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The broadening of atomic emission lines by high-velocity motion of gas near accreting supermassive black holes is an observational hallmark of quasars 1 . Observations of broad emission lines could potentially constrain the mechanism for transporting gas inwards through accretion disks or outwards through winds 2 . The size of regions for which broad emission lines are observed (broad-line regions) has been estimated by measuring the delay in light travel time between the variable brightness of the accretion disk  continuum and the emission lines 3 —a method known as reverberation mapping. In some models the emission lines arise from a continuous outflow 4 , whereas in others they arise from orbiting gas clouds 5 . Directly imaging such regions has not hitherto been possible because of their small angular size (less than 10 −4 arcseconds 3 , 6 ). Here we report a spatial offset (with a spatial resolution of 10 −5 arcseconds, or about 0.03 parsecs for a distance of 550 million parsecs) between the red and blue photo-centres of the broad Paschen-α line of the quasar 3C 273 perpendicular to the direction of its radio jet. This spatial offset corresponds to a gradient in the velocity of the gas and thus implies that the gas is orbiting the central supermassive black hole. The data are well fitted by a broad-line-region model of a thick disk of gravitationally bound material orbiting a black hole of 3 × 10 8 solar masses. We infer a disk radius of 150 light days; a radius of 100–400 light days was found previously using reverberation mapping 7 – 9 . The rotation axis of the disk aligns in inclination and position angle with the radio jet. Our results support the methods that are often used to estimate the masses of accreting supermassive black holes and to study their evolution over cosmic time. High-angular-resolution observations of the quasar 3C 273 reveal that it has a relatively small but thick disk, viewed nearly face-on, in which material is orbiting the central supermassive black hole.

The cool gas in the central parsec of the Galaxy is organized in the surrounding circumnuclear disk, made of neutral gas, and the internal minispiral, composed of dust and ionized gas. In order to study the transition between them we have investigated the presence of H2 neutral gas in this area, through NIR spectro-imaging data observed with SPIFFI. To preserve the spatial resolution we implemented a new method consisting of a regularized 3D fit. We concentrated on the supposedly fully ionized central cavity and the very inner edge of the CND. H2 is detected everywhere: at the boundary of the CND and in the central cavity, where it seems to split in two components, one in the background of the minispiral and one inside the Northern arm.

Owing to their similarities with giant exoplanets, brown dwarf companions of stars provide insights into the fundamental processes of planet formation and evolution. From their orbits, several brown dwarf companions are found to be more massive than theoretical predictions given their luminosities and the ages of their host stars 1 – 3 . Either the theory is incomplete or these objects are not single entities. For example, they could be two brown dwarfs each with a lower mass and intrinsic luminosity 1 , 4 . The most problematic example is Gliese 229 B (refs.  5 , 6 ), which is at least 2–6 times less luminous than model predictions given its dynamical mass of 71.4 ± 0.6 Jupiter masses ( M Jup ) (ref.  1 ). We observed Gliese 229 B with the GRAVITY interferometer and, separately, the CRIRES+ spectrograph at the Very Large Telescope. Both sets of observations independently resolve Gliese 229 B into two components, Gliese 229 Ba and Bb, settling the conflict between theory and observations. The two objects have a flux ratio of 0.47 ± 0.03 at a wavelength of 2 μm and masses of 38.1 ± 1.0 and 34.4 ± 1.5  M Jup , respectively. They orbit each other every 12.1 days with a semimajor axis of 0.042 astronomical units ( au ). The discovery of Gliese 229 BaBb, each only a few times more massive than the most massive planets, and separated by 16 times the Earth–moon distance, raises new questions about the formation and prevalence of tight binary brown dwarfs around stars. Analysis of the cool brown dwarf Gliese 229 B suggests that it is actually a close binary of two less massive brown dwarfs, explaining its low luminosity and settling the conflict between theoretical predictions and measurements.

The central pc around the super-massive black hole in the Galactic Center hosts a population of young and massive stars. The majority of these stars (outside the central 1 ) have been found to reside in disks. Here we develop a detailed statistical analysis method of the properties of these disks including a robust test of the significance of these disks versus an isotropic stellar population. We apply this method to the data set obtained with the AO assisted integral field spectrograph SINFONI on the ESO/VLT as of the end of 2007 and present preliminary results.

We report the definite spectroscopic identification of ≃ 40 OB supergiants, giants and main sequence stars in the central parsec of the Galaxy. Detection of their absorption lines have become possible with the high spatial and spectral resolution and sensitivity of the adaptive optics integral Held spectrometer SPIFFI/SINFONI on the ESO VLT. Several of these OB stars appear to be helium and nitrogen rich. Almost all of the ≃80 massive stars now known in the central parsec (central arcsecond excluded) reside in one of two somewhat thick (⟨|/R⟩ ≃ 0.14) rotating disks. These stellar disks have fairly sharp inner edges (R ≃ 1′′) and surface density profiles that scale as R−2. We do not detect any OB stars outside the central 0.5 pc. The majority of the stars in the clockwise system appear to be on almost circular orbits, whereas most of those in the counter-clockwise disk appear to be on eccentric orbits. Based on its stellar surface density distribution and dynamics we propose that IRS 13E is an extremely dense cluster (ρcore ≳ 3 × 108M⊙ pc−3), which has formed in the counter-clockwise disk. The stellar contents of both systems are remarkably similar, indicating a common age of ≃ 6±2 Myr. The K-band luminosity function of the massive stars suggests a top-heavy mass function and limits the total stellar mass contained in both disks to ≃ 1.5 × 104 M⊙. Our data strongly favor in situ star formation from dense gas accretion disks for the two stellar disks. This conclusion is very clear for the clockwise disk and highly plausible for the counter-clockwise system.

