by George Rhee
Fig. 10.8Spiral galaxies grow by swallowing smaller dwarf galaxies. As they are digested, these dwarf galaxies are severely distorted, forming stellar streams and more complex structures that surround their captors. For all images, the central part is an ordinary positive image. In the outer regions, the negative of the image is shown. In this way, the faint structures are more readily discerned. Wisps, plumes, stellar streams, partially disrupted satellites or stellar cloud are a result of these mergers (Credit: D. Martinez-Delgado et al. 2010, Astronomical Journal, 140, 962, Reproduced by permission of the American Astronomical Society)
Figure 10.9 shows an artist’s impression of a dwarf satellite experiencing the tidal gravity of our Milky Way. The yellow filament is the remains of the dwarf galaxy. The theory predicts that a Milky Way sized galaxy should have accreted 100–200 luminous satellites over the past 12 billion years, so there should be alot of tidal debris in the Milky Way halo. We observe the halo of the Milky Way from inside and the halo of M31 from outside. The Sloan Digital Sky Survey images can be used to select stars that have a high probability of belonging to the Milky Way halo according to their color and brightness. When the density and color of these stars are plotted on a map of the sky (as shown in Fig. 10.10), huge structures are revealed. These structures are the remnants of dwarf galaxies. Some of these structures, streams as they are called, wrap around the Milky Way more than once. This is direct evidence of our galaxy growing by tearing up smaller galaxies during merger events. These data allow us indirectly to map the orbits of dwarf galaxies which reveal the shape of the Milky Way dark matter halo.
Fig. 10.9Smaller satellite galaxies caught by a spiral galaxy are distorted into elongated structures consisting of stars, which are known as tidal streams, as shown in this artist’s impression. Figure 10.10 shows how these tidal streams appear to us from within the Milky Way galaxy (Credit: Image created for Galactic Starstream Survey, MPIA by jonlomberg.com ©2010)
Fig. 10.10This image, known as the field of streams, is a map of stars in the outer regions of the Milky Way Galaxy. It covers an area of the sky 90 by 140 ∘ . The color indicates the distance of the stars, while the intensity indicates the density of stars on the sky. The broad arcs visible in this map are streams of stars torn from the Sagittarius dwarf galaxy. A narrower ‘orphan’ stream crosses the Sagittarius streams. Further analysis reveals the presence of several faint dwarf galaxy companions to the Milky Way (Credit: V. Belokurov (University of Cambridge) and the SDSS Collaboration)
The M31 halo has been the subject of intense study. The image in Fig. 10.11 shows results of a survey made using the Canada France Hawaii Telescope on Mauna Kea. One can image the halo over an area of the sky 40 full moons on a side and the results are spectacular. The Andromeda galaxy spans only a couple of degrees in typical images, but by going to very faint light levels and selecting individual stars on the basis of their color we can map out structures many times the size of the main body of the galaxy. The faint wisps seen in the image are the remains of galaxies that have merged with the main spiral galaxy. Shell-like structures and streams are revealed as predicted by the simulations. The history of the assembly of spiral galaxies is partially written in the distribution of halo stars; we can learn how the Milky Way and Andromeda formed by mapping out the streams and structures left over from different merger events.
Fig. 10.11Map of stars selected to be in the halo and satellite galaxies of M31. M31 dwarf spheroidal galaxies are marked with blue circles. Five newly discovered dwarf spheroidal galaxies are highlighted in red. The green circle lies at a projected radius of 450,000 light years from the center of M31. In addition to the satellite galaxies numerous stellar streams and substructures are visible (Credit: Richardson et al. 2011, Astrophysical Journal, 732, 76, reproduced by permission of the American Astronomical Society)
Review
Observations of nearby galaxies yield clues to the very distant past. These data provide insight into the formation history of the Milky Way and Andromeda galaxies. Low mass galaxies in the neighborhood of the Milky Way and Andromeda can tell us about galaxy formation. The number of satellite dwarf galaxies was initially found to be much smaller than expected leading to the missing satellite problem. It now seems that there may be as many satellites present as predicted by the theory. Some of these dwarf galaxies may be fossils that formed before reionization. These would truly be the oldest galaxies in the universe since they were the first to form. They may be lurking in the suburbs of our galaxy and may be revealed by the next generation of digital sky surveys.
We can study the star forming history of nearby dwarf galaxies by observing the color and brightness of their more luminous stars. We can infer from these data whether stars were formed at a continuous rate or in single or multiple bursts of star formation. The results to date show a variety of star formation histories for dwarf galaxies. In the last decade we have been able to detect extreme examples of dwarf galaxies that consist of a few hundred stars at the center of dark matter halos. wells. The results suggest that we have not yet reached the low end mass end of the galaxy distribution. It is also possible that there may be dark matter halos that are devoid of stars, maybe just containing gas.
Finally we turned to the subject of tidal streams. These streams of stars originated in a galaxy that has been torn apart by the gravity of the Milky Way. We can use these streams to study the shape of the Milky Way dark matter halo.
