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Home , Palomar 5, Sloan Digital Sky Survey

Mapping the : Surveys of the Sky as Discovery Engines in Astronomy

Michael A. Strauss

Abstract: Astronomers can map the sky in many ways: observing in different regions of the electromagnetic spectrum, obtaining spectra of and to determine their physical properties and distances, and repeatedly observing to measure the variability, explosions, and motions of celestial objects. In this review Downloaded from http://direct.mit.edu/daed/article-pdf/143/4/93/1830845/daed_a_00309.pdf by guest on 25 September 2021 I describe recent surveys of the sky astronomers have carried out, focusing on those in the visible part of the spectrum. I describe in detail the , an ongoing imaging and spectroscopic survey of over one quarter of the celestial sphere. I also discuss some of the major surveys planned for the next decade, using telescopes both on the ground and in space.

Astronomy is an observational science. Unlike chemistry or biology, the objects of study in astron- omy are far removed, at distances to which we will not have the capability to travel using even the advanced foreseeable technology. This means that we cannot carry out experiments on the stars and galaxies that are the bread and butter of our disci- pline; all the information we can glean about them is the result of measuring the tiny fraction of the light that they emit that happens to fall on our eyes and our telescopes. We then interpret these data in the context of the laws of physics to draw conclu- sions about the nature of these distant bodies, allow- MICHAEL A. STRAUSS is Professor ing us to infer, for example, the conditions in the of Astrophysical Sciences and As- cores of stars, or the existence of new forms of matter sociate Chair of the Department of that are unknown from our experience and experi- Astrophysical Sciences at Princeton ments here on Earth. University. His research concerns The range of phenomena in the universe is vast, all aspects of extragalactic astron- and the rate of astronomical discovery today tells omy and observational . us that we are far indeed from a complete under- He has published over two hundred refereed papers on subjects ranging standing of all that the universe has to teach us. This from the large-scale distribution of essay describes one of the most productive approach- galaxies to the discovery of the most es we have toward astronomical discovery; namely, distant known. using our telescopes to map the heavens and create

© 2014 by the American Academy of Arts & Sciences doi:10.1162/DAED_a_00309 93 Surveys of a census of the objects we ½nd. Astronomi- ent regimes is a powerful tool for explor- the Sky as cal surveys have always been a key aspect ing these phenomena. However, normal Discovery Engines in of our ½eld: such surveys have much to stars (and thus galaxies, which are made Astronomy teach us about the formation and struc- up of stars) emit most of their radiation ture of the in which the at visible and near- wavelengths, sun sits; the expansion, future fate, and making this the most effective regime in origin of the universe as a whole; and the which to survey the sky for these objects. nature of stars and the planets that orbit them. Indeed, astronomy is the study of Go outside on a clear moonless night, origins; we ask (and occasionally answer!) far from the lights of civilization. Once the most fundamental questions about your eyes have adapted to the darkness, where planets, stars, galaxies, and the uni- between two and three thousand stars are Downloaded from http://direct.mit.edu/daed/article-pdf/143/4/93/1830845/daed_a_00309.pdf by guest on 25 September 2021 verse as a whole come from. With each discernable to the naked eye at any given advance in technology and new way to time. You will also see a silvery band cross- survey the heavens, we uncover new phe- ing the sky: the Milky Way. nomena that we did not anticipate, and was the ½rst to point a telescope to the we ½nd ourselves addressing questions heavens, and he discovered that the light that we previously did not have the imag- of the Milky Way comes from countless ination to ask. stars. Since that time, astronomers have With one hundred billion stars in our used ever larger telescopes to map objects Milky Way, and one hundred billion gal- in the sky. axies in the , our sur- However, the images we make with cam- veys have entered the realm of . eras placed at the back of our telescopes The biggest survey telescopes today can are two-dimensional. We have no depth gather terabytes of data in a single night, perception, and the stars look all to be and our catalogs of galaxies and stars in- equidistant, with no sense of which are clude over a billion objects, a number that closer and which are farther away. In fact, will increase by a factor of ten over the the nearest (other than the sun) is next decade or so. The discovery potential about four light years (or about forty tril- of our surveys is limited by a combina- lion kilometers) from us, a distance that tion of raw computer processing power, is completely outside our everyday expe- the cleverness of our algorithms, and our rience. It would take thirty thousand years imagination. Just as Google allows us to to cover that distance traveling at the speed query human databases to uncover facts of our fastest spacecraft (forty-½ve kilome- and the relations between them, astron - ters per second). While most of the indi- omers have been developing similar tech- vidual stars visible to the naked eye are nology to query the database of the uni- within a few hundred light years of us, the verse. bulk of stars in the Milky Way are much This essay will focus on surveys in visi- farther away, arrayed in a vast flattened ble light, which of course represents only spiral structure some one hundred thou- a small sliver of the full range of electro- sand light years across, containing roughly magnetic waves, from high-energy gamma one hundred billion stars. rays to long-wavelength radio waves. Very One hundred years ago, astronomers different physical phenomena are respon- understood the Milky Way galaxy to be sible for emission at different regions of the full extent of the universe. However, in the electromagnetic spectrum, and com- addition to the myriad stars apparent in parison of maps of the sky in these differ- astronomical images, one also sees fuzzy

