted by the shocked interstellar medium and the interaction region between the wind and CMZ gas. The thermal pressure and temperature obtained in our model are consistent with recent Suzaku observations.

The center of our Galaxy hosts almost two hundred very young stars, a subset of which is orbiting the central supermassive black hole (SMBH) in a relatively thin disk-like structure. First analyses indicated a power-law surface density profile of the disk, Σ∝R^β with β = −2. Recently, however, doubts about this profile arose. In particular, it now seems to be better described by a sort of broken power law. By means of both analytical arguments and numerical N-body modeling, we show that such a broken power-law profile is a natural consequence of the two-body relaxation of the disk. Due to the small relative velocities of the nearby stars in co-planar Keplerian orbits around the SMBH, two-body relaxation is effective enough to affect the evolution of the disk on timescales comparable to its estimated age. In the inner, densest part of the disk, the profile becomes rather flat (β ≈ −1) while the outer parts keep imprints of the initial state. Our numerical models show that the observed projected surface density profile of the young stellar disk can result from two-body relaxation driven evolution of a disk with initial single power-law profile with −2 ≲ β ≲ −1.5. In addition, we suggest that two-body relaxation may have caused a significant radial migration of the S-stars toward the central SMBH, thus playing an important role in their formation scenario.