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The Mysterious Milky Way
M. Colleen Gino
The center of our galaxy, the Milky Way, is a place of mystery. Veiled in dense clouds of gas and dust, the Galactic Center remained an enigma to astronomers for many decades after its location was determined. In this report I will present a historical overview of the methods used to determine the location of and distance to the Galactic Center, and discuss the ways in which technological advancements have given us a more detailed picture of the Galactic Center. I will review the evidence for the existence of a supermassive black hole at our galaxys core, and discuss a recently developed method for the determination of an accurate distance to the Galactic Center.
One of the first known efforts to plot our position in the galaxy was undertaken in the late 18th century by William Herschel. He incorrectly assumed that by counting the number of stars in different regions of the sky, he could easily identify the center of the galaxy by noting the location of the highest concentration of stars. Since Herschel did not find any region of the sky that had a higher concentration of stars than any other region, he deduced that we must be in the center of the galaxy.
Herschel was not the last astronomer to come to this conclusion. In 1906, Jacobus Kapteyn began a similar project, to map the size and the shape of the Milky Way Galaxy. The method he used was similar to Herschels method. He surveyed 206 stars in specific areas of the sky, analyzing their apparent brightness and proper motion. This project took Kapteyn 16 years to complete. In 1922, the year of his death, the results of his study were finally published: our galaxy was 30,000 light years across, 6000 light years thick, and the solar system was smack in the middle of it. Kapteyns model of the Milky Way was commonly accepted as accurate for many years.
Harlow Shapley began working at Mt. Wilson Observatory in 1914. Using the recently completed 60-inch telescope, the largest telescope in the world at that time, Shapley began observing globular clusters, compact spheres consisting of thousands of stars. During his research, he discovered a type a Cepheid variable, a star whose relationship between the period of its variability and its intrinsic luminosity allows the accurate measurement of its distance. Shapley discovered these variable stars in a large percentage of the globular clusters he observed. Aware of the period-luminosity relationship discovered just a few years earlier by Henrietta Swann Leavitt, he set out to determine the distances to all of the 93 clusters he observed.
His distance estimates to the globular clusters were shockingly large ranging from 50,000 to 220,000 light years away. While Shapley had previously accepted the Kapteyn model of the galaxy, he had to believe his own data. Shapley calculated the new diameter for our galaxy at 300,000 light years. More importantly, however, he determined the Galactic Center to be located in the constellation of Sagittarius, by mapping out the three dimensional distribution of the clusters. Shapley published his "Big Galaxy" theory in 1918.
While Shapleys "Big Galaxy" theory was not widely accepted, his placement of the Galactic Center in Sagittarius was. In 1927, Jan Oort published a paper on his research of the motion of stars in the vicinity of the Sun. As he had predicted, these stars exhibited differential rotation stars closer to the center of the galaxy traveled at higher velocities than stars farther away from the center. By determining the center of rotation of the stars, he pinpointed the center of the galaxy, which was within 2° of Shapleys estimate. The distance to the Galactic Center that Oort determined, however, was much less than Shapleys estimate just 19,000 light years, a figure that fit the Kapteyn model of the galaxy more closely than the "Big Galaxy" model.
Why did Kapteyn, Shapley and Oort produce such disparate estimates of the size of the Milky Way Galaxy and the location and distance to its center? Because they did not have an important piece of the puzzle the knowledge of interstellar extinction. Robert Trumpler of Lick Observatory was researching globular clusters in 1930. He was determining the distances to the globular clusters by deducing the absolute magnitude of individual stars in the cluster based on their color and spectra. Trumpler then compared the absolute magnitude to the apparent magnitude to calculate their distance. His study once again placed the solar system at the center of the galaxy, which had a diameter of 35,000 light years.
Trumpler then analyzed his data further. He compared the derived distances to the angular size of the globular clusters on photographic plates in order to determine the actual diameters of the clusters. The results were not what he had expected. The data showed that the more distant the cluster, the larger the diameter. Trumpler knew this was highly unlikely, and deduced that there must be some absorbing material throughout the galactic disk which was responsible for the dimming of starlight in proportion to the stars distance form the Earth. Trumpler made further calculations to test this theory, and found that if the light was dimmed by one magnitude for every 5000 light years of distance, the anomalous relationship between size and distance disappeared. He also recognized that the majority of the obscuring material must exist primarily in the galactic disk, as objects located above and below the plane of the galaxy were dimmed by a lesser amount.
Trumpler had discovered interstellar extinction, the dimming of starlight as it passes through the gas and dust of the interstellar medium. Without this knowledge, it was no wonder that Kapteyn and Trumpler incorrectly determined that we were at the center of the galaxy interstellar extinction places a limit on how far we can see into space in any direction. With this knowledge in hand, astronomers were on their way to determining a more accurate measurement of the Milky Way Galaxy and the distance to the Galactic Center.
