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

TL;DR

This paper presents the first horizon-scale image of the Milky Way's central black hole Sgr A* using the Event Horizon Telescope at 1.3 mm, revealing a ring-like shadow with diameter ≈ 51.8 μas and a central depression consistent with a Kerr black hole of mass ≈ 4 × 10^6 M⊙. Through extensive GRMHD modeling, the study finds that models with strong magnetic fields (MAD) and prograde spin at moderate inclination best reconcile the data with multiwavelength constraints, though many models fail at some criterion due to intrinsic variability. The inferred shadow diameter ≈ 48.7 μas and angular gravitational radius ≈ 4.8 μas align with stellar-dynamical mass measurements, providing GR consistency across three orders of magnitude in black hole mass when contrasted with M87*. Together, these results validate horizon-scale black hole imaging and set the stage for future polarimetric and time-resolved studies as the EHT expands.

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 $λ=1.3\,{\rm mm}$. 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.

Figures (6)

• Figure 1: The 2017 EHT array as seen from Sgr A$^*$. The array included eight observatories at six locations: the aa and the ap on the Llano de Chajnantor in Chile, the lm on Volcán Sierra Negra in Mexico, the jc and sm on Maunakea in Hawai'i, the Institut de Radioastronomie Millimétrique 30-m telescope (PV) on Pico Veleta in Spain, the az on Mt. Graham in Arizona, and the sp in Antarctica.
• Figure 2: EHT observations of Sgr A$^*$ on April 7. Top-Left: EHT baseline coverage, where dimensionless coordinates $\vec{u} = (u, v)$ give the projected baseline vector for each antenna pair in units of the observing wavelength $\lambda$. Top-Right: Calibrated visibility amplitudes of Sgr A$^*$ as a function of projected baseline length $|\vec{u}|$. Error bars show ${\pm}1\sigma$ thermal (statistical) uncertainties. Diamonds denote baselines to APEX and JCMT to distinguish them from baselines to their co-located observatories, ALMA and SMA, respectively. The visibilities have been coherently averaged in 120 second intervals. For comparison, the gray dashed line shows the Fourier transform of a thin ring with diameter $54\,\mu{\rm as}$ that has been convolved with a circular Gaussian kernel of FWHM $23\,\mu{\rm as}$. The red line and shaded region show the root-mean-square variability and associated 68% credible interval over the range of baselines for which it can be accurately measured [see][]PaperIV, while the blue horizontal ticks at zero baseline length show the range of variations in the total flux density. Bottom: The full light curve of Sgr A$^*$ on April 7, measured using ALMA and the SMA as stand-alone interferometers.
• Figure 3: Representative EHT image of Sgr A$^*$ from observations on 2017 April 7. This image is an average over different reconstruction methodologies (CLEAN, RML, and Bayesian) and reconstructed morphologies. Color denotes the specific intensity, shown in units of brightness temperature. The inset circle shows the restoring beam used for CLEAN image reconstructions ($20\,\mu{\rm as}$ FWHM). The bottom panels show average images within subsets with similar morphologies, with their prevalence indicated by the inset bars. The multiplicity of image modes reflects uncertainty due to the sparse baseline coverage; it does not correspond to different snapshots of the variable source. Nearly all reconstructed images show a prominent ring morphology. While the diameter and thickness of the ring are generally consistent across the reconstructions, the azimuthal structure of the ring is poorly constrained.
• Figure 4: Summary of constraints on our 200 fiducial GRMHD simulations. Color indicates combined EHT constraints apart from structural or flux variability, and hatching indicates combined non-EHT constraints. For each constraint category and parameter combination, we delineate whether all of the three simulation codes run with those parameters pass, whether only some pass, or whether none pass. These exclusions leave only two models, each a MAD with prograde spin, $30^\circ$ inclination, and ${R_\mathrm{high}}=160$. For details, see PaperV.
• Figure 5: Simulated images of Sgr A$^*$. Left: A single snapshot image of a numerical simulation of Sgr A$^*$ that passes 10 out of the 11 observational criteria described in PaperV. Middle: The average of this simulation with time sampling that matches the EHT observational cadence on April 7. Right: Representative image reconstruction using synthetic visibilities generated from the simulation in the adjacent panels [see Appendix H in][]PaperIII. This image has been averaged across methodologies and reconstructed morphologies, as in \ref{['fig:image']}. Each panel is shown on a linear brightness scale that is normalized to its peak.