High-Stakes Astronomy

Transcription

1 4 High-Stakes Astronomy AT LOW frequencies... Amidst the radio-frequency noise of microwave ovens, cell phones, and the occasional unfiltered stereo system, are astronomical features of our universe waiting to be found. In this article, Carolyn Collins Petersen conveys the enormous lengths to which astronomers are going in order to extract astronomical signals at low frequencies. 98 Image courtesy SKA Project Office and Xilostudios. 4 - High-Stakes Astronomy at Low Frequencies... Opening Up a New Wavelength Frontier

2 ...Opening Up a New Wavelength frontier T HE UNIVERSE is an equal-opportunity radio-frequencyemitter.pointadish(oraradio detector array) in any direction and you can find something interesting to study. This is because anything active in the universe gives off a range of wavelengths and frequencies, including lowfrequency signals below 400 megahertz (MHz). This includes our Sun, which puts out radio signals across a broad range of frequencies. The planet Jupiter does interesting things that we canstudybetween15-30mhz.explodingstars and the center of the Milky Way Galaxy are also among the millions of sources that astronomers have observed at low frequencies. Yet, until very recently, this range of the radio spectrum remained largely unexplored by astronomers because signals were difficult to detect through allthenoiseofourtechnology. Didyoutalkonacellphonetoday? Usea wirelesscomputer?transfersomemoneyfrom checking to savings using an ATM machine? Listentoaradiobroadcastinyourcar?Flyina plane? If so, you used technology that depended atsomepointontheuseofradiowaves.our lives are filled with gadgets that use radio waves, but these pieces of modern technology also put out radio frequency interference (RFI). This noise can and does interrupt or completely mask signals from space. Sifting them out from interference by both natural and human-made sourcesisachallenge. Left: Frequency allocations in the radio spectrum cover everything from 10 kilohertz (very low frequencies, LHF) to 30 gigahertz and above (extremely high frequency, EHF) wavelengths. All radioemitting technologies, from cable-locating equipment and power line carrier systems to aircraft navigation systems and industrial/business uses are shown on this spectrum chart. Radio astronomy peeks through at various wavelengths. Unfortunately, some of the most interesting and unexplored parts of the radio astronomy spectrum lie in the lower end of the spectrum, at frequencies up to a few hundred megahertz. These same frequencies are heavily populated by telecommunications and broadcast transmissions, which can partly or completely block off-planet signals. Image courtesy National Telecommunications and Information Administration, U.S. Department of Commerce. State of the Universe

3 Frequencies and Wavelengths In radio astronomy, signals are often stated in terms of frequency and/or wavelengths. Frequency is the term for the number of cycles a radio wave goes through per unit of time. You often see the terms megahertz or kilohertz used in relation to radio waves. One hertz (often abbreviated Hz) is one cycle per second. Since radio waves go through hundreds, thousands, millions, or billions of cycles per second, metric prefixes are used to describe those frequencies. Kilohertz is abbreviated khz and means thousands of hertz ; megahertz (MHz) is millions of cycles, and gigahertz (GHz) is billions of cycles. For purposes of low-frequency astronomy, astronomers concentrate on a frequency range between 10 and 400 MHz. For comparison, the radio wavelengths covered by all of radio astronomy range from 10 to 10,000 MHz, with VLBI experiments extending the range up to 245 GHz and the range of the Smithsonian Submillimeter Array topping out at around 800 GHz. The Atacama Large Millimeter/ submillimeter Array, which will be exploring the cool universe (radiation leftover from the Big Bang, molecular gas and dust clouds, planetary systems, and galaxies) will be sensitive at submillimeter wavelengths and frequencies between 80 and 900 GHz. In addition, you can express a radio signal in terms of its wavelength (literally, the length of the wave). A longer wave is a less-energetic signal; a shorter wave carries more energy. To derive a signal s wavelength in meters from a frequency, use the following formula: 300/ frequency (in MHz). Above right: Low-band (top) and High-band (middle) antenna elements in the Low-Frequency Array (LOFAR), located in the Netherlands. The Low-band antenna is sensitive to frequencies between MHz. The High-band antenna is sensitive to frequencies between MHz. These detectors should be able to trace signals from such celestial events as supernova explosions, black hole collisions, and signals from the earliest epochs after the Big Bang. The data will be processed in the STELLA supercomputer (bottom). Images courtesy ASTRON, LOFAR Consortium. The Low-Frequency Astronomy Race What kinds of signals are we talking about? Currently, radio astronomers are racing to the low-frequency end of the electromagnetic spectrum to search out everything from the oscillations of primordial hydrogen atoms that existed when the first stars began to shine, to the blasts of radio noise emitted when stars and galaxies collide. It also turns out that some distance-stretched signals from other civilizations,iftheyexist,couldbehidden in thenoise ofthelow-frequencyspectrum. All of these signals are in plain view of a newbreedofexistingandplannedradioarrays specifically sensitive to low-frequency radio signals that lie in sections of the frequency spectrum that we also use for radio, TV, military and civilian communications. The installations, suchastheverylargearrayinsocorro,new Mexico,containdozenstothousandsofdetector antennaunits.theyemployinterferometryto gatheragood picture ofanobjectinthesky High-Stakes Astronomy at Low Frequencies... Opening Up a New Wavelength Frontier