Penetrating the Veil
By the 1930s, the astronomical community had accepted the fact that the Galactic Center was located in the constellation of Sagittarius, as Shapley and Oort had determined. They also realized that the galactic nucleus was hidden from view. Visible light from the Galactic Center cannot penetrate the dense clouds of gas and dust surrounding the central region, so it is completely obscured from view. This is because the wavelength of visible light is comparable in size to the dust particles, so the light is absorbed. Less than a trillionth of the light from the stars at the center of the galaxy reaches us (1). Therefore, the early astronomers had no hope of ever seeing into the center of the galaxy. No one could have guessed that the first look into the heart of our galaxy would be made possible through radio waves.
Karl Jansky was an engineer employed by Bell Labs to study the source of annoying static that plagued transatlantic telephone calls. He constructed a radio receiver that was tuned to a frequency of 20.5 MHz (14.6m l ) in an effort to detect the unwanted noise. By 1930 Jansky had identified three distinct sources of radio emission:
After continued observations Jansky realized that the noise was not terrestrial in origin, but emanated from a particular location in the sky which rose four minutes earlier each day. Further analysis of the data enabled him to pinpoint the location of the radio emission to be in the constellation of Sagittarius, known to be the Galactic Center.
It is interesting to note that his choice of frequency for the observations was a serendipitous one. It just so happens that the Galactic Center emits copious amounts of radiation at 20 MHz. In addition, he happened to be carrying out his observations during a period of sunspot minimum, when the Sun was not very active. If he had observed during a period of sunspot maximum, the radio emission at 20 MHz would have been blocked by the Earths ionosphere.
Although Jansky was not an astronomer, he was aware of the importance of his discovery. His was the first detection of non-optical radiation from space. Unfortunately, his discovery went practically unnoticed, as most astronomers did not have the training to appreciate or even to understand radio observations. Although Jansky published his results in the Proceedings of the Institute of Radio Engineers in 1932, his findings were never published in an astronomical journal.
While professional astronomers were not interested in pursuing Janskys discovery, Grote Reber, a radio engineer and amateur astronomer, was. Envisioning a new field of astronomy unfolding before him, Reber constructed a 32-foot wire-mesh parabolic antenna in his back yard. He began his observations in 1937, in his spare time. Rebers first receiver was tuned to 3300 MHz (9cm l ) in order to detect thermal radiation. However, he had no success with this frequency. He next tried 900 MHz (33cm l ), and again was unsuccessful. Finally, Reber tuned his receiver to a frequency of 160 MHz (1.9m l ) and was successful at detecting non-thermal radio emission in 1938, nearly a year after he began observing.
Reber published his first results in The Proceedings of the Institute of Radio Engineers in 1940, just as Jansky had. Over the course of his observations he created the first radio maps of the Milky Way, revealing some detail in the galactic structure. He found the strongest source of radiation to be from Sagittarius, with "hot spots" in the constellations of Cygnus and Cassiopeia. With these results, Reber broke into the astronomical community and published a paper in the Astrophysical Journal.
While these early radio telescopes were capable of detecting radio emission, they did not have a high resolution and were therefore incapable of discerning much detail. It would be more than a decade before these instruments were improved upon, making it more popular to observe in radio wavelengths.
Meanwhile, astronomers began to realize the value of observing in infrared light, electromagnetic radiation with wavelengths slightly longer than visible light. Seth Nicholson and Edison Petit carried out the first systematic infrared observations of celestial objects in the 1920s. While the detectors were crude and plagued by interference, observations in this portion of the electromagnetic spectrum showed great promise. As previously mentioned, light from the Galactic Center is absorbed by the intervening dust particles because the dust particles are close to the same size as the wavelengths of visible light. However, radiation with longer wavelengths, such as radio and infrared, pass unimpeded through the clouds of gas and dust. Therefore, astronomers expected that infrared observations of the Milky Way Galaxy and its central portion would reveal details that could not be observed in visible light.
In 1947 Joel Stebbins and A. E. Whitford conducted infrared observations of the Galactic Center, and found a previously undetected nucleus. These observations were confirmed by other astronomers, and by the 1950s it was an accepted fact that the central region of the galaxy had an extended nucleus visible only in infrared light. This period marked the beginning of modern infrared observing, due in large part to the development of special interference filters and cryogenic cooling systems to improve the quality of the IR detectors.
