Fig. 1: High-resolution images of M87 at 3.5 mm obtained on 14–15 April 2018.

a, Uniformly weighted CLEAN (ref. ⁶) image. The filled ellipse in the lower-left corner indicates the restoring beam, which is an elliptical Gaussian fitted to the main lobe of the synthesized beam (fullwidth at half-maximum = 79 μas × 37 μas; position angle = −63°). Contours show the source brightness in the standard radio convention of flux density per beam. The contour levels start at 0.5 mJy per beam and increase in steps of factors of 2. The peak flux density is 0.18 Jy per beam. b, The central region of the image as shown in a, but the image is now restored with a circular Gaussian beam of 37 μas size (fullwidth at half-maximum), corresponding to the minor axis of the elliptical beam in a. The peak flux density is 0.12 Jy per beam. The contour levels start at 0.4 mJy per beam and increase in steps of factors of 2. c, A magnification of the central core region using regularized maximum likelihood (RML) imaging methods. Contours start at 4% of the peak and increase in steps of factors of 2. The solid blue circle of diameter 64 μas denotes the measured size of the ring-like structure at 3.5 mm, which is approximately 50% larger than the EHT 1.3-mm ring with a diameter of 42 μas (dashed black circle)⁴. For each panel, the colour map denotes the brightness temperature T in kelvin, which is related to the flux density S in jansky as given in the equation T = λ²(2k_BΩ)⁻¹S, where λ is the wavelength, k_B is the Boltzmann constant and Ω is the solid angle (shown on a square-root scale). The CLEAN images are the mean of the best-fitting images produced independently by team members, and the RML image is the mean of the optimal set of SMILI images (Supplementary Information section 3). dec, declination; RA, right ascension. Scale bars, 0.5  mas (a), 0.2 mas (b) and 50  μas (c).