A ring-like accretion structure in M87 connecting its black hole and jet

Abstract

The nearby radio galaxy M87 is a prime target for studying black hole accretion and jet formation^1,2. Event Horizon Telescope observations of M87 in 2017, at a wavelength of 1.3 mm, revealed a ring-like structure, which was interpreted as gravitationally lensed emission around a central black hole³. Here we report images of M87 obtained in 2018, at a wavelength of 3.5 mm, showing that the compact radio core is spatially resolved. High-resolution imaging shows a ring-like structure of [Formula: see text] Schwarzschild radii in diameter, approximately 50% larger than that seen at 1.3 mm. The outer edge at 3.5 mm is also larger than that at 1.3 mm. This larger and thicker ring indicates a substantial contribution from the accretion flow with absorption effects, in addition to the gravitationally lensed ring-like emission. The images show that the edge-brightened jet connects to the accretion flow of the black hole. Close to the black hole, the emission profile of the jet-launching region is wider than the expected profile of a black-hole-driven jet, suggesting the possible presence of a wind associated with the accretion flow.

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

a, Uniformly weighted CLEAN (ref. ) 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). 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).

Fig. 2. RML images and model images at 3.5 mm and 1.3 mm.

a–f, RML images (a,d) and model images (b,c,e,f) obtained at 3.5 mm (a–c) and 1.3 mm (d–f). a, The 3.5-mm image obtained on 14–15 April 2018 is the same as in Fig. 1c but shown on a linear brightness scale. b,e, The thermal synchrotron model from the accretion flow assumes synchrotron emission from electrons with a Maxwellian energy distribution. c,f, The non-thermal synchrotron model from the jet region assumes synchrotron emission from electrons with a power-law energy distribution. d, The 1.3-mm EHT image obtained on 11 April 2017, reconstructed with the publicly available data and imaging pipeline using the EHT-imaging library. Note that the differences in the azimuthal intensity distribution in the two observed images are probably because of time variability and/or blending effects with the underlying jet footpoints. Although the morphology of both models is consistent with the observations at 1.3 mm (e and f), the larger and thicker ring-like structure at 3.5 mm can be understood by the opacity effect at longer wavelengths, preferentially explained by thermal synchrotron absorption from the accretion flow region (b). For comparison, reconstructed and simulated images are convolved with a circular Gaussian beam of 27 μas (3.5 mm) and 10 μas (1.3 mm) and are shown in a linear colour scale. The blue circle denotes the measured ring diameter of 64 μas at 3.5 mm and 42 μas at 1.3 mm.

Fig. 3. Jet collimation profile.

Red filled circles mark the measured jet transverse width for the observations reported here. The error bars (1σ) are within the symbols (see Supplementary Information section 8 for more details on measuring the jet width). Grey filled squares, dots and triangles denote previous measurements of the width on larger scales^,,, for which a power-law fit with a fixed power-law index of 0.58 is shown by the dashed line. The vertical dashed line marks the position at which the intrinsic half-opening angle θ of the fitted parabolic jet equals the jet viewing angle of θ_v = 17° (that is, boundary condition for a down-the-pipe jet). The horizontal blue solid line marks the measured diameter of the ring at 3.5 mm, whereas the horizontal black dashed line marks the ring diameter measured with the EHT at 1.3 mm. In each case, the shaded area denotes the corresponding measurement uncertainty. The light-grey-shaded area denotes the outermost streamlines of the envelope of the parabolic jet from theoretical simulations (projected for θ_v = 17°; ref. ) that are anchored at the event horizon for a range of black hole spins (dimensionless spin parameters, a = 0.0–0.9). The lower and upper boundaries of this shaded area correspond to the highest (a = 0.9) and lowest (a = 0.0) spin, respectively. As the jet footpoint is anchored at the event horizon, some flattening of the jet width profile is expected near the black hole. This is further enhanced by geometrical projection effects in the region where the intrinsic jet half-opening angle (θ) is larger than the jet viewing angle (θ_v). The quasi-cylindrical shape in region I requires some change in the physical conditions to connect the innermost Blandford–Znajek jet from the event horizon to the upstream jet (region II).