An intense radio source located in the Galactic Center was discovered by J. Piddington and H. Minnett in 1951 (3). Because their radio data agreed with the assumption that the nucleus of our galaxy should be about 20 parsecs in diameter, the International Astronomical Union defined this strong source as the official center of our galaxy, and in 1959 a galactic coordinate system was developed using this point as its origin (4).
In 1959 Frank Drake used the Green Bank 85-foot radio telescope to observe at 22cm and 3.75cm simultaneously, resolving the Galactic Center into four separate components, named Sagittarius A, B, B2 and C. Observing at these higher frequencies resulted in a fairly high resolution, revealing the complexity of the nucleus. Two of the components, resolved at 2.75cm, were small thermal sources, while the other two sources were not thermal. Drake believed the emission from the non-thermal sources to be attributed to synchrotron radiation, electrons spiraling near the speed of light around magnetic field lines. His data also indicated that the size of the region was about 20 parsecs in diameter, a figure that agreed with previous estimates. He concluded that the brightest of the four sources within this 20 parsec region was probably the Galactic Center (5).
Eric Becklin and Gerry Neugebauer observed the Galactic Center in 1968 in the infrared and near-infrared at a high angular resolution. These high resolution images of unprecedented detail revealed a structure consisting of a dominant source, a point-like source centered on the dominant source, an extended background and additional discreet background sources (6). It was discovered that in Sagittarius A (as the whole structure is generically referred to), both radio and IR radiation occurred at the same coordinates and were of similar sizes. However, Becklin and Neugebauer found the infrared radiation to be orders of magnitude greater than its optical or radio counterparts.
Evidence for a Massive Black Hole
The theory of the existence of a black hole at the center of our galaxy was put forth by Martin Rees and Donald Lynden-Bell in 1971. They believed that a black hole could be one possible explanation for the tremendous amount of energy that had been detected coming from the Galactic Center. Rees and Bell predicted that if the radiation was intense emission resulting from gasses spiraling into a black hole, it would be detectable as a compact synchrotron radio source.
Just three years later, Bruce Balick and Robert Brown detected such a compact source in the inner one parsec core of the galactic nucleus (7). They found the location of this extremely energetic source, Sgr A* (Sagittarius A star), to coincide with the dynamical center of the galaxy. Balick and Brown had detected the first evidence that a massive black hole resided in the heart of our galaxy.
In 1981 Mike Watson led a team that observed the Galactic Center with the orbiting Einstein Observatory in search of X-ray emission (8). They detected diffuse X-ray emission and 12 distinct point sources within 20 arcminutes of the galactic nucleus, Sgr A West. A number of subsequent X-ray observations between 1980 to 1990 revealed a number of transient sources in the region as well.
The detection of X-ray is an important aspect in the identification of black holes. The material falling in toward a black hole forms an accretion disk. As the matter in the accretion disk spirals in toward the black hole, the gravitational energy gets converted into heat, causing the disk material to heat up and emit X-rays. However, when the Galactic Center was imaged in hard X-rays at high resolution with the XRT instrument on Spacelab2 in 1987, Sgr A* appeared much fainter than was expected for a massive black hole. This low emission of X-rays remains unexplained.
Charles Townes and Reinhard Genzel conducted a series of observations in the late 1980s using infrared telescopes to search for evidence of a black hole at the center of our galaxy. They were able to track the movement of giant clouds of gas in the innermost three light years of the Galactic Center. The gas clouds closest to the center moved faster than those farther away, suggesting that the clouds were being gravitationally pulled by a massive black hole. While Genzel felt that this was concrete evidence for the existence of a black hole, the astronomical community in general did not.
Genzel joined forces with Andreas Eckart, and continued the search for a central black hole. Instead of tracking the motion of gas clouds around the Galactic Center, they focused their efforts on tracking the movement of individual stars. Beginning in 1990, Genzel and Eckart used a technique called speckle imaging (a large number of very short exposures are reconstructed into high resolution image) in the near-infrared in an effort to resolve individual stars near the central portion of the galaxy. The resulting data were so heavily processed, however, that although they detected motion in the stars that enabled them to calculate a mass for the central object in the galaxy, their results were not taken too seriously.
Eckart and Genzel continued to develop their technique. By 1999 they had determined the proper motion of more than 40 stars in the central portion of the galaxy (9). Using Newtons laws of gravitation, they were able to estimate the total mass of Sgr A* to be at least 2.6 million solar masses and contained in a relatively small region, clearly indicating the presence of a black hole.
Further evidence for the existence of a massive black hole in the Galactic Center has been introduced by Mark Reid. In 1999 he used the VLBA to conduct radio observations to measure the motion of the Sun around the Galactic Center, and the motion of Sgr A* using distant quasars as reference points (10). If Sgr A* was home to a black hole, it would remain still compared to the Sun moving in its orbit. Two years of measurements show that this is exactly the case Sgr A* remains virtually motionless, while the Sun is moving at 500 kilometers per hour.
