Issues in Physics & Astronomy

Issues in Physics & Astronomy Board on Physics and Astronomy · The National Academies · Washington, D.C. · 202-334-3520 · nationalacademies.org/bpa · Winter 2010 Unveiling the Black Hole and its Environs at the Center of the Milky Way A. Ghez, UCLA he proximity of our galaxy’s center presence of a million solar masses (Mo) • How do stars interact with super- presents us with a unique oppor- of dark matter and confined it to within a massive black holes? tunity to study a galactic nucleus radius of 0.1 pc—to a probability—when • What is the nature of the matter Twith orders of magnitude higher spatial proper motion velocity dispersion mea- flows induced by the black hole in its resolution than can be brought to bear on surements increased the inferred dark neighborhood? any other galaxy. After more than a decade mass density by 3 orders of magnitude It has been known for some time that 12 3 of diffraction-limited imaging with the to 10 Mo/pc and eliminated a cluster there is a population of young stars around rudimentary technique of speckle imag- of dark objects as a possible explana- the center of the Milky Way. The presence ing at Keck and NTT/VLT, the case for tion of the galaxy’s central dark mass of these young stars was used initially to a supermassive black hole at the galactic concentration—and finally to a certainty— argue that there could not be a black hole; center has improved dramatically. The case when individual stellar orbits confined this argument went as follows. Premise: has moved from a possibility—when the the central dark mass to within 0.0004 pc the tidal forces around a black hole must earlier, low angular resolution, dynami- (90 AU) and thereby increased the dark be overcome for stars to form. Premise: cal measurements of the gas and stars at mass density by another four orders of the observed gas densities at the galatic the center of the Milky Way suggested the magnitude. This dark mass density has center are simply not high enough for the eliminated the fermion ball hypothesis as self-gravity of the gas to overcome the tidal an alternative and now the center of the forces and coalesce into stars. Conclusion: Milky Way presents not only the closest if there are young stars present, there can- In this issue: example of a supermassive black hole, but not be a black hole. • Black Hole. Page 1 the strongest case for their existence. Our As previously stated, however, the case • Astro2010 Update. Page 2 galaxy has therefore become an exquisite for the supermassive black hole is much laboratory for learning by example about stronger today than at the time of the first • Spectrum Release. Page 2 supermassive black holes and their envi- discovery of young stars. Furthermore, • RIPLS Release. Page 4 rons at the centers of other galaxies. adaptive optics observations have revealed The recent advent of adaptive optics additional young stars much closer where • DUSEL. Page 6 (AO) has expanded significantly the the tidal forces demand 1,000 times higher • Science & Entertainment. Page 9 scientific reach of high-spatial-resolution gas densities to form stars, greatly enhanc- infrared studies of the center of our galaxy ing “the paradox of youth” at the center of • NP2010. Page 10 through the addition of diffraction-limited our galaxy. With observations that span a • BPA Fall Meeting. Page 11 spectroscopy and significantly improved greater time-baseline and are more precise, imaging capabilities. This has made it it has been possible to estimate the orbits • Standing Committees. Page 13 possible to address a number of questions, of young stars at larger radii than those • Science & Congress. Page 15 including: used to reveal the black hole. At these • How do young stars come to reside • Staff News. Page 15 in the vicinity of a supermassive black hole? See “Black Hole” on page 2 BPA News · Winter 010 Black Hole Spectrum Management for (continued from page 1) Science in the 1st Century larger distances, a disk of young stars has D. Lang, BPA Staff been revealed suggesting that they formed from a pre-existing dense gas disk. In this ecognizing the growing impor- in-person meetings, four town hall meet- case, it is quite likely that when these stars tance of radio observations to ings, and numerous teleconferences in the were forming our black hole was much their respective missions and the development of its report. The committee’s more active than it is today. Rincreasing potential for interference from work was aided by presentations from a Adaptive optics observations of stars new wireless technologies, NASA, the De- number of outside experts who provided in the vicinity of the Milky Way’s super- partment of Commerce, and NSF commis- detailed information at in-person meet- massive black hole have revealed a second sioned the NRC to identify the spectrum ings. The below text is taken from the surprise. Theoretical work suggests that needs of today’s scientific activities and summary of the committee’s final report, old