by George Rhee
Big Science: The Age of Surveys
Key developments in recent years have been provided by surveys that systematically map the sky. The Sloan Digital Sky Survey for example, consisted of a survey of the sky visible from Apache Point Observatory in southern New Mexico. What was revolutionary about this project was the way the whole database was made available to the science community and indeed the general public.
With the development of the internet, surveys of the sky have transformed the way astronomy is carried out. One can access archived survey data to select samples of objects for further study. One can also compare the properties of hundreds of thousands of objects.
In the era of photographic plates this was more difficult as one had to have direct access to the plates which were stored in specific observatories. Today the data are accessed with a browser. The surveys are driven by scientific questions. The original goal of the Sloan Survey was to map the large scale structure of the universe on scales large enough to determine whether the universe was homogeneous.
The Sloan Digital Sky Survey imaged the sky in five color bands and carried out a spectroscopic redshift survey of about one million galaxies. The Sloan Survey website has had over 800 million hits in 9 years with over 1 million distinct users (far larger than the number of professional astronomers in the world). The survey produced 30 TB of data which is comparable in size to all the information stored in books in the Library of Congress.
Another survey, the Two Micron All Sky Survey made use of two 1.3 m telescopes locates in Arizona and Chile to produce the first high resolution survey of the complete infrared sky. These data have been used to address a number of topics such as the structure of the Milky Way, the distribution of galaxies in the nearby universe and also to support observations by more sophisticated telescopes such as NASA’s Spitzer Space Telescope.
Projects like these have ushered in a new era that one might call the age of surveys. Data that are accumulated for one purpose can be used on different projects. By making the data public, people who are not members of the original team can answer entirely new questions that may not even have occurred when the project was started.
The Large Synoptic Survey Telescope
The Decadal survey recommended the Large Synoptic Survey Telescope as the first priority for ground based astronomy. The word synoptic derived from the Greek describes observations that give a broad view of a subject. The telescope (Fig. 11.1) has an 8.4 m diameter mirror and will be sited in Chile. It is uniquely designed to produce excellent images over a 3. 5 ∘ field of view (seven moon diameters). The plan is to observe the sky repeatedly in six colors in and near the visible part of the spectrum. Over its 10 year lifetime the telescope will image each region of the sky 1,000 separate times. These images can be used to produce a deep map of the entire sky. The data will be made available to the astronomical community and the general public. The telescope will essentially make a movie of the sky. This movie will be ideal for investigations of time-variable phenomena such as supernovas, variable stars and near earth objects. The use of a groundbreaking camera will undoubtedly bring about new and surprising discoveries. The synoptic telescope will make use of a three billion pixel camera. By comparison the latest ipad screen has three million pixels. It will take less than 2 s to read out the data and this will be done every 15 s.
Fig. 11.1The 8.4-m Large Synoptic Survey Telescope will use a special three-mirror design, creating an exceptionally wide field of view and will have the ability to survey the entire sky in only three nights (Credit: LSST Corporation)
The telescope will produce an astonishing amount of data, about 100 PB of data (1017), which is almost a billion bytes of data. About 20 TB (20,000 GB) of data per night will be generated, an amount of information equivalent to the entire information stored in all the books in the Library of Congress. Coping with this vast amount of data presents a special challenge for astronomers. What a long way we are from Galileo’s first telescopic observations 400 years ago.
The telescope will provide a catalog of 20 billion astronomical objects. In two nights of imaging the synoptic telescope will obtain more data than the Sloan Digital Sky Survey gathered in 8 years. The telescope is expected to start operating in 2015. The camera will take two 15 s exposures at each location and survey the entire sky visible from the site every 4 days. A quantity called the etendue is used to compare instruments used to survey the sky. We use the product of the mirror collecting area and the telescope field of view to compare telescopes. By this measure the synoptic telescope is a factor of 30 ahead of its immediate competitors.