4 Interferometry combines and correlates data fromthetelescopesinanarraytoformmultiple pixelsofthesky.thepixelsizedeterminedby thesizeofthebaselinebetweenarrayelements. Still, just to have a ghost of a chance of reliably detecting astronomically meaningful low-frequency signals buried by our radio and TV broadcasts, at least two of these observatories the the Mileura Wide-Field Array (MWA) and thesquarekilometrearray(ska) arebeing planned for installation in very radio-quiet areas in the southern hemisphere. Australia is hosting MWA, and as of this writing, the SKA consortium is considering Australia and South Africaforthefinalsite. Whileyoumaythinkthatallradioarrays are out there in the desert somewhere, like the MWA, or the Very Large Array, several of these detector collections are sited near local farms, or at observatories nestled up to people s backyards. Operators of these systems, such as the Low- Frequency Array (LOFAR) in the Netherlands, maintain they can also do the same science as theirmoredistantcounterparts,buttoachieve it,theymustdevelopsophisticatedalgorithms and filtering techniques to tease out the lowfrequencywhispersofexcitingcosmicevents. An Array in My Corn Field? Whathappenswhenanarraymustbebuiltin a populated place (for political, financial, or scientific reasons)? It doesn t automatically mean that science can t be done. This is a challenge the builders of LOFAR (currently under construction in the Netherlands), are working to overcome in their detector designs. Thisarrayisunderlocatedonfarmlandsand open spaces across the Netherlands, and will ultimately spread 15,000 antennas distributed over 77 stations across that country and possibly into Germany. When fully deployed, LOFAR could have a maximum baseline between its mostdistantelementsofabout360kilometers, and will be sensitive to frequencies between 30 and240mhz.theinteriorportionofthearray will be able to look in as many as 200 directions simultaneously. The antennas will pump data to collectorsites,thentoaseriesofremotestations, and ultimately to a core computing center where systems will correlate the data and send it along to scientists for study. The builders of LOFAR and other arrays close to human habitation must operate in what its designers call a hostile RFI environment. For LOFAR, mitigating interference by using filteringtechnologywillbeakeytosuccess.the firstarrayelementsareinplace,andin2006, LOFAR scientists and engineers began reporting test results. Deuterium from the Backyard Advances in filtering technology for lowfrequency radio astronomy got a demonstrable boostin2005whentheoperatorsofaseriesof antennas called the Deuterium Array, located at MassachusettsInstituteofTechnology shaystack Observatory in northern Massachusetts, reported the detection of oscillations of deuterium atoms at327mhz.thissignalisextremelysensitiveto interference, and so in order to see deuterium at that wavelength, the receivers had to be shielded from outside RFI. Top: The first high quality deep wide-field image with a LOFAR station, released 25 April The data were collected with 96 low-band antennas located in four fields at the heart of the array in the province of Drenthe in the North-East of the Netherlands, and transported over a dedicated glass-fiber link to a central processing facility at the University of Groningen, some 60 km away. The image was made at a frequency of about 50 MHz and is centered on the bright radio source Cassiopeia A. At least 40 other sources can be seen in this image. Image courtesy ASTRON, LOFAR Consortium. Above: Early results from the LOFAR ITS test site show a map of the sky as seen between 29.5 and 30.5 MHz. Some easily recognizable structures, such as the Perseus A region, quasar 3C123, the Cygnus X object, and other quasars and the cores of star-forming regions stand out at these wavelengths. Image courtesy Stefan Wijnholds (ASTRON). State of the Universe

