Supermassive Black Hole at Center of Milky Way Imaged

A team of astronomers led by scientists at the Center for Astrophysics | Harvard & Smithsonian who produced the first direct image of a black hole three years ago have now produced a portrait of a second, this time a much-anticipated glimpse of the supermassive blck hole at the center of the Milky Way.

The new picture was captured by researchers from the Event Horizon Telescope (EHT) Collaboration who unveiled their first image in 2019. The group targeted both black holes at the outset but focused their attention on one at a time, owing to a difference in the complexity of the two projects.

The work was published in the Astrophysical Journal Letters. It provides the first direct visual evidence that the giant lurking 27,000 light-years away at the center of Earth’s galaxy is, in fact, a supermassive black hole. It also bolsters theories of where these cosmic monsters exist and may help to explain how galaxies are formed.

The researchers produced the picture with observations from the Event Horizon Telescope, a worldwide network of radio telescopes that link together to form a single Earth-sized virtual instrument. In April 2017, eight radio observatories on six mountains on four continents stared on and off at a pair of black holes for 10 days — Sgr A* and a second that lies at the heart of the elliptical galaxy M87.

First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way

We present the first Event Horizon Telescope (EHT) observations of Sagittarius A* (Sgr A*), the Galactic center source associated with a supermassive black hole. These observations were conducted in 2017 using a global interferometric array of eight telescopes operating at a wavelength of λ = 1.3 mm. The EHT data resolve a compact emission region with intrahour variability. A variety of imaging and modeling analyses all support an image that is dominated by a bright, thick ring with a diameter of 51.8 ± 2.3 μas (68% credible interval). The ring has modest azimuthal brightness asymmetry and a comparatively dim interior. Using a large suite of numerical simulations, we demonstrate that the EHT images of Sgr A* are consistent with the expected appearance of a Kerr black hole with mass ∼4 × 106 M⊙, which is inferred to exist at this location based on previous infrared observations of individual stellar orbits, as well as maser proper-motion studies. Our model comparisons disfavor scenarios where the black hole is viewed at high inclination (i > 50°), as well as nonspinning black holes and those with retrograde accretion disks. Our results provide direct evidence for the presence of a supermassive black hole at the center of the Milky Way, and for the first time we connect the predictions from dynamical measurements of stellar orbits on scales of 103–105 gravitational radii to event-horizon-scale images and variability. Furthermore, a comparison with the EHT results for the supermassive black hole M87* shows consistency with the predictions of general relativity spanning over three orders of magnitude in central mass.

SOURCES – Astrophysical Journal Letters, Harvard
Written by Brian Wang,

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