Tycho could not have anticipated that his instruments would lead to a revolution in physics and eventually a new theory of motion and the discovery of gravity as a fundamental force. This only happened after the many years Kepler spent analyzing Tycho’s data. To search for hidden treasure in the huge synoptic telescope database, astronomers will need to develop new methods. The effort to simultaneously push the envelope in the fields of optics, electronics, and software at once is one of the fascinating aspects of this project.
The Next Generation of Radio Telescopes
Observations of the redshifted 21 cm line provide a picture of the universe at a time when the first stars and galaxies were forming, the era known as the dark ages. The dark ages started about 400,000 years after the big bang and ended about 1 billion years after the big bang when reionization was complete. The 21 cm radiation is redshifted to wavelengths of a few meters as it travels towards our telescopes. These large wavelengths correspond to low frequencies and new telescopes are being built to operate at these frequencies. The Square Kilometer Array is one such telescope (Fig. 11.2). It is to be completed by 2024 and will be 50 times more sensitive than any other radio telescope. The telescope consists of many separate radio dishes whose data are combined to make images of the sky using a technique called interferometry. The cost of the project is estimated at 2 billion dollars. It is a collaboration involving 20 countries. The telescope is scheduled to be built on sites located in Australia and South Africa.
Fig. 11.2Artist’s impression of dishes that will make up the The Square Kilometer Array (SKA) radio telescope. The telescope will give astronomers remarkable insights into the formation of the early Universe, including the emergence of the first stars, galaxies and other structures (Credit: Swinburne Astronomy Productions/SKA Program Development Office)
The telescope will produce 1 PB (1015 bytes) of data per day about 500 times the entire amount of information stored in books in the library of congress. New technology to cope with this deluge of data is being developed. As with the Synoptic Telescope the idea is to vastly improve the sensitivity of previous radio surveys and provide insights into current key problems of cosmology.
Several other radio telescopes are being built. A project, which is due to start operations in 2016, is the single dish 50 m Aperture Spherical radio Telescope known as FAST. The telescope will be located in China and will aim to measure the hydrogen distribution in galaxies, such as the faint dwarf galaxies described in the previous chapter. In particular, it may be possible to detect ‘galaxies’ which contain dark matter and gas but have not formed stars. The FAST project is an up-scaling of the Arecibo telescope that was featured in the Bond movie GoldenEye. The Very Large Array in Socorro, featured in the movie Contact, consists of 27 radio dishes each 25 m in diameter that can be moved on railroad tracks into various configurations. The Very Large Array is being upgraded to improve sensitivities by a factor 5–20. The Giant Meter Wave Telescope is located near Pune, India and is an array of 30 fully steerable radio telescopes. This telescope operates at low frequencies and is well suited to searches for neutral hydrogen gas at high redshifts discussed above.
Many of these projects are international collaborations. FAST is a Chinese project influenced by a US observatory located in Puerto Rico, SKA is being planned and designed by scientists at 70 institutes spread over
20 countries. Science inspires us by the fantastic discoveries that are made but also by showing how people of different cultures and races can work together towards a common goal.
The Next Step for Infrared and Sub-millimeter Telescopes
Infrared imaging of distant galaxies gives us information on the visible light that they emit. We have a wealth of information on nearby galaxies at visible wavelengths, so we can compare high redshift galaxies with nearby galaxies in the same emitted wavelength range by carrying observations at very different wavelengths. These studies have concluded that the merging of galaxies was more frequent in the past than it is today. The sub-millimeter detector on the James Clerk Maxwell telescope has revealed a new population of far infrared luminous high redshift galaxies. These galaxies form stars at a much higher rate than our Milky Way galaxy. The positions of these galaxies are not known well enough to find their optical counterparts; there are dozens of candidate galaxies in the Hubble Deep Field image consistent with the location of one sub-millimeter galaxy.
