This project will support the development of wideband Very Long Baseline Interferometry arrays through deployment of enhanced backend and array-phasing instrumentation. The Very Long Baseline Interferometry arrays enabled by this work will bring significant new capabilities to the astronomy community through upgrades of national facilities, and will enable millimeter/submillimeter wavelength Very Long Baseline Interferometry arrays with the ability to resolve the event-horizon of a super-massive black hole.
The primary goal of this NSF funded project was to build instrumentation that would allow astronomers to make the first image of a black hole. Black holes are the most extreme objects predicted by Einstein's General Theory of Relativity. They are born during the death of stars and in collisions of galaxies, when sufficient matter is condensed into a small enough volume that nothing can prevent a complete gravitational collapse into a singularity of incomprehensible density. These objects are surrounded by an event horizon: a one-way membrane in space-time where gravity is so strong that once matter and energy fall through they can never return. Though exotic in nature, black holes that are millions of times more massive than the Sun are now believed to reside at the heart of most galaxies. There they convert the gravitational energy of in-falling gas into heat and intense radiation that can outshine the combined light from all stars in the host galaxy. When viewed from afar by any conventional telescope, a black hole thus appears to be exceptionally bright: a paradox created by its own intense gravitational pull. If, however, it were possible to zoom in to the event horizon and capture an image, General Relativity predicts that we would see a ‘shadow’ against a backdrop of glowing super-hot gas as the black hole absorbs and bends the light surrounding it. Since first predicted in the 1970’s, direct observation of this shadow and the motion of matter as it plunges inward have been long-standing goals in astronomy and physics, because the shape of the shadow and dynamics near the black hole hold answers to some of the most fundamental questions in astrophysics: Do Event Horizons, one-way portals in space-time, exist? Was Einstein right about gravity? Do his theories hold when gravity is dominant? How do super massive black holes affect the evolution of galaxies? To observe this 'shadow' our group focused on a technique called Very Long Baseline Interferometry (VLBI) in which radio dishes around the world are combined to create an Earth-sized virtual telescope. With a telescope this large, we achieve the magnification necessary to make out fine details of the emission from hot gas at the black hole boundary. The nearest supermassive black holes that we can hope to image are Sagittarius A* (SgrA*), a 4 million solar mass black hole at the center of the Milky Way, and M87, a 6 billion solar mass black hole at the center of a giant elliptical galaxy that is about 50 million light year away. The shadows of these black holes should both measure about 50 micro arcseconds across - that is the same angle a grapefruit would present to us if it were on the moon. But these two black holes are faint, so in order to achieve the sensitivity required to observe them, our team developed very broadband VLBI systems that were able to capture black hole emission over wide ranges of frequency. We deployed these new systems at three geographic sites: the top of the Mauna Kea volcano in Hawaii, at the summit of Mt. Graham in Arizona, and in the White Mountains of Northern California. In our first observations of SgrA*, we were able to measure the size of the emission surrounding the black hole to be only about four times the size of the event horizon. Since the black hole shadow is theoretically about five times the size of the event horizon, we had observed sub-horizon-scale features. This extremely exciting result has provided us with the most powerful arguments for the existence of black holes, and confirmed that with more telescopes added to the global array, we will be able to make true images in the future. In subsequent observations, we also measured the size of the emission towards M87. This much more massive black hole powers highly directed jets of material that accelerate to near light-speeds, tearing through the entire surrounding galaxy. In this case, we have likely measured the size of base of the jet, and we found it to be only about 5.5 times the size of the event horizon. We used this size, combined with calculations of how light bends near a black hole, to argue that the M87 black hole must be spinning. Such spinning black holes are thought to twist magnetic fields and accelerate charged particles, such as electrons, into the high-speed jets we see from this source. This work involved the efforts of a broad international team, and we are grateful to all the facilities and hard-working scientists who made these pioneering observations possible. It is hard to describe the sense of excitement and wonder we felt when we first realized we had measured the size of black hole.
