Stars orbiting the compact radio source Sgr A* in the Galactic Center serve as precision probes of the gravitational field around the closest massive black hole. In addition to adaptive ...optics-assisted astrometry (with NACO/VLT) and spectroscopy (with SINFONI/VLT, NIRC2/Keck and GNIRS/Gemini) over three decades, we have obtained 30–100 μas astrometry since 2017 with the four-telescope interferometric beam combiner GRAVITY/VLTI, capable of reaching a sensitivity of
m
K
= 20 when combining data from one night. We present the simultaneous detection of several stars within the diffraction limit of a single telescope, illustrating the power of interferometry in the field. The new data for the stars S2, S29, S38, and S55 yield significant accelerations between March and July 2021, as these stars pass the pericenters of their orbits between 2018 and 2023. This allows for a high-precision determination of the gravitational potential around Sgr A*. Our data are in excellent agreement with general relativity orbits around a single central point mass,
M
•
= 4.30 × 10
6
M
⊙
, with a precision of about ±0.25%. We improve the significance of our detection of the Schwarzschild precession in the S2 orbit to 7
σ
. Assuming plausible density profiles, the extended mass component inside the S2 apocenter (≈0.23″ or 2.4 × 10
4
R
S
) must be ≲3000
M
⊙
(1
σ
), or ≲0.1% of
M
•
. Adding the enclosed mass determinations from 13 stars orbiting Sgr A* at larger radii, the innermost radius at which the excess mass beyond Sgr A* is tentatively seen is
r
≈ 2.5″ ≥ 10× the apocenter of S2. This is in full harmony with the stellar mass distribution (including stellar-mass black holes) obtained from the spatially resolved luminosity function.
GRAVITY is a new instrument to coherently combine the light of the European Southern Observatory Very Large Telescope Interferometer to form a telescope with an equivalent 130 m diameter angular ...resolution and a collecting area of 200 m2. The instrument comprises fiber fed integrated optics beam combination, high resolution spectroscopy, built-in beam analysis and control, near-infrared wavefront sensing, phase-tracking, dual-beam operation, and laser metrology. GRAVITY opens up to optical/infrared interferometry the techniques of phase referenced imaging and narrow angle astrometry, in many aspects following the concepts of radio interferometry. This article gives an overview of GRAVITY and reports on the performance and the first astronomical observations during commissioning in 2015/16. We demonstrate phase-tracking on stars as faint as mK ≈ 10 mag, phase-referenced interferometry of objects fainter than mK ≈ 15 mag with a limiting magnitude of mK ≈ 17 mag, minute long coherent integrations, a visibility accuracy of better than 0.25%, and spectro-differential phase and closure phase accuracy better than 0.5°, corresponding to a differential astrometric precision of better than ten microarcseconds (μas). The dual-beam astrometry, measuring the phase difference of two objects with laser metrology, is still under commissioning. First observations show residuals as low as 50 μas when following objects over several months. We illustrate the instrument performance with the observations of archetypical objects for the different instrument modes. Examples include the Galactic center supermassive black hole and its fast orbiting star S2 for phase referenced dual-beam observations and infrared wavefront sensing, the high mass X-ray binary BP Cru and the active galactic nucleus of PDS 456 for a few μas spectro-differential astrometry, the T Tauri star S CrA for a spectro-differential visibility analysis, ξ Tel and 24 Cap for high accuracy visibility observations, and η Car for interferometric imaging with GRAVITY.
Using VLTI/GRAVITY and SINFONI data, we investigate the subparsec gas and dust structure around the nearby type 1 active galactic nucleus (AGN) hosted by NGC 3783. The
K
-band coverage of GRAVITY ...uniquely allows simultaneous analysis of the size and kinematics of the broad line region (BLR), the size and structure of the near-infrared(near-IR)-continuum-emitting hot dust, and the size of the coronal line region (CLR). We find the BLR, probed through broad Br
γ
emission, to be well described by a rotating, thick disc with a radial distribution of clouds peaking in the inner region. In our BLR model, the physical mean radius of 16 light-days is nearly twice the ten-day time-lag that would be measured, which closely matches the ten-day time-lag that has been measured by reverberation mapping. We measure a hot dust full-width at half-maximum (FWHM) size of 0.74 mas (0.14 pc) and further reconstruct an image of the hot dust, which reveals a faint (5% of the total flux) offset cloud that we interpret as an accreting or outflowing cloud heated by the central AGN. Finally, we directly measure the FWHM size of the nuclear CLR as traced by the Ca
VIII
and narrow Br
γ
line. We find a FWHM size of 2.2 mas (0.4 pc), fully in line with the expectation of the CLR located between the BLR and narrow line region. Combining all of these measurements together with larger scale near-IR integral field unit and mid-IR interferometry data, we are able to comprehensively map the structure and dynamics of gas and dust from 0.01 to 100 pc.