The Atacama Large Millimeter Array (ALMA) shown in Fig. 11.3 will be able to solve this problem. It has started to produce images that are as detailed as those of ground based optical telescopes. However it will do this for the first time at wavelengths of half a millimeter. ALMA consists of a giant array of 6,612 m antennas spread over an area 16 km in diameter. It is located at 5,000 m altitude.
Fig. 11.3The ALMA (Atacama Large Millimeter/submillimeter Array) site. Ten 12-m antennas are in position in this photo taken in 2011. The site is located at 5,000 m altitude on the Chajnator plateau. The ALMA main array will have fifty 12-m antennas (Credit: ALMA, ESO/NAOJ/NRAO, J. Guarda)
There are also two major satellites operating in the sub-millimeter and the infrared. They were launched simultaneously (on the same rocket) by the European Space Agency. Planck is a 4 m telescope operating in the centimeter to sub-millimeter range with the goal of making detailed studies of the microwave background. The idea is follow-up on the results of NASA’s COBE and WMAP satellites. Planck is more sensitive and has better spatial resolution, it will also measure to high accuracy the polarization of the cosmic microwave background which will help determine when the first stars and galaxies were formed.
Launched in tandem with Planck, the Herschel Space Observatory covers the far infrared range of the electromagnetic spectrum from roughly 50–700 μm. Herschel is being used for galaxy surveys.
The most ambitious future project is the James Webb Space Telescope to which we devote the final chapter.
Thinking Small
The projects we have discussed so far each cost more than 1 billion dollars. Can cutting edge astronomy be done for less than 1 billion dollars? NASA believes so and runs the explorers program that is designed to provide flight opportunities for cheaper science missions (Fig. 11.4). Many of these have been huge successes. The COBE satellite was the first telescope to detect variations in the cosmic background radiation intensity. COBE’s successor, WMAP, another small NASA mission, showed that one can obtain precise information about the age and content of the universe from detailed measurements of these fluctuations. The NASA SWIFT satellite followed up on the discovery of gamma-ray afterglows and produced many identifications of the galaxies in which these bursts occur and a deeper understanding of the whole phenomenon of gamma-ray bursts. It takes about 5 years to develop and launch an explorer class mission. The total cost of three explorer missions is less than the price of one Hubble Space Telescope. NASA’s major observatories that explored the sky from space at infrared (Spitzer), optical (Hubble), X-ray (Chandra) and gamma-ray wavelengths were hugely successful, but NASA has also demonstrated that smaller budget missions can produce great science. Recent ideas for new smaller missions include a radio mission that would be placed in orbit around the Moon to measure the spectrum of neutral hydrogen emission and absorption from the time of reionization. An infrared satellite to search for quasars in the redshift range 7–11 using infrared imaging of the sky has also been proposed. There are many creative ideas coming from the community for these missions.
Fig. 11.4Two examples of competitively selected astrophysics explorer missions launched since 2000 (Credit: WMAP MIDEXNASA/WMAP Science Team (top) and WISE MIDEXNASA/JPL-Caltech/WISE Team; NASA/JPL-Caltech (bottom))
X-Ray and Gamma-Ray Astronomy
There are many ways that X-ray and gamma-ray astronomy can contribute to cosmology. Cosmological models suggest that black holes formed at the same time or before the formation of galaxies. One would like to find evidence of these black holes at redshifts greater than ten when the first galaxies were forming. It is also becoming clear that black holes play a role in regulating star formation in massive galaxies. The next generation of X-ray telescopes will help determine the amount of and chemical composition of matter expelled from the surroundings of black holes.
The hot X-ray emitting gas in clusters of galaxies is also of interest to cosmology. We can measure the mass of clusters using X-ray observations, we can measure the composition of the hot gas and we can see evidence of cluster and galaxy merging in the images of the hot gas. The next generation of telescopes will push the study of this gas up to redshifts of two (3 billion years after the Big Bang). Proposals for future telescopes include the idea of a two spacecraft telescope. The mirror would be contained on one spacecraft and the detectors on another, requiring the two spacecraft to fly in formation with an accuracy of a few millimeters. The funding for this mission is at present uncertain. A number of ideas for new telescopes have appeared and disappeared on the NASA and ESA websites and the situation is fairly fluid. We currently have the XMM-Newton telescope that is carrying out X-ray observations.
