When surrounded by a transparent emission region, black holes are expected to reveal a dark shadow caused by gravitational light bending and photon capture at the event horizon. To image and study this phenomenon, we have assembled the Event Horizon Telescope, a global very long baseline interferometry array observing at a wavelength of 1.3 mm. This allows us to reconstruct event-horizon-scale images of the supermassive black hole candidate in the center of the giant elliptical galaxy M87. We have resolved the central compact radio source as an asymmetric bright emission ring with a diameter of 42±3 μas, which is circular and encompasses a central depression in brightness with a flux ratio 10:1. The emission ring is recovered using different calibration and imaging schemes, with its diameter and width remaining stable over four different observations carried out in different days. Overall, the observed image is consistent with expectations for the shadow of a Kerr black hole as predicted by general relativity. The asymmetry in brightness in the ring can be explained in terms of relativistic beaming of the emission from a plasma rotating close to the speed of light around a black hole. We compare our images to an extensive library of ray-traced general-relativistic magnetohydrodynamic simulations of black holes and derive a central mass of M=(6.5±0.7)×10 9 M e . Our radiowave observations thus provide powerful evidence for the presence of supermassive black holes in centers of galaxies and as the central engines of active galactic nuclei. They also present a new tool to explore gravity in its most extreme limit and on a mass scale that was so far not accessible.

We present measurements of the properties of the central radio source in M87 using Event Horizon Telescope data obtained during the 2017 campaign. We develop and fit geometric crescent models (asymmetric rings with interior brightness depressions) using two independent sampling algorithms that consider distinct representations of the visibility data. We show that the crescent family of models is statistically preferred over other comparably complex geometric models that we explore. We calibrate the geometric model parameters using general relativistic magnetohydrodynamic (GRMHD) models of the emission region and estimate physical properties of the source. We further fit images generated from GRMHD models directly to the data. We compare the derived emission region and black hole parameters from these analyses with those recovered from reconstructed images. There is a remarkable consistency among all methods and data sets. We find that >50% of the total flux at arcsecond scales comes from near the horizon, and that the emission is dramatically suppressed interior to this region by a factor >10, providing direct evidence of the predicted shadow of a black hole. Across all methods, we measure a crescent diameter of 42±3 μas and constrain its fractional width to be <0.5. Associating the crescent feature with the emission surrounding the black hole shadow, we infer an angular gravitational radius of GM/Dc 2 =3.8±0.4 μas. Folding in a distance measurement of -+ 16.8 Mpc 0.7 0.8 gives a black hole mass of =  ´ | | M M 6.5 0.2 0.7 10 stat sys 9. This measurement from lensed emission near the event horizon is consistent with the presence of a central Kerr black hole, as predicted by the general theory of relativity.

Abstract. We present the completed results of a high resolution radio imaging survey of all (∼200) low-luminosity active galactic nuclei (LLAGNs) and AGNs in the Palomar Spectroscopic Sample of all (∼488) bright northern galaxies. The high incidences of pc-scale radio nuclei, with implied brightness temperatures > ∼ 10 7 K, and sub-parsec jets argue for accreting black holes in > ∼ 50% of all LINERs and low-luminosity Seyferts; there is no evidence against all LLAGNs being mini-AGNs. The detected parsec-scale radio nuclei are preferentially found in massive ellipticals and in type 1 nuclei (i.e. nuclei with broad Hα emission). The radio luminosity function (RLF) of Palomar Sample LLAGNs and AGNs extends three orders of magnitude below, and is continuous with, that of "classical" AGNs. We find marginal evidence for a low-luminosity turnover in the RLF; nevertheless LLAGNs are responsible for a significant fraction of present day mass accretion. Adopting a model of a relativistic jet from Falcke & Biermann, we show that the accretion power output in LLAGNs is dominated by the kinetic power in the observed jets rather than the radiated bolometric luminosity. The Palomar LLAGNs and AGNs follow the same scaling between jet kinetic power and narrow line region (NLR) luminosity as the parsec to kilo-parsec jets in powerful radio galaxies. Eddington ratios l Edd (=L Emitted /L Eddington ) of ≤10 −1 −10 −5 are implied in jet models of the radio emission. We find evidence that, in analogy to Galactic black hole candidates, LINERs are in a "low/hard" state (gas poor nuclei, low Eddington ratio, ability to launch collimated jets) while low-luminosity Seyferts are in a "high" state (gas rich nuclei, higher Eddington ratio, less likely to launch collimated jets). In addition to dominating the radiated bolometric luminosity of the nucleus, the radio jets are energetically more significant than supernovae in the host galaxies, and are potentially able to deposit sufficient energy into the innermost parsecs to significantly slow the gas supply to the accretion disk.