The GRAVITY instrument on the ESO VLTI pioneers the field of high-precision near-infrared interferometry by providing astrometry at the 10−100
μ
as level. Measurements at this high precision ...crucially depend on the control of systematic effects. We investigate how aberrations introduced by small optical imperfections along the path from the telescope to the detector affect the astrometry. We develop an analytical model that describes the effect of these aberrations on the measurement of complex visibilities. Our formalism accounts for pupil-plane and focal-plane aberrations, as well as for the interplay between static and turbulent aberrations, and it successfully reproduces calibration measurements of a binary star. The Galactic Center observations with GRAVITY in 2017 and 2018, when both Sgr A* and the star S2 were targeted in a single fiber pointing, are affected by these aberrations at a level lower than 0.5 mas. Removal of these effects brings the measurement in harmony with the dual-beam observations of 2019 and 2020, which are not affected by these aberrations. This also resolves the small systematic discrepancies between the derived distance
R
0
to the Galactic Center that were reported previously.
Context.
The central parsec of the Galaxy contains a young star cluster embedded in a complex interstellar medium. The latter mainly consists of a torus of dense clumps and streams of molecular gas ...(the circumnuclear disk) enclosing streamers of ionized gas (the Minispiral).
Aims.
In this complex environment, knowledge of the local extinction that locally affects each feature is crucial to properly study and disentangle them. We previously studied molecular gas in this region and inferred an extinction map from two H
2
lines. Extinction appears to be correlated with the dereddened flux in several contiguous areas in the field of view. Here, we discuss the origin of this local correlation.
Methods.
We model the observed effect with a simple radiative transfer model. H
2
emission arises from the surfaces of clumps (i.e., shells) that are exposed to the ambient ultraviolet (UV) radiation field. We consider the shell at the surface of an emitting clump. The shell has a varying optical depth and a screen of dust in front of it. The optical depth varies from one line of sight to another, either because of varying extinction coefficient from the shell of one clump to that of another or because of a varying number of identical clumps on the line of sight.
Results.
In both scenarios, the model accurately reproduces the dependence of molecular gas emission and extinction. The reason for this correlation is that, in the central parsec, the molecular gas is mixed everywhere with dust that locally affects the observed gas emission. In addition, there is extinction due to foreground (“screen”) dust.
Conclusions.
This analysis favors a scenario where the central parsec is filled with clumps of dust and molecular gas. Separating foreground from local extinction allows for a probe for local conditions (H
2
is mixed with dust) and can also constrain the three-dimensional (3D) position of objects under study.
Aims. We have investigated neutral gas in the central cavity of the circumnuclear disk (CND) at the Galactic center, where the ionized minispiral lies, to describe the H2 distribution and properties ...in this ionized environment. Methods. This study was carried out through a spectro-imaging data cube of the central cavity obtained with SPIFFI on the VLT. The observed field of view is 36″ × 29″, with a spectral resolution R = 1300 in the near-infrared. These observations cover several H2 lines. To preserve the spatial resolution and avoid edge effects, we applied a new line-fitting method that consists of a regularized 3D fitting. We also applied a more classical 1D fitting to compare the relative strength of the H2 lines. Results. We present high spatial and spectral resolution maps of the intensity, velocity, and width of five H2 lines and an extinction map derived from H2. Molecular gas is detected everywhere in the field. In particular, in addition to the known CND features, we detected an emission from the northern arm cloud and from the minicavity. The excitation diagrams allow us to estimate the temperature, mass, and density of these features. Conclusions. We interpret the CND emission as coming from a hot, thermalized, thin layer at the surface of the clouds. The observed H2 corresponds only to a small fraction of the total H2 mass. The emission remains fairly strong in the whole central cavity, but it is not thermalized. A strong deviation from thermal equilibrium is detected near the minicavity. We suggest that this emission is caused by constantly forming H2 that is destroyed again before it reaches ortho/para equilibrium.
