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  1. Abstract

    The groundbreaking image of the black hole at the center of the M87 galaxy has raised questions at the intersection of observational astronomy and black hole physics. How well can the radius of a black hole shadow be measured, and can this measurement be used to distinguish general relativity from other theories of gravity? We explore these questions using a simple spherical flow model in general relativity, scalar Gauss–Bonnet gravity, and the Rezzolla and Zhidenko parameterized metric. We assume an optically thin plasma with power-law emissivity in radius. Along the way we present a generalized Bondi flow, as well as a piecewise analytic model for the brightness profile of a cold inflow. We use the second moment of a synthetic image as a proxy for EHT observables and compute the ratio of the second moment to the radius of the black hole shadow. We show that corrections to this ratio from modifications to general relativity are subdominant compared to corrections to the critical impact parameter, and we argue that this is generally true. In our simplified model the astrophysical parameter uncertainty dominates the gravity theory parameter uncertainty, underlining the importance of understanding the accretion model if EHT is to be used to successfully test theories of gravity.

     
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  2. Abstract

    We present a detailed study of the Planck-selected binary galaxy cluster PLCK G165.7+67.0 (G165;z= 0.348). A multiband photometric catalog is generated incorporating new imaging from the Large Binocular Telescope/Large Binocular Camera and Spitzer/IRAC to existing imaging. To cope with the different image characteristics, robust methods are applied in the extraction of the matched-aperture photometry. Photometric redshifts are estimated for 143 galaxies in the 4 arcmin2field of overlap covered by these data. We confirm that strong-lensing effects yield 30 images of 11 background galaxies, of which we contribute new photometric redshift estimates for three image multiplicities. These constraints enable the construction of a revised lens model with a total mass ofM600 kpc= (2.36 ± 0.23) × 1014M. In parallel, new spectroscopy using MMT/Binospec and archival data contributes thirteen galaxies that meet our velocity and transverse radius criteria for cluster membership. The two cluster components have a pair-wise velocity of ≲100 km s−1, favoring an orientation in the plane of the sky with a transverse velocity of 100–1700 km s−1. At the same time, the brightest cluster galaxy (BCG) is offset in velocity from the systemic mean value, suggesting dynamical disturbance. New LOFAR and Very Large Array data uncover head-tail radio galaxies in the BCG and a large red galaxy in the northeast component. From the orientation and alignment of the four radio trails, we infer that the two cluster components have already traversed each other, and are now exiting the cluster.

     
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