The Event Horizon Telescope (EHT) has mapped the central compact radio source of the elliptical galaxy M87 at 1.3 mm with unprecedented angular resolution. Here we consider the physical implications of the asymmetric ring seen in the 2017 EHT data. To this end, we construct a large library of models based on general relativistic magnetohydrodynamic (GRMHD) simulations and synthetic images produced by general relativistic ray tracing. We compare the observed visibilities with this library and confirm that the asymmetric ring is consistent with earlier predictions of strong gravitational lensing of synchrotron emission from a hot plasma orbiting near the black hole event horizon. The ring radius and ring asymmetry depend on black hole mass and spin, respectively, and both are therefore expected to be stable when observed in future EHT campaigns. Overall, the observed image is consistent with expectations for the shadow of a spinning Kerr black hole as predicted by general relativity. If the black hole spin and M87's large scale jet are aligned, then the black hole spin vector is pointed away from Earth. Models in our library of non-spinning black holes are inconsistent with the observations as they do not produce sufficiently powerful jets. At the same time, in those models that produce a sufficiently powerful jet, the latter is powered by extraction of black hole spin energy through mechanisms akin to the Blandford-Znajek process. We briefly consider alternatives to a black hole for the central compact object. Analysis of existing EHT polarization data and data taken simultaneously at other wavelengths will soon enable new tests of the GRMHD models, as will future EHT campaigns at 230 and 345 GHz.

Abstract. This paper presents the results of a high resolution radio imaging survey of all known (96) low-luminosity active galactic nuclei (LLAGNs) at D ≤ 19 Mpc. We first report new 2 cm (150 mas resolution using the VLA) and 6 cm (2 mas resolution using the VLBA) radio observations of the previously unobserved nuclei in our samples and then present results on the complete survey. We find that almost half of all LINERs and low-luminosity Seyferts have flat-spectrum radio cores when observed at 150 mas resolution. Higher (2 mas) resolution observations of a flux-limited subsample have provided a 100% (16 of 16) detection rate of pc-scale radio cores, with implied brightness temperatures > ∼ 10 8 K. The five LLAGNs with the highest core radio fluxes also have pc-scale "jets". Compact radio cores are almost exclusively found in massive ellipticals and in type 1 nuclei (i.e. nuclei with broad Hα emission). Only a few "transition" nuclei have compact radio cores; those detected in the radio have optical emission-line diagnostic ratios close to those of LINERs/Seyferts. This indicates that some transition nuclei are truly composite Seyfert/LINER+H II region nuclei, with the radio core power depending on the strength of the former component. The core radio power is correlated with the nuclear optical "broad" Hα luminosity, the nuclear optical "narrow" emission-line luminosity and width, and with the galaxy luminosity. In these correlations LLAGNs fall close to the low-luminosity extrapolations of more powerful AGNs. The scalings suggest that many of the radio-non-detected LLAGNs are simply lower power versions of the radio-detected LLAGNs. The ratio of core radio power to nuclear optical emissionline luminosity increases with increasing bulge luminosity for all LLAGNs. Also, there is evidence that the luminosity of the disk component of the galaxy is correlated with the nuclear emission-line luminosity (but not the core radio power). About half of all LLAGNs with multiple epoch data show significant inter-year radio variability. Investigation of a sample of ∼150 nearby bright galaxies, most of them LLAGNs, shows that the nuclear (≤150 mas size) radio power is strongly correlated with both the black hole mass and the galaxy bulge luminosity; linear regression fits to all ∼150 galaxies give: log P 2 cm = 1.31(±0.16) log M MDO + 8.77 and log P 2 cm = 1.89(±0.21) log L B (bulge) − 0.17. Low accretion rates (≤10 −2 −10 −3 of the Eddington rate) are implied in both advection-and jet-type models. In brief, all evidence points towards the presence of accreting massive black holes in a large fraction, perhaps all, of LLAGNs, with the nuclear radio emission originating in either the accretion inflow onto the massive black hole or from jets launched by this black hole-accretion disk system.