5 As Massachusetts Institute of Technology scientist Alan Rogers and his team of researchers built the Deuterium Array, they rose to the challengeofweedingoutradionoisebysniffing out sources of RFI from nearby houses in the neighboring towns of Westford and Groton. By replacing such things as answering machines andstereopartstomakeaquietenvironment, they cleared the way for the array to search out the signals from primordial deuterium located at the anti-center of the Milky Way Galaxy. The practice paid off. After a year of data-gathering, the team reported the first unambiguous detectionofdeuteriumat327mhz,inapaper by Rogers et al. entitled Deuterium Abundance in the Interstellar Gas of the Galactic Anticenter from the 327 MHz Line published in the 1 September 2005 issue of Astrophysical Journal, one of the bibles of astronomy science results. Whatisdeuteriumandwhyallthefussover detecting it at the low end of the frequency spectrum? Deuterium is an isotope of hydrogen, andmostofthenaturallyoccurringkindwas createdinthebigbang,some13billionyears ago. It ssomethingofa fragile elementbecause deuterium can be destroyed very easily in stellar interiors, for example, or by the heat generated in the starbirth process. Areas in galaxies where starformationisactive(orhasbeeninthepast) donothaveveryhighamountsofprimordial deuterium. Thus, deuterium (or the lack of it) is whatscientistscalla tracer ofstellaractivity. The Deuterium Array specifically zeroed in on a region of the Milky Way Galaxy where the scientists expected that some primordial deuterium would still exist in the interstellar gas.theyfocusedonthe327mhztransitionof deuterium because it allows them to measure the relative amounts of deuterium to hydrogen (called the D/H ratio) and apply it to sources anywhere in ourgalaxy.asmallamountofdeuteriumrelative to hydrogen means that most of the deuterium that mayhaveexistedintheareahasbeendestroyedor locked in a chemical bond with another element. TheDeuteriumArrayresultshowedaD/Hratio of 23 parts per million in the region of the galactic anti-center(aregionwellawayfromthecenterof the galaxy, where relatively little star formation has taken place). Above right: Two views of the MIT Haystack Observatory deuterium array. This was an electronically steerable multi-beam array of twenty-five 5 x 5 crossed dipole stations sensitive to deuterium emissions at 327 MHz. It performed its tasks within a few hundred feet of a multitude of houses in Westford and Groton, Massachusetts, communities about 30 miles northwest of Boston. The small silver trailer (arrowed in the aerial photograph, and also shown in the close-up view) is a radio frequency interference monitor that kept track of stray radio noise during the array s operation. Images courtesy MIT Haystack Observatory. In larger cosmic terms, astronomers have known for some time that the amount of deuterium in the universe has a bearing on how much regular matter (called baryonic matter) thereis,aswellashowmuchphotonic matter (light) there is. Because of the way it was created in the Big Bang, accurate measurements of deuterium allow astronomers to understand more about the mechanics of creation. In addition, knowing how much deuterium existed both then and now will let astronomers calculate the densityofcosmicbaryons(regularmatter)inthe universe. That density would indicate whether ordinary matter is bound up in black holes, gas clouds,orbrowndwarfs,orismoreluminous andtiedupinmakingstars. However, there is a substantial amount of something else in the cosmos that we can t detect (yet), but we can sense its gravitational pull on ordinary matter. It s called dark matter, and measurements of deuterium at as many wavelengths as possible will also give scientists ahandleonjusthowmuchdarkmatterthere is out there. Thanks to the Deuterium Array, theyareafewstepsclosertothatgoal. An Array Out Back in the Outback Itmayseemanunlikelyplaceforradioastronomy, butalivestockstationinfarwesternaustralia isthesiteofanotheruniquearraythatisalso underconstruction,butisalreadystudyingthe universe at frequencies below 300 MHz. Unlike thelofararraysitedinaradio-noisypartof Northern Europe, the Mileura Wide-field Array takesadvantageofoneofthefewremainingradio High-Stakes Astronomy at Low Frequencies... Opening Up a New Wavelength Frontier

