First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole

TL;DR

The paper reports the first EHT images of M87 at 1.3 mm, revealing a ring-like structure with a diameter of about $40\,\mu\mathrm{as}$ consistent with the black hole shadow. A two-stage imaging strategy—four independent blind reconstructions followed by parameter surveys on synthetic data—demonstrates the robustness of the ring feature across imaging methods and assumptions. Quantitative analysis of ring properties (diameter, width, orientation, asymmetry) shows a persistent ring with southern brightness and evidences intrinsic temporal variability, while flux budgets and calibration systematics are carefully constrained. These results validate the EHT's capability to image event-horizon-scale structures around a SMBH and provide a foundation for testing general relativity and accretion-jet physics in M87.

Abstract

We present the first Event Horizon Telescope (EHT) images of M87, using observations from April 2017 at 1.3 mm wavelength. These images show a prominent ring with a diameter of ~40 micro-as, consistent with the size and shape of the lensed photon orbit encircling the "shadow" of a supermassive black hole. The ring is persistent across four observing nights and shows enhanced brightness in the south. To assess the reliability of these results, we implemented a two-stage imaging procedure. In the first stage, four teams, each blind to the others' work, produced images of M87 using both an established method (CLEAN) and a newer technique (regularized maximum likelihood). This stage allowed us to avoid shared human bias and to assess common features among independent reconstructions. In the second stage, we reconstructed synthetic data from a large survey of imaging parameters and then compared the results with the corresponding ground truth images. This stage allowed us to select parameters objectively to use when reconstructing images of M87. Across all tests in both stages, the ring diameter and asymmetry remained stable, insensitive to the choice of imaging technique. We describe the EHT imaging procedures, the primary image features in M87, and the dependence of these features on imaging assumptions.

Figures (38)

• Figure 1: Top panels: aggregate baseline coverage for EHT observations of M87, combining observations on all four days. The left panel shows short-baseline coverage, comprised of ALMA interferometer baselines and intra-site EHT baselines (SMA-JCMT and ALMA-APEX). These short baselines probe angular scales larger than$0.1^{\prime \prime}$. The right panel shows long-baseline coverage, comprised of all inter-site EHT baselines. These long baselines span angular scales from 25 to $170 \mu$ as. Each point denotes a single scan, which range in duration from 4 to 7 minutes. Bottom panels: the full baseline coverage on M87 for each observation. In all panels, the dashed circles show baseline lengths corresponding to the indicated fringe spacings ( $0.2^{\prime \prime}$ for the upper-left panel; 25 and $50 \mu$ as for the remaining panels).
• Figure 2: Left panel: S/N as a function of projected baseline length for EHT observations of M87 on April 11. Each point denotes a visibility amplitude coherently averaged over a full scan ($4-7$ minutes). Points are colored by baseline. Right panel: visibility amplitudes (correlated flux density) as a function of projected baseline length after a priori and network calibration. The amplitudes are corrected for upward bias from thermal noise (Equation (6)). Error bars denote $\pm 1 \sigma$ uncertainty from thermal noise and do not include expected uncertainties in the a priori calibration (see Paper III and Section 4.1).
• Figure 3: Selected closure phases from coherently averaged visibilities on three triangles as a function of Greenwich Mean Sidereal Time (GMST) using data from all four days. Error bars denote$\pm 1 \sigma$ uncertainties from thermal noise. The trivial ALMA-APEX-SMT triangle (left panel) has closure phases near zero on all days, as expected because this triangle includes an intra-site baseline. Deviations from zero arise from a combination of thermal and systematic errors (Paper III). The ALMA-LMT-SMT triangle (middle panel) shows persistent structure across all days, while the large LMT-SMA-SMT triangle (right panel) shows source evolution between the first two days and last two days.
• Figure 4: The first EHT images of M87, blindly reconstructed by four independent imaging teams using an early, engineering release of data from the April 11 observations. These images all used a single polarization (LCP) rather than Stokes$I$, which is used in the remainder of this Letter. Images from Teams 1 and 2 used RML methods (no restoring beam); images from Teams 3 and 4 used CLEAN (restored with a circular $20 \mu$ as beam, shown in the lower right). The images all show similar morphology, although the reconstructions show significant differences in brightness temperature because of different assumptions regarding the total compact flux density (see Table 2) and because restoring beams are applied only to CLEAN images.
• Figure 5: The four simple geometric models and synthetic data sets used in the parameter surveys (see Appendix C for details). Top: linear scale images, highlighting the compact structure of the models. Middle: logarithmic scale images, highlighting the larger-scale jet added to each model image. Bottom: one realization of simulated visibility amplitudes corresponding to the April 11 observations of M87. We indicate the conventions for cardinal direction and position angle used throughout this Letter on the upper-right panel. Note that east is oriented to the left, and position angles are defined east of north.