Collaborative Research: Building an Event Horizon Telescope: (Sub)millimeter VLBI from the South Pole Telescope

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A research topic of great current interest is the supermassive black hole of about 4 million solar masses that is now known to exist at the center of our Milky Way galaxy. Similar objects are common in other galaxies. Most of what is known about the Milky Way's exotic object comes from studies of the kinematics of stars influenced by its strong gravitational field. No possibility has heretofore existed for directly imaging the black hole, or more technically its event horizon, the gravitational "point of no return" from within which no particle or photon can escape the black hole. The galactic center region is distant, about 8 kpc, making it extraordinarily difficult to obtain high-spatial-resolution images of the supermassive black hole by any direct technique. If such images could be obtained, they would be the very first of their kind, and could reveal information not yet imagined about black holes and the dynamics in the cores of galaxies. To attack this challenging scientific problem, Drs. D. Marrone of the University of Arizona, S. Doeleman of the Northeast Radio Observatory Corporation, and J. Carlstrom of the University of Chicago intend to enhance an existing array of radio telescopes operating at the very highest radio frequencies (the submillimeter, with wavelengths somewhat shorter than a millimeter) and in a mode where telescopes separated by continental baselines coherently combine their data to achieve unprecedented spatial resolution. The general technique is termed Very Long Baseline Interferometry, or VLBI, and the particular array of radio telescopes to be used is referred to as the Event Horizon Telescope (EHT), developed in part through a previous award from NSF. Through a new NSF proposal the researchers plan to add the South Pole Telescope (SPT) as a new station to the EHT. Adding stations (telescopes) in this manner enhances sensitivity to radio signals and also improves the spatial response of the instrument as an imaging device, so that better-quality maps may be made of fainter objects. In addition to the addition of a key observing station, the technology will feature a new, sensitive dual-band millimeter-wave VLBI receiver, optimized for the needs of this project, that will employ superconducting mixer technology developed for the Atacama Large Millimeter Array (ALMA) project by the National Radio Astronomy Observatory (NRAO). Finally, the proven 230 GHz design will be adapted for use at 345 GHz.Funding for this work is being provided by NSF's Division of Astronomical Sciences through its Advanced Technologies and Instrumentation program.