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

TL;DR

This paper presents the first direct images of M87's supermassive black hole by the Event Horizon Telescope, revealing a persistent bright ring consistent with theoretical predictions of the black hole shadow.

Contribution

It introduces a robust two-stage imaging process involving multiple independent teams and synthetic data validation to reliably image the black hole's shadow.

Findings

Ring diameter and shape are consistent across methods.

Images show a bright, asymmetric ring around the black hole.

Results are stable against different imaging assumptions.

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.