The most conclusive evidence to date has come from a team of astronomers led by Andrea Ghez (11). They have made observations in the near-infrared with the 10-meter Keck telescope on Mauna Kea, which is equipped with an adaptive optics system, a complex optical system capable of canceling out the image-blurring effects of the atmosphere, thereby producing extremely high-resolution images. These images have allowed a very precise determination to be made of the proper motion of the stars, which are moving at velocities up to 1400 km/s, and therefore a precise determination of the mass of Sgr A* in which the stars are in orbit around. The most recent calculations give the mass Sgr A* as 2.6 million solar masses. Since the object must fit within the orbits of the fast-moving stars, an area with a diameter of about 16 light days, the objects density is so high that it simply must be a black hole.
Distance to the Center of the Galaxy
Since the days of Herschel, astronomers have attempted to determine the size of our galaxy and the distance to its center. This is an important measurement, as many of the other measured parameters of galactic objects such as distance, mass and luminosity, are directly related to Ro. The exact distance is still a point of debate, ranging from 8.0 to 8.5 kpc. But this situation may soon come to an end.
Samir Salim and Andrew Gould of Ohio State University have developed a method that will allow a direct and extremely accurate measurement of the distance to the Galactic Center. They plan to solve for the Keplerian orbit of individual stars in orbit around the black hole (12). A Keplerian orbit is a closed ellipse that occurs when an object orbits a point mass, such as a planet orbiting the Sun or a binary star system. In general, the orbits of stars in a galaxy are not Keplerian, since they dont orbit around a point mass. In the case of the stars around the black hole in our galaxy however, they are clearly orbiting around a point mass, so must be in a Keplerian orbit.
According to Salim and Gould, the Keplerian orbit can be solved from radial velocity and proper motion measurements of the precision currently obtained by Ghez. They claim that this method, which has been commonly used to measure the masses and distances to binary stars for decades, will allow them to determine the distance to the Galactic Center to an accuracy within 4% as early as the year 2002. One advantage of this system is that it is not a statistical method, and does not suffer from systematic errors. Each star observed will lead to a value of Ro independently, with the accuracy dependent only upon the precision of the measurements of proper motion and radial velocity. Salim and Gould predict they can determine a value of Ro to an accuracy of 1-5% after 15 years of observations, and to an accuracy of 0.5-1% after 30 years of observations. If they are correct, we will very soon have the most accurate measurement of the distance to the Galactic Center than ever before.
For centuries, astronomers have been striving to learn more about the Milky Way Galaxy we call home, and to determine our place in it. The existence of an exotic beast at the heart of our galaxy, a supermassive black hole, has almost certainly been determined. Advancements in technology have allowed astronomers to break through many barriers in order to obtain an ever closer look at the Galactic Center, and will no doubt continue to do so.
(1) Verschur, G.L, 1987, The Invisible Universe Revealed: The Story of Radio Astronomy, Springer-Verlag, New York, 1987
(2), (4) Malphrus, B.K., 1996, The History of Radio Astronomy and the National Radio Astronomy Observatory, Kreiger Publishing Company
(3) Piddington, J.H., Minnett, 1951, Australian Journal of Scientific Research 4A, p.459
(5) National Radio Astronomy Observatory Annual Report, July 1, 1959
(6) Becklin, E.E., Neugebauer, G., 1968, Infrared Observations of the Galactic Center, Astrophysical Journal 151, p 145-161
(7) Balick, B., Brown, R.L., 1974, Intense Sub-arcsecond Structure in the Galactic Center, Astrophysical Journal 194, p 265
(8) Watson, M., et al, 1981, An X-ray Study of the Galactic Center, Astrophysical Journal, Part 1, vol. 250, p142-154
(9) Eckart, A., Genzel, R., 1999, The Galactic Center Black Hole, The Physics and Chemistry of the Interstellar Medium, Proceedings of the 3rd Cologne-Zermatt Symposium
(10) Reid, M. et al, 1999, The Proper Motion of Sagittarius A*, First VLBA Results, The Astrophysical Journal, Volume 524, Issue 2, p. 816-823
(11) Ghez, A.M. et al, 2001, Towards Complete Stellar Orbits Around the Galaxy's Central Black Hole: The First Acceleration Measurements, Black Holes in Binaries and Galactic Nuclei. Proceedings of the ESO Workshop held at Garching, Germany
(12) Salim, S., Gould, A., Sgr A* "Visual Binaries" A Direct Measurement of the Galactocentric Distance, submitted to Astrophysical Journal 2001
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