stars should be highly concentrated in to assist spectrum managers in balancing Spectrum Management for Science in the a cusp around the black hole, providing the requirements of scientific uses of the 21st Century. both an alternative method (searches for spectrum with those of other interests. The Natural radio emissions from objects as cusps) for identifying supermassive black committee discussed the original charge to diverse as hurricanes and distant galaxies holes in cases where dynamical studies are the committee at length, and chose to con- yield vital information about Earth and its not possible and predictions of how stars sider only the passive (“receive-only”) sci- place in the universe. Radio observations are accreted onto the black hole. However, entific applications of the radio spectrum, of Earth are central to weather forecast- recent observations show a dearth of old and specifically how the requirements stars compared to what is expected at our of spectrum could be expected to evolve galactic nuclei. These observations are pro- over the next two decades. This decision Committee on Scientific Use of the viding unique insight into how black holes stemmed from the committee’s recognition Radio Spectrum Membership interact with their environment and hence that passive scientific uses involve unique Marshall H. Cohen, Co-chair, California how black holes grow over time, and they issues. Institute of Technology are modifying our views of the possible To address its task, the NRC’s Com- Albin J. Gasiewski, Co-chair, University of populations of supermassive black holes. mittee on Scientific Use of the Radio Colorado at Boulder Adaptive optics observations of the Spectrum—comprising representatives Donald Backer, University of California, galactic center have also resulted in the first of universities, private industry, and Berkeley detection of infrared emission associated nonprofit organizations—employed four Roberta Balstad, Columbia University with the central supermassive black hole. Steven W. Ellingson, Virginia Polytechnic Along with providing the best constraints Institute and State on theoretical models for low accretion Darrel Emerson, National Radio Astronomy rate flows—important for galactic nuclei Astro010 Update Observatory and also for x-ray binaries—those observa- Aaron S. Evans, Stony Brook University Joel Johnson, The Ohio State University tions also provide detection of the galatic he Astro2010 Decadal Survey Paul Kolodzy, Kolodzy Consulting, LLC center’s dramatic short-timescale infrared on Astronomy and Astrophysics David Kunkee, The Aerospace Corporation continues to move forward. The variations showing red noise behavior and Molly K. Macauley, Resources for the a power-law break consistent with the x-ray Tnine panel reports recently entered the Future, Inc. variability seen in active galactic nuclei. NRC’s peer-review process and the survey James M. Moran, Harvard-Smithsonian With high spatial resolution capabili- committee held their last two (closed) Center for Astrophysics ties on large-ground based telescope, data meetings at the end of January and Feb- Lee G. Mundy, University of Maryland at analysis techniques continuing to improve, ruary. The survey committee’s report is College Park and larger telescopes on the horizon, the scheduled to enter NRC review in the Timothy J. Pearson, California Institute of next decade promises new discoveries in spring. The prepublication versions of the Technology galactic center research. Specifically, future survey committee report and the panel re- Christopher Ruf, University of Michigan measurements are expected to reveal ports are expected to be released later this Frederick S. Solheim, Radiometrics Corporation deviations from Keplerian orbits, enabling summer. Check the survey’s webpage at David H. Staelin, Massachusetts Institute of both tests of general relativity and mea- www.nationalacademies.org/astro2010 for Technology further updates on the release of the report surements of the extended dark-matter Alan B. Tanner, Jet Propulsion Laboratory distribution. n over the summer months. n BPA News · Winter 010 ing and climate studies, and observa- vices (Earth Exploration-Satellite Service The current regulatory regime is strain- tions of the cosmos are similarly critical [EESS] and Radio Astronomy Service ing to enable the capabilities and meet the for increasing our understanding of the [RAS]), which do not radiate. Transmitters needs of the various communities of users. universe and answering grand questions for the active services create an artificial As the active and the passive services are such as that on the origin of planets. Such “electronic fog,” which can cause confusion increasing their use of the spectrum, the information is gathered by geoscientists and, in severe cases, totally blind the EESS potential for interference, already strong, using complex Earth-orbiting satellites and RAS receivers. is also increasing.

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