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

TLDR

The study presents the first Event Horizon Telescope images of M87 and outlines a two‑stage, blind imaging procedure to assess their reliability. The two‑stage method first had four independent teams reconstruct M87 images with CLEAN and regularized maximum likelihood, then used a large synthetic‑data survey to objectively select imaging parameters. The resulting images reveal a ~40 µas ring matching the expected photon‑orbit shadow, with persistent south‑side brightness and stable diameter and asymmetry across all imaging tests.

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.