Measuring the Ellipticity of M 87* Images

The Event Horizon Telescope (EHT) images of the supermassive black hole at the center of the galaxy M 87 provided the first image of the accretion environment on horizon scales. General relativity predicts that the image of the shadow should be nearly circular, given the inclination angle of the black hole M 87*. A robust detection of ellipticity in the image reconstructions of M 87* could signal new gravitational physics on horizon scales. Here we analyze whether the imaging parameters used in EHT analyses are sensitive to ring ellipticity and measure the constraints on the ellipticity of M 87*. We find that the top set is unable to recover ellipticity. Even for simple geometric models, the true ellipticity is biased low, preferring circular rings. Therefore, to place a constraint on the ellipticity of M 87*, we measure the ellipticity of 550 simulated data sets of GRMHD simulations. We find that images with intrinsic axis ratios of 2:1 are consistent with the ellipticity seen from the EHT image reconstructions.