The Galactic Center A Case Study in Technology Driving Science

The center of our galaxy has remained an enigma for decades. It was first discovered in the early 20th century that we live in a galaxy, which must have a core like those we see in galaxies around us. Detecting that core has been a challenge. At optical wavelengths it is obscured by at least 30 magnitudes due to intervening dust along the plane of the galaxy. Even the cumulative light from billions of stars along the line of site to the Galactic center cannot penetrate to our position in the galaxy. As a result, the advent of recording devices like photographic plates, photoelectric sensors, and even CCDs, were of little use in observing the Galactic center.

The era of Galactic center research began in earnest only with the use of infrared sensors. These first seriously emerged in the 1960's in astronomy and led to pioneering research which was later perfected with "advanced" infrared optimized telescopes and observing techniques in the 1970's. The challenge in using these sensors was to detect targets that were millions of times fainter than the thermal glow of the earth's atmosphere. This required techniques that previous generations of optical astronomers never encountered. It was only through rather ingenious techniques involving fast chopping on the sky and lock-in amplifiers to sample and subtract several times a second object from sky beams that background removal to the level of about 1 part per million was possible.

Figure 13 shows a watershed result in the early days of infrared astronomy, which involved the use of some of the first simple single element PbS infrared detectors in astronomy. This is the discovery strip chart of the Galactic center, which is seen as a blip toward the left of the chart. This first faint infrared signal from the Galactic center sparked a revolution in this field of astronomy and soon led to the first crude maps of the complex star field circling the chaotic central gravitational well of the Milky Way. These maps were made by step-scanning the telescope across the field (see Figure 14) and led to the discovery of massive young stars orbiting the Galactic center, their formation mechanism still unknown in the highly dynamic and energetic region which characterizes the core of our Galaxy. It wasn't until the 1980's that the first infrared arrays were used to image the Galactic center from the summit of Mauna Kea. Images were obtained utilizing a detector multiplex gain that at the time only optical astronomers had tasted through their much larger format CCDs, which were of course still blind to the radiation from the ~10 kpc distant stars in the Galactic center. It was then possible to achieve in a matter of minutes with an array detector what previously took hours through tedious step-scanning techniques, which were susceptible to the vagaries of seeing, humidity, and photometric conditions while discrete observations were being compiled into maps. Suddenly it became "easy" to achieve impressive results in infrared astronomy. Information could be extracted much faster through direct infrared imaging than ever before, and astronomers finally had the upper hand in studying the Galactic center. When used in infrared spectrometers, these new arrays allowed spectroscopic studies of the stars in the Galactic center for the first time, revealing unusual populations of stars unlike those in our immediate neighborhood. More importantly, these studies strongly suggested the presence of an unseen extremely massive object or cluster of dark objects that were clearly dominating the local dynamics of the visible stars in the Galactic center. An unseen monster was lurking in their midst.

Figure 13. The discovery strip chart of the Galactic center by Becklin et al. [1] is shown. Adapted from McLean [2].

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Figure 14. Soon after the discovery of the center of the Galaxy, Becklin and Neugebauer [1] made the first maps by step scanning the telescope beam across the field.

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Figure 14. Soon after the discovery of the center of the Galaxy, Becklin and Neugebauer [1] made the first maps by step scanning the telescope beam across the field.

In the beginning of the next decade the push was on at telescopes around the world for ever higher resolution images of the Galactic center. This was performed using larger format infrared detectors and techniques like "shift and add", various deconvolution algorithms (all fraught with problems to some extent), speckle observations, and even lunar occultations with fast arrays to meticulously drill-down to finer spatial resolutions. Sites like

Mauna Kea had the advantage in the early 1990's, thanks to the intrinsically better seeing offered by the smooth high altitude slopes of this volcano situated in the middle of the Pacific. The next breakthrough came in the mid to late 1990's, when the first adaptive optics systems were used to make observations with nearly an order of magnitude higher resolution than anything achieved before. Though these AO systems relied upon visible sensors to lock onto natural guide stars and compensate for atmospheric turbulence, fortuitously, a foreground star along the line-of-sight to the Galactic center provided astronomers with the "break" they needed to use AO systems with Galactic center observations. Finally, in the late 1990's and early 2000's, the use of AO systems on the 8-10 m class telescopes of the time put astronomers "over the top" - they were at last armed to probe the dynamics of the core of our galaxy on spatial scales comparable to our own solar system but a distance of nearly 30,000 light years. From this great distance, using infrared detectors then containing over a million pixels, astronomers could map with exquisite precision the motions of stars as they danced around an unseen companion. Using nothing more than orbital measurements and Kepler's Laws, it was possible to directly measure the mass of the unseen companion, proving that a black hole with a mass well over a million times that of our sun resides at the center of the Milky Way [3]. These observations led to an even more spectacular result though, as serendipity came into play and astronomers imaged a bright flash from the exact location of the black hole. This was soon recognized as the signature of hot gas as it spiraled under the black hole's inexorable pull, through its event horizon (see Figure 15), and slipped into a space-time singularity that still defies our understanding of physics. The stuff of science fiction just became science fact.

Figure 15. The flare from matter falling into the black hole at the center of the galaxy is shown in these 2 frames recorded at the VLT. Adapted from Genzel et al. [4].

None of this would have been possible without the advent of infrared array technology. In the ~25 year history of infrared observations of the Galactic center, technology laid the course of discovery from a blip on a strip-chart to movies of matter falling into the abyss of a super-massive black hole. The engineers, technicians, material scientists, physicists, etc. that are collectively responsible for the advent of infrared arrays deserve as much credit for these discoveries as the astronomers who made these remarkable observations over the past couple of decades. Amazingly, this same technology path is advancing our capabilities at an exponential rate. What took 25 years to achieve before might only take a decade in the future as our understanding of the universe accelerates forward. As a case-study, Galactic center research beautifully illustrates where we've been and how fast we're going. We know our trajectory, but what lies ahead on the horizon?

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