Further Reading
Dwarf-Galaxy Cosmology. R. Schulte-Ladbeck, U. Hopp, E. Brinks, and A. Kravtsov Eds, Advances in Astronomy, 2010.
Galaxies and the Cosmic Frontier. W. Waller and P. Hodge, Cambridge, Harvard University Press, 2003.
George RheeAstronomers' UniverseCosmic Dawn2013The Search for the First Stars and Galaxies10.1007/978-1-4614-7813-3_11© Springer Science+Business Media, LLC 2013
11. Looking Ahead in Wonder: Telescopes at the Cosmic Frontier
George Rhee1
(1)Department of Physics & Astronomy, University of Nevada, Las Vegas, Nevada, USA
Abstract
Astronomy is big science, the costs of major observing facilities run into the hundreds of millions of dollars. How does one justify such expenditures into basic scientific research which is mainly motivated by curiosity?
The Dutch physicist Hendrik Casimir has argued that innovations originating in fundamental research have a huge impact on the economy. He implies that these innovations would not have happened were it not for basic research.
A Dutch historian recently wrote a book on her life called Omzien in Verwondering (Looking Back in Astonishment). My own life has been, and still is, one of marveling about what lies ahead. If I were to write a book on it, I would rather call it Looking Ahead in Wonder.
J. H. Oort, Some Notes on my life as an astronomer
The Value of Basic Scientific Research
Astronomy is big science, the costs of major observing facilities run into the hundreds of millions of dollars. How does one justify such expenditures into basic scientific research which is mainly motivated by curiosity?
The Dutch physicist Hendrik Casimir has argued that innovations originating in fundamental research have a huge impact on the economy. He implies that these innovations would not have happened were it not for basic research.
Certainly, one might speculate idly whether transistors might have been discovered by people who had not been trained in and had not contributed to wave mechanics or the quantum theory of solids. It so happened that the inventors of transistors were versed in and contributed to the quantum theory of solids.
One might ask whether basic circuits in computers might have been found by people who wanted to build computers. As it happens, they were discovered in the thirties by physicists dealing with the counting of nuclear particles because they were interested in nuclear physics.
One might ask whether there would be nuclear power because people wanted new power sources or whether the urge to have new pow
er would have led to the discovery of the nucleus. Perhaps - only it didn’t happen that way.
One might ask whether an electronic industry could exist without the previous discovery of electrons by people like Thomson and H.A. Lorentz. Again it didn’t happen that way.
One might ask even whether induction coils in motor cars might have been made by enterprises which wanted to make motor transport and whether then they would have stumbled on the laws of induction. But the laws of induction had been found by Faraday many decades before that.
Or whether, in an urge to provide better communication, one might have found electromagnetic waves. They weren’t found that way. They were found by Hertz who emphasized the beauty of physics and who based his work on the theoretical considerations of Maxwell. I think there is hardly any example of twentieth century innovation which is not indebted in this way to basic scientific thought.
Astronomy as well as physics benefits society in practical ways. Andy Fabian in his presidential address to the Royal Astronomical Society has presented a few examples; charge couple devices were not invented by astronomers but were developed for imaging purposes by astronomers and are now used in all phones. The methods we use to access wireless computer networks were discovered by an Australian astronomer. X-ray astronomers developed techniques that are used on security scanners at airports. The atomic clocks used in GPS satellites were developed to check Einstein’s prediction that clocks run differently in different gravitational fields. Basic science such as astronomy provides training in problem solving that can be applied in any field. Basic science also can get children interested in science and engineering fields through the excitement that new discoveries create in the general public.
Can we plan ahead for new discoveries? Fred Chaffee, the first director of the Keck Telescopes made a list of the most important discoveries made by the Keck Telescopes in their first decade. He noted that none of these discoveries was anticipated when the case was made for building the telescopes. Astronomy and science in general abound with examples of discoveries made by accident.
Henri Becquerel discovered radioactivity through a series of accidents. First the Sun didn’t come out on the day he wanted to do an experiment so he stored the equipment in a desk drawer. He then for some reason developed the photographic plate he had left in his drawer. He had for no apparent reason stored the plate with a phosphorescent material containing uranium. All three of these coincidences together enabled the discovery of radioactivity. Rutherford discovered the atomic nucleus by accident. Most recently the discovery of dark energy (Nobel Physics Prize 2011) was made during a campaign to calculate the density of matter in the universe. The discovery of dark energy was completely unexpected.
Immanuel Kant has presented the idea of the orderly progress of science;
Reason must not approach nature in the character of a pupil who listens to everything the teacher has to say, but as an appointed judge who compels the witness to answer questions that he himself has formulated.
The physicist Sheldon Glashow disagrees;
Reason may act as an appointed judge who compels the witness to answer questions that he himself has formulated, but reason must approach nature in the character of a pupil who listens to everything the teacher has to say.
Indeed serendipity has played a key role in many astronomical discoveries, from quasars to dark energy.