6 quietareasontheplanet,aregionofaustralia nearly 200 miles (300 kilometers) inland from the country s western coast and safely located well away from major cities and sources of radio frequency interference. It has excellent sky access, especially to the center of our Milky Way, and the Large and Small Magellanic Clouds (our galactic neighbors). The array is being built in two parts: a Low-Frequency Demonstrator (LFD, active in the MHz range) and the New Top left: Outback Australia, at the Mileura Wide Field Array site, is the sort of rugged, radio-quiet region where low-frequency astronomy can be carried out more easily. Image courtesy Colin Lonsdale, MIT Haystack Observatory. Bottom left: Students help construct a prototype of the 500 tiles of the MWA in Australia. Each tile has 16 antennas sensitive to frequencies between 80 and 300 MHz. Image courtesy Merv Lynch, Curtin University of Technology, Australia. Above: An image sequence showing the galactic center passing overhead at Mileura. The frequency is 108 MHz and the images were made by steering a single-tile beam to 40 grid locations in rapid succession. The zenith is at the center of the image. Image courtesy Judd Bowman, MIT Haystack Observatory. Low-Frequency Astronomy Targets The ambitious science programs that low-frequency astronomers want to pursue echo a broad range of astronomical topics. Examine the web sites for the science goals of the MWA, SKA, and the Allen Telescope Array (planned for full deployment in California), and the United States Navy s Low-Frequency Array, and you ll see many of the same science targets. Let s examine some of them in more detail. First, there s the Epoch of Reionization (often called EOR, for short). This is something of a Holy Grail at nearly all frequencies. It s a period early in the universe about a billion years after the Big Bang when the first luminous sources (such as stars, galaxies, or quasars) began to light up the intergalactic medium (filled with neutral hydrogen gas). By studying the EOR at many frequencies, astronomers can learn about how structures like galaxies and galaxy clusters formed in the tumultuous times after the Big Bang. It casts insight into the distribution of matter and how today s highly structured universe may have evolved. Low-frequency detection of events at these early epochs depends on being able to sift out the signal of what astronomers call the 21-cm hyperfine transition line of neutral hydrogen redshifted to frequencies below 200 MHz. Theoretically, lowfrequency arrays that can detect this line will be able to probe the reionization of the early universe in great detail, giving astronomers a feeling for the density, temperatures, and velocities of material that existed back then. A wide field of view, as possible from an array, would show huge parts of the early universe. Second on everybody s hit list of cosmic study targets are transient radio sources, the so-called transient sky. These are things like cosmic rays, pulsars, gamma-ray burst afterglows, the aftermath of supernova explosions, the huge bursts of energy released when massive objects (like black holes or neutron stars) collide, and the radio emissions from extrasolar Jupiter-like planets. Next are low-frequency surveys of distant galaxies to count and classify them, followed by studies of the molecules in the interstellar and intergalactic mediums. The abundances of certain molecules, such as oxygen, hydrogen, carbon, and compounds made with combinations of those elements, reveal a great deal about the processes that created them, such as starbirth and stardeath. Planetary science is also a big part of any program, including the study of possible naturally occurring low-frequency signals from Jupiter-sized (and larger) exoplanets with strong magnetic fields. In our own neighborhood, solar activity spurs a tremendous amount of space weather - defined as the events that occur as a result of interactions between material ejected from the Sun and our planet s upper atmosphere and extended magnetic field. State of the Universe

