First Images of a Black Hole from the Event Horizon Telescope

Astronomers have used a telescope that spans the globe to capture the first detailed images of a black hole: the nearby supermassive black hole in the Messier 87 galaxy. The first results from the Event Horizon Telescope (EHT) are detailed in six articles that make up a new Focus Issue in The Astrophysical Journal Letters.Hubble image of the elliptical galaxy M87, dominated by the visible jet extending to the northwest of the supermassive black hole at the galaxys center (click to enlarge). [NASA/ESA/Hubble Heritage Team (STScI/AURA)/P. Cote (Herzberg Institute of Astrophysics)/E. Baltz (Stanford University)]Why M87?Obtaining an up-close view of a black hole is a goal that has long remained out of reach. When astronomers first developed a plan to image a supermassive black holes event horizon the close-in boundary from which not even light can escape two sources were selected as targets: the black hole at the center of our galaxy, Sgr A*; and the gargantuan, jet-producing black hole in the neighboring galaxy M87.Sgr A*, while the nearest supermassive black hole, suffers from a host of complicating factors for imaging. This monsters rapid variability and its position in the galactic plane where its blurred by the interstellar medium provide additional challenges that must be overcome to accurately capture it.M87 may be further away, but its black hole is a thousand times larger than Sgr A*, giving it longer and more manageable variability timescales. Were also peering through much less of the interstellar medium when pointed at M87.These factors contribute to making the neighboring behemoth in M87 comparatively accessible for imaging and, consequently, its the first source for which the EHT is presenting images.A Planet-Scale TelescopeSo how do we zoom in on this distant object? We need a telescope unlike any other.The eight stations of the EHT 2017 campaign over six geographic locations. [EHT Collaboration et al 2019]The EHT is an extensive virtual telescope created by combining simultaneous observations from radio arrays and dishes all around the planet. For the images of M87 released today, the observations were made by eight ground-based telescopes in Arizona, Hawaii, Mexico, Chile, Spain, and the South Pole.The EHT works by performing very-long-baseline interferometry; by combining different telescopes around the world, the EHT can function like a telescope with an effective size thats the same as its longest baseline the distance between component telescopes. In this way, the EHT is able to achieve unprecedented resolution: it can theoretically resolve down to 25 millionths of an arcsecond at its observing wavelength of 1.3 mm.Its been more than a decade since the EHTs bold imaging plans were first begun. Scientists have patiently waited as existing facilities were upgraded and new facilities have been built and in April 2017, conditions were finally right to obtain the first good look at M87s event horizon.Images of a ShadowM87s black hole was observed on four days in April 2017. Weather was uniformly good planet-wide! during those observations, allowing EHT scientists to combine the data from the eight telescopes and reconstruct images of the black hole.EHT observations of M87 taken over 4 days reveal a bright, asymmetric ring; north is up and east is left. The images remain consistent over the observations, providing evidence of a stable source. [EHT Collaboration et al 2019]What they saw was spot-on with predictions: a ring of light spanning 3844 as, with the southern part of the ring appearing brighter than the rest.While a naked black hole would simply appear dark in an image, an active black hole like M87 is surrounded by dust and gas that forms an accreting disk, as well as funnels at the base of its powerful jets. Images of such an encased black hole were predicted to reveal a dark region the black holes shadow surrounded by a ring of emission produced by the distorted paths of light from the surrounding material. The EHT observations of M87 beautifully confirm this picture!Why is the southern part of the ring brighter? As fast-moving material rotates around the black hole, it speeds toward us on one side and away from us on the other. On the side of the ring where matter moves toward us, a relativistic effect beams light in our direction, causing this region to appear brighter.From the asymmetry of the ring, EHT scientists determine that matter on the south side of M87s black hole is moving toward us. Combined with previous observations of M87s jet, which show its inclined at an angle of 17 relative to our line of sight, this tells us that M87s black hole likely spins clockwise from our point of view, with its spin axis pointed away from us at an angle.Three of the authors example models of spinning black holes (top row) and their simulated observations (bottom row). All three models produce similar simulated observations that well-match the real observations, indicating a single good fit doesnt imply that one model is preferred over the others. [EHT Collaboration et al 2019]Models and MeasurementsThe EHTs images of M87s black hole were compared to an extensive library of synthetic observations obtained from simulations of this source under varying models and conditions. The simulated observations show remarkable consistency with the actual observations, confirming a picture of a spinning supermassive black hole as the source of the emission. The models also suggest the most likely way that M87s jets are launched: via energy extraction directly from the black holes spin.Previous estimates of the mass for M87s black hole ranged from 37 billion solar masses, depending on the method used to measure it. From the EHTs images of M87, the authors were able to estimate a mass for the black hole of 6.5 0.7 billion solar masses, providing an independent check of past measurements.Future PlansWhats next for the EHT? These images of M87 are just the start! The team still plans to perform polarimetric analysis of their images, which will probe the magnetic field and help determine the rate of accretion onto M87s black hole. Future observations will test the stability, shape, and depth of its shadow more accurately. And higher-resolution images will be possible with the addition of new telescopes to the EHT and a possible push to shorter-wavelength observations.Continued improvements to EHTs observational technology and analysis techniques should also bring our own supermassive black hole, Sgr A*, into reach. Welcome to a new world of black-hole exploration!For more information, check out the fullApJL Focus Issue here:Focus on the First Event Horizon Telescope ResultsCitationFirst M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole, EHT Collaboration et al 2019 ApJL 875 L1. doi:10.3847/2041-8213/ab0ec7