94 Dædalus, the Journal ofthe American Academy of Arts & Sciences extended objects, termed nebulae (the Latin tells us that galaxies are made of stars. Michael A. word for cloud). demon- However, there is an important difference: Strauss strated in the 1920s that these nebulae were the absorption lines in galaxy spectra are other “island ,” as large as our shifted systematically to longer (redder) own Milky Way but at much greater dis- wavelengths. Hubble found that the spec- tances. This discovery enormously expand - tra of essentially all galaxies are redshifted, ed our understanding of the size of the and the degree of is proportional universe. The nearest big galaxy to our own to the distance of the galaxy. is about two million light years distant, This relationship between redshift and and the number of galaxies seen in the distance is a consequence of the expan- deep est images with the epon ymous Hub- sion of the universe and, as explained in ble Space Telescope imply that there are David Spergel’s companion article in this Downloaded from http://direct.mit.edu/daed/article-pdf/143/4/93/1830845/daed_a_00309.pdf by guest on 25 September 2021 about one hundred billion of them in the volume, it leads directly to our modern observable universe. understanding of the . For our purposes, however, this becomes a valu- Surveys of galaxies based on photo- able tool for mapping the universe in three graphic plates show that they are far from dimensions: measuring the spectrum of uniformly distributed in the sky: clusters a galaxy allows us to determine its redshift, a few million light years across containing and thus, by Hubble’s law, its distance. hundreds of galaxies are apparent, with Photographic ½lm records the presence hints of larger structures yet. But to really of only (at best) a few percent of the pho- map the distribution of galaxies in three tons that fall upon it. Thus, even with the dimensions, we need an unambiguous way largest telescopes available in Hubble’s to measure their distances. Hubble’s sec- time, measuring the spectrum of a faint ond great discovery–that the universe is galaxy in order to determine its redshift expanding–gives us the way to do so. was enormously time-consuming, requir- Consider the spectrum of an astronomical ing exposure times of many hours for even object, which measures the intensity of the nearest galaxies. Modern electronic its light as a function of wavelength. This detectors, such as those in your digital spectrum gives much more detailed infor - camera, are far more sensitive, detecting mation about the physical nature of the close to 100 percent of the photons that object than the properties (size, brightness, fall on them. Their development and adop- and color) measureable from an astronom- tion by the astronomical community ical image. For example, the wavelength starting in the late 1970s meant that ap - at which the spectrum of a star peaks is a preciable numbers of galaxy spectra, and measure of its surface temperature, which thus and distances, could be mea- for most stars in turn indicates their mass. sured. As described in Anna Frebel’s article in The galaxy distribution based on these this volume, there are characteristic wave- early surveys of a few thousand redshifts lengths at which the star is dimmer (absorp- were stunning and surprising. These maps tion lines); these are due to absorption of showed that most galaxies are strung along light by atoms in the atmosphere of the long ½lamentary structures that connect star, and they reflect the star’s chemical the rich clusters. These ½laments, up to comp osition. hundreds of millions of light years across, As illustrated in Figure 1, the spectra of surround enormous volumes, essentially galaxies show many of the same absorp- devoid of galaxies. These pictures raised tion lines as do stars like the sun, which many questions: How large can these

