Stars with short orbital periods at the center of our galaxy offer a powerful probe of a supermassive black hole. Over the past 17 years, we have used the W. M. Keck Observatory to image the galactic center at the highest angular resolution possible today. By adding to this data set and advancing methodologies, we have detected S0-102, a star orbiting our galaxy's supermassive black hole with a period of just 11.5 years. S0-102 doubles the number of known stars with full phase coverage and periods of less than 20 years. It thereby provides the opportunity, with future measurements, to resolve degeneracies in the parameters describing the central gravitational potential and to test Einstein's theory of general relativity in an unexplored regime.
The orbits of stars within the central arcsecond of our Galaxy. In the background, the central portion of a diffraction-limited image taken in 2012 is displayed. The orbits have been inferred from images taken with the primitive technique of speckle imaging (1995 - 2005) and with the more sophisticated adaptive optics (2005-2012). While several stars can be seen in their motion through this region, only two stars - S0-2 and the newly discovered S0-102 - have been traced through a complete orbit. They are the most tightly bound to the black hole and therefore comprise the most information about it. S0-2, which has an orbital period of 16 years, proved the existence of a black hole. The addition of S0-102, with a period of 11.5 years, will for the first time allow us to test the warping of space and time this close to a black hole. Stars that have been observed through at least one turning point in their orbit are shown in blue.
Credit: These images/animations were created by Prof. Andrea Ghez and her research team at UCLA and are from data sets obtained with the W. M. Keck Telescopes
The two W. M. Keck Telescopes on Mauna Kea, Hawaii, observing the galactic center. The lasers are used to create an artificial star in Earth's upper atmosphere, which is then employed to measure the blurring effects of the lower atmosphere (the unfortunate effect that makes the stars twinkle in the night sky). The blurring gets corrected in real time with the help of a deformable mirror. This is the so-called adaptive
Picture credit: Ethan Tweedie.
The story: For the past 17 years our group has been looking for stars with orbital periods of less than 20 years (i.e. those deep enough in the gravitational potential to be strongly affected by the black hole), using the 10-m telescopes at Keck Observatory. While our initial goal was to prove the existence of supermassive black holes, our goal posts are much more ambitious today, thanks to the tremendous leap in technological development. Today, having proved that black holes exist, our research aims to understand the nature of black holes and how they warp space and time. The challenge in observing our galactic center is to overcome the blurring of Earth's atmosphere. In the early '90s, a technique called Speckle imaging, which uses very short exposures to effectively freeze out the atmosphere's blurring effects, led to the identification of individual stars at the center of the galaxy. While early images reveal the presence of stars that appear to be close to the black hole, you can't know if they are on short-period orbits. One needs to watch for at least half of the orbit to know that it has a short period. With Speckle, the "winning" star was S0-2, which orbits around a central dark mass in 16 years. This proved the existence of a black hole at the galactic center. But S0-2 was the only short-period (<20 years) star revealed with Speckle. To carry out a fundamental test of general relativity, you need at least two stars with short orbits. Fortunately, since then a revolutionary technology called adaptive optics has arrived. This technique corrects for the effects of the atmosphere in real-time, distorting a mirror in exactly the opposite way that the atmosphere distorts the starlight. This groundbreaking development enabled our discovery of S0-102, which is orbiting in a mere 11.5 years around the black hole. With adaptive optics we can now go beyond the existence proof of black holes and learn about their nature. Einstein's theory of general relativity predicts precisely how they warp space and time. We will be able to test this fundamental theory in a new regime with adaptive optics observations of S0-102 and S0-2.