Colloquium

  • First M87 Event Horizon Telescope Results – The Shadow of the Supermassive Black Hole

  • Speaker : 손봉원(Bong Won Sohn)
    Affiliation : Korea Astronomy and Space Science Institute (KASI)
    Date : October 30, 2019 4:00 PM
    Place : Bldg.110 Rm. N103
    Contact : dsryu@unist.ac.kr
    Host : 류동수(Dongsu Ryu)
  • Abstract

  • I will summarize the recent EHT results on behalf of the Event Horizon Telescope Collaboration.
    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 (EHT), a global very long baseline
    interferometry (VLBI) 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 micor-arcsec, 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) x 10^9 M_{Sun}.
    Our radio-wave 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.