143 (4) Fall 2014 95 Surveys of Figure 1 the Sky as Spectra of Objects from the Sloan Digital Sky Survey Discovery Engines in Astronomy Downloaded from http://direct.mit.edu/daed/article-pdf/143/4/93/1830845/daed_a_00309.pdf by guest on 25 September 2021

The upper panel shows a star similar to the sun; the pair of dips (circled) at a wavelength just below 4000 A is due to absorption by calcium atoms in the atmosphere. The other panels show spectra of galaxies at ever greater distances (marked in each panel in units of millions of light years, or Mly). The calcium absorption feature is cir- cled in each case; the more distant the galaxy, the larger the redshift. Source: The Sloan Digital Sky Survey, http:// www.sdss.org/.

struc tures be? How did they form? What of which was the Sloan Digital Sky Sur- can they tell us both about the nature of vey (sdss).1 This program was the scien- galaxies and the structure of the universe ti½c vision of astron- overall? omer , who realized in the late 1980s that advances in electronic de - All this motivated the next generation tector technology, telescope design, and of redshift surveys, the most ambitious computer processing power made it pos-

96 Dædalus, the Journal ofthe American Academy of Arts & Sciences sible to design a dedicated telescope proj- Kirshner’s memorable phrase: there are Michael A. ect to map the universe much more thor- no structures even larger than this; and if Strauss oughly than had been done to date. The we step back far enough, the distribution initial stated goal of the project was to of galaxies appears uniform. measure redshifts for one million galax- As Spergel’s article describes, these data, ies in the local universe in order to char- together with measurements of the cos- acterize the nature of the galaxy distribu- mic microwave background and other cos- tion on the largest scales. The sdss built mological probes, lead to a comprehen- a specialized telescope with a two-and-a- sive model for the structure and makeup half-meter-diameter primary mirror, as of the universe. In particular, we can un- well as an astronomical camera with a derstand the richness of the distribution total of 145 megapixels (the largest ever at of galaxies in a model in which ordinary Downloaded from http://direct.mit.edu/daed/article-pdf/143/4/93/1830845/daed_a_00309.pdf by guest on 25 September 2021 the time it was built). The telescope swept matter (in the form of atoms, mostly hy - the sky essentially every clear moonless drogen, which make up the stars that cause night for ten years, covering twenty square galaxies to shine) represents just under 5 degrees every hour in ½ve broad ½lters. percent of the mass-energy density of the The resulting color images, which survey universe, with the remainder being in the the region of sky far from the band of the form of and . Milky Way (whose dust obscures the light One lesson of astronomy surveys is that of more distant galaxies) now cover about by gathering data necessary to answer one one-third of the celestial sphere and con- question (in this case, wide-½eld imaging tain data on almost half a billion stars, gal- and to measure the large- axies, quasars, and asteroids. scale distribution of galaxies), astronomers On the less than pristine nights, the gain the ability to address other problems imaging camera was replaced with a in the ½eld, many of which were unantic- spectrograph, fed by optical ½bers from ipated at the beginning of the survey. The the focal plane of the telescope, allowing sdss is no exception to this rule, making spectra of 640 objects selected from the fundamental discoveries in the structure images (increased to 1000 objects after a of the Milky Way galaxy, the most distant major upgrade to the system in 2009) to quasars and the nearest stars, and many be measured at a time. To date, the survey other topics.2 has measured the spectra, and thus red- About half the objects visible in the sdss shifts and distances, of over two million images are stars in our own Milky Way, at s galaxies, creating a stunning and detailed typical distances of thousands to tens of r map of the universe in which we live. Fig- thousands of light years, appearing as / ure 2 covers only a few percent of the full sharp points of light; most of the rest are sample; each of the roughly ½fty thou- galaxies, which appear fuzzy in the images. sand dots shown represents an entire Away from the densest part of the Milky galaxy, as large as the Milky Way, con- Way, a flattened rotating disk, stars lie in taining one hundred billion stars. The what is thought to be a roughly spherical ½laments and voids apparent in the ½rst distribution around the center of our redshift surveys of the early 1980s appear galaxy, termed the halo of the Milky Way, in all their glory here. The largest struc- extending to tens of thousands of light ture in the map, dubbed “The Sloan Great years. It was long thought that the halo Wall,” is 1.4 billion light years across. was smooth, with stellar density falling Interestingly, there appears to be an “end steadily as one moves from the galactic to greatness,” to use astrophysicist Bob center. But the maps of the stars from the