We have observed a bright flare of Sgr A* in the near infrared with the adaptive optics assisted integral field spectrometer SINFONI. Within the uncertainties, the observed spectrum is featureless and can be described by a power law. The associated power law index is subject to systematic effects, namely the determination of the background level. We explore these effects and can show that while the absolute value of the spectral power law index is relatively uncertain, our data nevertheless suggest that the spectral index is correlated with the instantaneous flix. Both quantities experience signifbant changes within less than one hour. We argue that the near infrared flares from Sgr A* are due to synchrotron emission of transiently heated electrons, the emission being affected by orbital dynamics and synchrotron cooling, both acting on timescales of ≈20 minutes.

In this article we present and discuss the latest results from the observations of stars ( S-stars ) orbiting Sgr A*. With improving data quality the number of observed S-stars has increased substantially in the last years. The combination of radial velocity and proper motion information allows an ever more precise determination of orbital parameters and of the mass of and the distance to the supermassive black hole in the centre of the Milky Way. Additionally, the orbital solutions allow us to verify an agreement between the NIR source Sgr A*and the dynamical centre of the stellar orbits to within 2 mas.

We present the adaptive optics assisted, near-infrared VLTI instrument GRAVITY for precision narrow-angle astrometry and interferometric phase referenced imaging of faint objects. With its two fibers per telescope beam, its internal wavefront sensors and fringe tracker, and a novel metrology concept, GRAVITY will not only push the sensitivity far beyond what is offered today, but will also advance the astrometric accuracy for UTs to 10 μas. GRAVITY is designed to work with four telescopes, thus providing phase referenced imaging and astrometry for 6 baselines simultaneously. Its unique capabilities and sensitivity will open a new window for the observation of a wide range of objects, and — amongst others — will allow the study of motion within a few times the event horizon size of the Galactic Center black hole.

The EHT collaboration released in 2019 the first horizon-scale images of a black hole accretion flow, opening a novel route for plasma physics comprehension and gravitational tests. Although the present unresolved images deeply depend on the astrophysical properties of the accreted matter, GR predicts that they contain highly lensed observables, the \"photon rings\", embodying the effects of strong-field gravity. Focusing on the supermassive black hole M87* and adopting a geometrically thin, equatorial disc as a phenomenological configuration for the accreting matter, our goal is to study the degeneracy of spacetime curvature and of physically-motivated emission processes on EHT-like images observed at 230 and 345 GHz. In a parametric framework, we simulate adaptively ray-traced images using GYOTO in various spherically-symmetric spacetime geometries, for a comprehensive class of disc velocities and realistic synchrotron emission profiles. We then extract the width and the peak position of 1D intensity cross sections on the direct image and the first photon ring. We show that, among the investigated quantities, the most appropriate observables to probe the geometry are the peak positions of the first photon ring. Small geometric deviations can be unequivocally detected, regardless of the motion of the disc, ranging from a Keplerian to a radially infalling one, if the black hole mass-to-distance estimate is accurate up to around 2%; the current uncertainty of 11% being sufficient just to access extreme deviations. The equatorial set-up of this paper, favoured by present EHT observations of M87*, is adapted to model future measurements at higher observing frequencies, where absorption effects are negligible, and with higher resolution, indispensable to resolve the photon rings. Additional work is needed to investigate if our conclusions hold for more realistic disc configurations.

Measuring the astrometric and spectroscopic data of stars orbiting the central black hole in our galaxy (Sgr A*) offers a promising way to measure relativistic effects. In principle, the \"no-hair\" theorem can be tested at the Galactic Center by monitoring the orbital precession of S-stars due to the angular momentum (spin) and quadrupole moment of Sgr A*. Closer-in stars, more strongly affected by the black hole's rotation, may be required. GRAVITY+ could detect such stars that are currently too faint for GRAVITY. We aim to analytically and numerically characterize orbital reorientations induced by spin-related effects of Sgr A* up to the second post-Newtonian (2PN) order. We use the two-timescale method to derive the 2PN analytical expressions of the secular evolution of the orbital parameters that are related to the observer. To study the interaction between the orbital and spin orientations, we introduce observer-independent quantities that offer insight into the Kerr geometry. We also use the post-Newtonian code OOGRE to simulate hypothetical stars orbiting closer to Sgr A*, where spin and quadrupole effects are stronger. This enables comparison with our analytical predictions. We exhibit three orbital-timescale precession rates that encode the in-plane pericenter shift and the out-of-plane redirection of the osculating ellipse. We provide the 2PN expressions of these precession rates and express the orbit-integrated associated angular shifts of the pericenter and of the ellipse axes. We relate these orbital-timescale precession rates to the secular-timescale precession of the orbital angular momentum around the black hole spin axis. We consider that the theoretical insight we provide in this article will be useful in constraining the spin effect of Sgr A* with GRAVITY+ observations.