The Fermi satellite is the current state of the art gamma-ray observatory. The satellite was launched in 2008. Fermi can measure the spectra of gamma-ray bursts from a few kiloelectron-volts to hundreds of gigaelectron-volts. The Large Hadron Collider at CERN collides protons at energies ten times large than the highest energy gamma rays observed with Fermi. Some gamma-ray bursts are associated with the most energetic bursts of light in the universe and also with the highest redshift objects known to us. The highest redshift gamma-ray burst is at a redshift of 8.2, which means it is observed when the universe was less than 5% of its present age. The redshifts are estimated using the Lyman-break technique. Observations of the burst afterglow are made simultaneously at optical and infrared wavelengths. If we see infrared emission but no optical emission we infer that the optical emission has been absorbed by neutral hydrogen at high redshift. These high redshift events help estimate the rate of star formation and potentially the abundance of elements other than hydrogen at high redshift. It is possible that the first population of stars which are thought to be much more massive than the average stars formed today might leave their imprint in the gamma-ray burst absorption lines. The high redshift gamma-ray bursts might also pinpoint places in the universe where something ‘interesting’ is happening so that we can follow up with observations at that location after the burst has taken place.
The Next Generation Giant Optical Telescopes
Optical astronomers are also busy planning for the next big step in their field. Bigger is usually better for optical telescopes, so the idea is to ramp up from the current 8 and 10 m mirror giants to mirrors that are between 30 and 40 m in diameter. Figure 11.5 illustrates the increase in the collecting area of the largest telescopes since the telescope was invented. The collecting area has increased by roughly a factor of 10 every 100 years. The diameter of the current largest planned optical telescope mirrors will be 6,000 times larger than that of the human eye. Optical telescopes were first used by Galileo and Lippershey about 400 years ago. Their telescopes used lenses a few centimeters in diameter. We want to increase the telescope size in order to gather the light of increasingly faint stars and galaxies for analysis. The diameter increase was initially achieved by manufacturing large lenses which in turn required very long tu
bes. This is because light could only be brought to a focus a long distance from the lens to avoid image distortions. Isaac Newton built a reflecting telescope in 1688 (with a 3.3 cm mirror diameter). William Herschel scaled things up to 1.3 m and the Earl of Rosse built a mirror of almost 2 m diameter for his telescope in 1845. George Ellery Hale had the Mount Wilson 100 in. built in 1928 and the Palomar 200 in. in 1949. The Mount Wilson and Palomar telescopes are still used as active research tools. William Huggins in the late 1880s established spectroscopy as a tool in astrophysics. It is the combination of large telescopes and the tools of spectroscopy that brought about many of the discoveries in cosmology. For example Slipher, Hubble and Lemaitre established the expansion of the universe using galaxy spectra. Schmidt showed in 1963 that quasars were distant luminous sources of radiation that play a key role in galaxy formation. In the 1970s, analysis of quasar spectra revealed the presence of neutral hydrogen in clouds at high redshifts. A major step forward in the 1980s was the development of multi object spectrographs that could take spectra of a few tens of galaxies simultaneously. This development led surveys such as the Sloan Digital Sky Survey in the US and the Two Degree Field Redshift Survey in Australia. These surveys delivered spectra of several hundred thousand nearby galaxies. The current state of the art is a 3,200 fiber spectrograph that has a 1.5 ∘ field of view and will be used on the 8 m Subaru telescope on Mauna Kea. This instrument will deliver about 100,000 redshifts per night for galaxies out to redshift 1.6. The technology is such that the 3,200 fibers can be repositioned in 40 s.
