First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way (2311.08680v2)

Abstract: We present the first Event Horizon Telescope (EHT) observations of Sagittarius A* (Sgr A$^*$), the Galactic center source associated with a supermassive black hole. These observations were conducted in 2017 using a global interferometric array of eight telescopes operating at a wavelength of $\lambda=1.3\,{\rm mm}$. The EHT data resolve a compact emission region with intrahour variability. A variety of imaging and modeling analyses all support an image that is dominated by a bright, thick ring with a diameter of $51.8 \pm 2.3$\,\uas (68\% credible interval). The ring has modest azimuthal brightness asymmetry and a comparatively dim interior. Using a large suite of numerical simulations, we demonstrate that the EHT images of Sgr A$^*$ are consistent with the expected appearance of a Kerr black hole with mass ${\sim}4 \times 10^6\,{\rm M}_\odot$, which is inferred to exist at this location based on previous infrared observations of individual stellar orbits as well as maser proper motion studies. Our model comparisons disfavor scenarios where the black hole is viewed at high inclination ($i > 50^\circ$), as well as non-spinning black holes and those with retrograde accretion disks. Our results provide direct evidence for the presence of a supermassive black hole at the center of the Milky Way galaxy, and for the first time we connect the predictions from dynamical measurements of stellar orbits on scales of $10^3-10^5$ gravitational radii to event horizon-scale images and variability. Furthermore, a comparison with the EHT results for the supermassive black hole M87$^*$ shows consistency with the predictions of general relativity spanning over three orders of magnitude in central mass.

• The paper presents the first EHT observations of Sagittarius A*, revealing a bright, circular shadow with a diameter of approximately 51.8±2.3 μas.
• It employs high-resolution VLBI imaging and GRMHD simulations to validate that the radiatively inefficient accretion flow matches Kerr black hole predictions.
• Findings strongly support general relativity by constraining the black hole's physical parameters while ruling out alternative non-physical models.

Testing the Black Hole Metric with the Event Horizon Telescope: Insights into Sagittarius A*

The paper "First Sagittarius A* Event Horizon Telescope Results: Testing the Black Hole Metric" by the EHT Collaboration addresses the observations of Sagittarius A* (\sgra) using the Event Horizon Telescope (EHT) and analyzes these observations to test the properties of the black hole metric at the heart of our galaxy. The EHT's 2017 observational campaign focused on revealing the structure and dynamics of the supermassive black hole at the center of the Milky Way by achieving unprecedented angular resolution through very long baseline interferometry (VLBI) techniques at millimeter wavelengths.

Observations and Imaging

Utilizing a global set of radio telescope observatories, the EHT resolved the emission surrounding \sgra into a bright, circular structure, approximately $51.8 \pm 2.3\,\mu{\rm as}$ in diameter, with a central brightness depression indicative of the black hole shadow. These imaging results are consistent with a radiatively inefficient accretion flow (RIAF) around a Kerr black hole, affirming theoretical predictions of such accretion models.

The observation methodology included compensating for variability and interstellar scattering. \sgra exhibits significant intrinsic variability on short timescales; thus, the team developed innovative methods to disentangle this variability from the dataset. Through a comprehensive suite of simulations, they derived the size and properties of the emission ring, enabling a comparison with the expected properties derived from general relativity's Kerr metric.

Strong Numerical Results and Constraints on Models

The results presented in the paper assert a strong resemblance to predicted black hole images according to general relativity. Utilizing the observed emission features, the paper rules out non-physical models such as non-spinning or retrograde accretion disks with high inclinations, and some alternative theories of black holes. The diameter of the shadow showed consistency with a non-spinning black hole model prediction, providing key evidence supporting the existence of a black hole rather than alternative compact objects.

The researchers leveraged GRMHD simulations to provide constraints on the black hole's physical parameters, including mass and spin, and examined the implications of these results using multiple lines of sight, magnetic field configurations, and electron-to-proton temperature ratios. This analysis resulted in a constraint on the deviation from the expected shadow size by no more than a few percent, a finding consistent with the null-hypothesis predictions of general relativity for a Kerr black hole.

Implications for Black Hole Physics and General Relativity

The paper extends the test of general relativity to an unprecedented small curvature scale in the regime of strong gravity, concluding that the observed shadow size of \sgra corresponds closely with the predictions from stellar orbit dynamics observed in the Milky Way's center. Thus, \sgra acts as a profound celestial laboratory to test alternative gravity theories awaiting scrutiny at close proximity to black holes.

These observations also provide critical insights into the physics of accretion and outflow processes in supermassive black hole environments, aligning with the broader paradigm understanding of AGNs across different scales and luminosities. The near-horizon view presented by the EHT accentuates our understanding of accretion dynamics, jet launching regions, and relativistic feedback processes.

Future Directions

The paper sets a foundation for future datasets with improved sensitivity, scrutinizing the models of accretion flow structures and further constraining the spin parameter and inclination angle of \sgra more rigorously. Future EHT observations, including multi-frequency and polarimetric datasets, will continue to test the properties of black holes within the framework of general relativity more stringently and explore the parameter space of possible extensions.

The work highlights the EHT's capability in probing the extreme environment of \sgra, bolstering empirical evidence for black holes as described by Einstein's theory in the heavy mass region and stimulating nuanced discussions for potential deviations or corrections that might be unveiled with forthcoming observations and theoretical advancements.