7 An artist s impression of an Earthlike planet with a radio-loud civilization. If communicative life existed on another planet, it s possible that some portion of its radio communications could be detected by low-frequency-sensitive arrays on Earth. Image courtesy David A. Aguilar, Harvard University Center for Astrophysics. Technology Demonstrator (NTD, sensitive to frequenciesbetween800and1600mhz). The MWA-LFD is designed to detect and characterize highly redshifted 21-centimeter emission from hydrogen molecules in the early universe. One of its most important scientific goalsistocreateathree-dimensionalmapof ionized bubbles that formed as the first quasars andgalaxiesfloodedspacewithultravioletlight billions of years ago. Along with the NTD, whichisalsoservingasatestbedfortechnology to be used in the upcoming Square Kilometre Array (SKA), MWA will track transient radio sources such as pulsars, low-frequency signals from molecules in the interstellar medium, and propagationeffectsduetothesun sinfluenceon our planet s upper atmosphere. Eavesdropping on the Universe An interesting use of the Low-Frequency Demonstratorwasproposedatthe2007American Astronomical Society meeting in Seattle, by theoristaviloeboftheharvard-smithsonian Center for Astrophysics. He argues that while most Search for Extraterrestrial Intelligence (SETI) programs are looking for specific signals deliberately beamed across space, searchers should also be on the lookout for signals that leak from regular communications channels. Loeb and co-author Martias Zaldarriaga (also of CFA) suggest that such accidental leakage from military radars, as well as broadcast TV and radio on another planet would be detectable withthemwa-lfd.asetiprogramusingthe array could detect Earth-like radio signals from civilizations up to 30 light-years away during a one-month-long staring session. If these signals were found, then the arrays (along with other SETI surveys) could make more observations to measure the rotation rate of the source planet andthelengthofitsyear. TheMWAisbeingcreatedinapartnership between institutions in Australia and the United States, including the Massachusetts Institute of Technology,theHarvard-SmithsonianCenterfor Astrophysics, the Australia Telescope National Facility, the Australian National University andcurtinuniversity,andthegovernmentof Western Australia. Funding also comes from the U.S. National Science Foundation. The Low-Frequency Demonstrator and the New Technology Demonstrator will start extensive operations in A second stage of the project, with more sensitivity and resolution will follow thesuccessfuldeploymentofltdandntd. Other institutions from Europe and Asia are also cooperating in using and developing the LFD, which has been tested in the field and is now taking data. The current collaboration is focused on measuring interplanetary scintillations, radio bursts, and ionospheric perturbations caused by coronal mass ejections from the Sun. These are aimed directly at understanding space weather phenomena and how they can affect life on our planet. SKA and the Future of Large Arrays ThefutureoflargearraysisinarguablytheSquare KilometreArray(SKA),tobelocatedineither AustraliaorSouthAfrica. Itsfrequencyrange is 100 MHz to 25 GHz, so it will be covering part of the low-frequency radio spectrum. SKA is designed to complement the Atacama Large Millimeter Array (located on the Atacama Desert of Chile), and the James Webb Space Telescope, the orbital follow-up to Hubble Space Telescope, and planned to be mostly sensitive to infrared light with some visible-light capability. Both of these are currently in development for use within the next decade. SKA will consist of several thousand antennas combined to give a radio aperture equivalent to a million square meters of collecting area with a sensitivity nearly a hundred times that of existinglargearrays!theexactconfigurationis still under design, but one possibility is for High-Stakes Astronomy at Low Frequencies... Opening Up a New Wavelength Frontier

8 Left: An artist s concept showing one possible configuration of the Square Kilometre Array antenna farm shows a collection of small dishes side-by-side with focal plane arrays that will give SKA a million square meters of collecting surface. Image courtesy SKA Project Office and Xilostudios. stations,eachwithacollectingareaofa200- metertelescope,andanother150stationseach with the collecting area of a 90-meter telescope. Thiswouldgivethearrayanincrediblyhighresolution view of the low-frequency universe. DevelopmentofSKAbeganin1991.Itisstill in the planning stages, including the technology test bed in Australia. The international SKA consortium (which includes more than 50 institutions in 17 countries) oversees all science and technical developments. It makes all decisions on location and funding, as well as construction.ifallgoeswell,constructioncould start as early as What will SKA look at? As with the other arrays, this huge installation will focus its highly tunedradiosensorsonthebigpicture literally! Itsplannersexpecttomakenewdiscoveriesin astroparticle physics, cosmology, fundamental physics, galactic and extragalactic astronomy, and solar system science. Like its forerunners, SKAwillfocusontheEpochofReionization, using what it finds to understand how the firstgalaxiesassembledthemselves,howearly black holes formed, and how these objects influenced the primordial cosmic environment. Other topics for study include the origin and evolutionofcosmicmagnetism,thesearchfor astrobiologicalprecursorstolifeandearth-like planets, and the evolution of galaxies and the large-scale structure of the universe. With their high resolution, advanced filtering technologies and/or radio-quiet locations, coupled withextremelyhighdataratesandcomputerized correlation, the low-frequency observatories in existencenowandthoseplannedforthenear future are giving astronomers unprecedented chances to study hundreds of square degrees of sky at difficult-to-observe frequencies. These arrays are peering across time, into places we could never see, at astrophysically important signalsfromobjectsthatgiveoffsomethemost fleeting and hard-to-observe flickers of radio wavesintheuniverse. For further information, consult these websites: Allocation of radio spectrum: spectrum.html Background on radio astronomy in general: resources/basicresources.html ALMA web site: Haystack Observatory Radio Arrays web site: index.html LOFAR web site: SKA web site: Carolyn Collins Petersen is a Massachusettsbased science writer specializing extensively in astronomy and space science. She is first author on Visions of the Cosmos (coauthored with John C. Brandt), a book that explores the multi-wavelength universe. She was the senior science writer for the Los Angeles, California-based Griffith Observatory astronomy exhibition (which opened in late 2006), and is currently working on a series of vodcasts about space weather, and documentary scripts for planetarium and science center use. State of the Universe