143 (4) Fall 2014 97 Surveys of Figure 2 F the Sky as A Slice through the Three-Dimensional Distribution of Galaxies Mapped T Discovery by the Sloan Digital Sky Survey Engines in Astronomy Downloaded from http://direct.mit.edu/daed/article-pdf/143/4/93/1830845/daed_a_00309.pdf by guest on 25 September 2021

T b h

Each of the more than ½fty thousand dots in this ½gure represents a galaxy as large as the Milky Way. The Milky Way itself sits in the center of the ½gure; the outer circle represents a distance of two billion light years from the sun. The ½lamentary nature of the galaxy distribution is readily apparent in this ½gure. The segments devoid of data are regions of the sky that the sdss did not survey. Source: The Sloan Digital Sky Survey, http://www.sdss.org/.

sdss and from earlier photographic sur- than ten thousand light years long. It is veys show that things are more interesting now un derstood that as the globular clus- than that. A taste of the results is shown in ter orbits our Milky Way, gravitational tidal Figure 3, which maps the distribution of forces (the difference in the gravitational stars in the vicinity of a so-called globular acceleration between the near and far sides cluster (named Palomar 5), a conglomer- of the ) are tearing the clus- ation of roughly one hundred thousand ter apart, pulling streams of stars from it. stars 150 light years across. Palomar 5 is Indeed, the star maps from the sdss have accompanied by a stream of stars more shown that such star streams are com-

98 Dædalus, the Journal ofthe American Academy of Arts & Sciences Figure 3 Michael A. The Distribution of Stars in the Vicinity of the Globular Cluster Palomar 5 Strauss

5

4

3

2

1 Downloaded from http://direct.mit.edu/daed/article-pdf/143/4/93/1830845/daed_a_00309.pdf by guest on 25 September 2021

0 Decl. [deg. J2000]

–1

–2

–3 236 235 234 233 232 231 230 229 228 227 226 225 224

R.A. [deg. J2000]

The globular cluster itself is the dark region to the lower-right of the center of the ½gure. It is accompanied by both leading and trailing streams of stars, stretching over ten thousand light years. This stream is believed to have been pulled out from the globular cluster by tidal forces from the Milky Way. Source: Michael Odenkirchen et al., “The Extended Tails of Palomar 5: A 10° Arc of Globular Cluster Tidal Debris,” The Astro- nomical Journal 126 (2003): 2385. Reprinted with permission.