Context.
The central region of NGC 1068 is one of the closest and most studied active galactic nuclei. It is known to be type 2, meaning that its accretion disk is obscured by a large amount of dust ...and gas. The main properties of the obscuring structure are still to be determined.
Aims.
We aim to model the inner edge of this structure, where the hot dust responsible for the near-infrared emission reaches its sublimation temperature.
Methods.
We used several methods to interpret the
K
-band interferometric observables from a GRAVITY/VLTI observation of the object. At first, we used simple geometrical models in image reconstructions to determine the main 2D geometrical properties of the source. In a second step, we tried to reproduce the observables with
K
-band images produced by 3D radiative transfer simulations of a heated dusty disk. We explore various parameters to find an optimal solution and a model consistent with all the observables.
Results.
The three methods are consistent in their description of the image of the source, an elongated structure with ∼4 × 6 mas dimensions and its major axis along the northwest–southeast direction. The results from all three methods suggest that the object resembles an elongated ring rather than an elongated thin disk, with the northeast edge appearing less luminous than the southwest one. The best 3D model is a thick disk with an inner radius
r
= 0.21
−0.03
+0.02
pc and a half-opening angle
α
1/2
= 21 ± 8° observed with an inclination
i
= 44
−6
10
° and PA = 150
−13
8
°. A high density of dust
n
= 5
−2.5
+5
M
⊙
pc
−3
is required to explain the contrast between the two edges by self-absorption from the closer one. The overall structure is itself obscured by a large foreground obscuration
A
V
∼ 75.
Conclusions.
The hot dust is not responsible for the obscuration of the central engine. The geometry and the orientation of the structure are different from those of the previously observed maser and molecular disks. We conclude that a single disk is unable to account for these differences, and favor a description of the source where multiple rings originating from different clouds are entangled around the central mass.
We report the detection of continuous positional and polarization changes of the compact source SgrA* in high states (“flares”) of its variable near-infrared emission with the near-infrared ...GRAVITY-Very Large Telescope Interferometer (VLTI) beam-combining instrument. In three prominent bright flares, the position centroids exhibit clockwise looped motion on the sky, on scales of typically 150 μas over a few tens of minutes, corresponding to about 30% the speed of light. At the same time, the flares exhibit continuous rotation of the polarization angle, with about the same 45(±15) min period as that of the centroid motions. Modelling with relativistic ray tracing shows that these findings are all consistent with a near face-on, circular orbit of a compact polarized “hot spot” of infrared synchrotron emission at approximately six to ten times the gravitational radius of a black hole of 4 million solar masses. This corresponds to the region just outside the innermost, stable, prograde circular orbit (ISCO) of a Schwarzschild–Kerr black hole, or near the retrograde ISCO of a highly spun-up Kerr hole. The polarization signature is consistent with orbital motion in a strong poloidal magnetic field.
Context. The central parsec of the Galaxy contains a young star cluster embedded in a complex interstellar medium. The latter mainly consists of a torus of dense clumps and streams of molecular gas ...(the circumnuclear disk) enclosing streamers of ionized gas (the Minispiral). Aims. In this complex environment, knowledge of the local extinction that locally affects each feature is crucial to properly study and disentangle them. We previously studied molecular gas in this region and inferred an extinction map from two H2 lines. Extinction appears to be correlated with the dereddened flux in several contiguous areas in the field of view. Here, we discuss the origin of this local correlation. Methods. We model the observed effect with a simple radiative transfer model. H2 emission arises from the surfaces of clumps (i.e., shells) that are exposed to the ambient ultraviolet (UV) radiation field. We consider the shell at the surface of an emitting clump. The shell has a varying optical depth and a screen of dust in front of it. The optical depth varies from one line of sight to another, either because of varying extinction coefficient from the shell of one clump to that of another or because of a varying number of identical clumps on the line of sight. Results. In both scenarios, the model accurately reproduces the dependence of molecular gas emission and extinction. The reason for this correlation is that, in the central parsec, the molecular gas is mixed everywhere with dust that locally affects the observed gas emission. In addition, there is extinction due to foreground (“screen”) dust. Conclusions. This analysis favors a scenario where the central parsec is filled with clumps of dust and molecular gas. Separating foreground from local extinction allows for a probe for local conditions (H2 is mixed with dust) and can also constrain the three-dimensional (3D) position of objects under study.
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