What are astronomers to make of this? Astronomers can focus on questions they regard as important in planning future facilities but can they also plan for the unexpected? As Fabian points out;
You can discover things in astronomy by looking deeper in space, by looking at fainter objects, by looking for longer times - or even shorter times if you want to discover pulsars - you can look in finer detail, with better spatial or spectral resolution, you can use other wavebands, such as X-rays, gamma rays, TeV, or polarization, and so on. That’s how we tend to discover things.
Future Plans for Astronomy: Tycho and NASA
Tycho was one of the first scientists to take part in what we might call big science. Tycho Brahe was born in 1546 and became fascinated by astronomy at an early age. He realized in 1563 that the prediction of the best astronomical tables for the alignment of Jupiter and Saturn was off by about 1 month. Tycho found a patron in Frederick II of Denmark who offered him the island of Hveen with buildings, assistants, and income to carry out his researches. It has been estimated that Tycho received an annual income of about 1% of the King’s income; comparable to the percentage of the federal budget that goes to NASA. Tycho had a number of skills required of scientists in the era of big science. One must convince wealthy individuals or governments of the value of the project. This involves communicating the scopes, goals and benefits to non-scientists. One needs managerial skills to deal with the many aspects of running a large operation. If these skills are combined with wise scientific decisions, the results as in Tycho’s case will be remembered and celebrated more than 500 years later. Using Tycho’s observations, Kepler, after much hard work came up with a series of equations or laws describing the motion of the planets. The product of Tycho’s labors was the Rudolphine Tables published by Kepler in 1627. In the time since then our ability to generate data has vastly increased. Tycho’s observatory produced about 100 bytes of data per night, the LSST optical telescope alone will produce 100 billion times as much data per night of observing.
Like Tycho we build new telescopes and scientific instruments to increase the accuracy of scientific measurements and thus lead to new insights. Tycho improved the precision of position measurements on the sky by a factor of more than 10. He used his instruments to systematically measure planetary positions over a period of 20 years.
In the twenty-first century, funding large projects is often more complex than going to a single very wealthy man and convincing him to part with a small percentage of his fortune. There are several thousand professional astronomers in the world and they can’t all go to governments individually. It is easier for the astronomical community to make itself heard if it comes up with a coherent plan for future facilities and research.
In the United States this task is carried out by the National Academy of Sciences. The Academy forms a committee of astronomers whose task is to recommend a list of priorities for astronomy every decade. The resulting document known as the Decadal survey is published and available to the public (see Further Reading section at the end of this chapter). The most recent Decadal survey was published in 2010. The main organizations that fund astronomy in the USA are the National Science Foundation, NASA and the Department of Energy. It is interesting to look at some numbers. The US tax revenue was about 2,000 billion dollars in 2010. The NASA astrophysics budget is about 1 billion dollars currently. The National Science Foundation budget is about 7 billion dollars of which 250 million goes to astronomy research and half to maintaining research facilities. The Department of Energy is also involved in funding some astronomy research in the field of particle astrophysics.
The Decadal Survey came up with three top priorities for the decade 2012–2021. The first priority is the search for the first stars, galaxies and black holes. The goal is to find out when and how the first galaxies formed out of cold clumps of hydrogen and started to shine.
The second priority is the search for habitable planets. Since the detection in 1995 of a Jupiter mass planet orbiting a sun-like star every 4 days, several hundred planets beyond our solar system have been discovered. With current technology we can detect planets that have a mass comparable to the Earth. Most recently telescopes have directly imaged some planets as point sources. The Hubble and Spitzer Space telescopes have found spectral lines revealing the presence of carbon dioxide, water and methane in the atmosphere of orbiting planets. Astronomers are searching for nearby habitable rocky planets with liquid water and oxygen. These earth-like planets are hard to detect because they are so small and dim, so this presents a technological challenge.
The third pri
ority for US astronomy is to understand the fundamental physics of the universe. The 2011 Nobel prize in physics was awarded for the discovery of dark energy and the accelerating universe. By studying how the expansion rate changes with time we can hope to learn more about the nature of dark energy. The report also recommends funding experiments to directly detect gravitational waves which would allow us to explore the nature of extreme gravitational fields.
Having laid out these three themes, astronomers have made specific recommendations regarding which projects to fund. These can broadly be divided into two general categories; space telescopes and ground based telescopes. The top recommendation in the space category is the Wide Field Infrared Survey Telescope WFIRST, designed to see if the nature of dark energy is changing with time. The top recommendation for ground telescopes is the Large Synoptic Survey Telescope (LSST), a wide-field optical survey telescope which will survey the entire sky to faint limits every 4 days. This revolutionary project will transform observations of the variable universe and will address broad questions that range from indicating the nature of dark energy to determining whether there are objects that may collide with Earth.
The European Space Agency (ESA) has conducted a similar exercise and published a document entitled Cosmic Vision; Space Science for Europe 2015–2025. The themes are by and large similar to those of the Decadal Survey. For each of these themes about six candidate projects have been selected. ESA has had a number of great successes in space science with satellites such as Hipparcos, XMM-Newton in the X-ray, Planck, and the Herschel mission in the infrared.