mon: in a process predicted by some the- history of the universe. Of course, very dis- orists and now con½rmed by these obser- tant galaxies will be tremendously faint vations, the halo appears to be made as seen from Earth, so, all else being equal, y largely of the debris of small galaxies and the easiest distant galaxies to see will be e e globular clusters that have fallen into, those with the highest intrinsic luminosi- and been torn apart by, the gravity of our ties. These include quasars: galaxies with Milky Way.3 a (a black hole with a mass up to several billion times that Surveying large swaths of sky to very of the sun) in their center; as described in faint levels makes one sensitive to very rare Scott Tremaine’s article in this volume, gas (and therefore interesting) objects, such orbiting close to the black hole heats up as quasars at very great distances. Because tremendously and glows enough to out- of the ½nite speed of light, we see a distant shine the hundred billion stars of the gal - galaxy not as it is today, but as it was when axy by a factor of one hundred or more. the universe was signi½cantly younger. Because the light from a is dom- Astronomers thus can use their telescopes inated by the tiny region in the vicinity of as time machines to directly observe the the black hole, they are usually unresolved

143 (4) Fall 2014 99 Surveys of in sdss images, and thus look like stars. we found ourselves stumbling over some the Sky as The sdss was designed to identify quasars of the nearest stars to our own! Discovery Engines in by their distinctive colors in the images. Despite these problems (and the seren - Astronomy In particular, due to absorption by neutral dipity of ½nding brown dwarfs was excit- hydrogen in their spectra, the highest- ing and scienti½cally important in its own red shift quasars are extremely red, ap pear - right), we broke the record multiple times ing only in the longest-wavelength ½lter for the most distant quasar discovered, in - that the sdss measures (the “z” band). cluding one whose light we observe today Soon after the sdss had garnered its ½rst was emitted less than a billion years after images of the sky, my student Xiaohui Fan the Big Bang. This is remarkable: the cos- (now a professor at the University of Ari- mic microwave background, emitted by zona) and I started searching for record- gas in the universe four hundred thousand Downloaded from http://direct.mit.edu/daed/article-pdf/143/4/93/1830845/daed_a_00309.pdf by guest on 25 September 2021 breaking quasars. years after the Big Bang is smooth to one Finding these most distant quasars is part in one hundred thousand, and yet a straightforward in principle: simply iden- billion years later, a supermassive black tify those objects that appear only in the z hole, the densest conceivable object with band, and con½rm that they are quasars a mass billions of times that of the sun, by taking a spectrum. Making this work had managed to form. It is still poorly un - in practice, however, required a very de - derstood how this happened, and discov- tailed understanding of every way the data eries of yet more distant quasars may shed could fool us; even extremely rare glitches some light into this process. Our record that affect one in a million stars would has since been broken in spectacular fash- swamp our search for these objects. We ion with a quasar seen one hundred mil- learned that doing science from surveys re - lion years earlier than our own, found in quires exquisite quality control and a deep data from the United Kingdom Infrared understanding of the nature of the data! Telescope Deep Sky Survey. But we were stymied in our search by One of the most important lessons from another problem, this one astrophysical. the sdss and other surveys is that if the Extremely cool stars also appear quite red. data are of high quality and are made avail- About the time that the sdss was taking able to the world, they enable science far its ½rst data, astronomers were using near- beyond the initial goals of the survey. The infrared surveys of the sky to discover sdss, which is now in its ½fteenth year of new classes of very cool red stars, called full operation, has plans to continue gath - brown dwarfs. These have very low mass es, ering data through the year 2020. Almost so low that their gravity is inadequate to six thousand refereed articles have been ignite thermonuclear fusion in their cores. written to date by scientists all over the Indeed, these are only a bit more massive world using the sdss data, a level of pro- than many of the planets described in ductivity that rivals that of much larger Gáspár Bakos’s article in this vol ume on telescopes, such as the ten-meter Keck . Brown dwarfs have surface Tele scopes in Hawaii. Thus astronomers temperatures below 2000 K (for com par - are motivated to consider the next gener- ison, the sun has a surface temperature of ation of surveys beyond the sdss. The 6000 K), making them dim (and thus only sdss telescope has a primary mirror with visible at very small distances from Earth, a diameter of only two-and-a-half meters, typically thirty light years or less) and very which is small compared to the largest op- red, just like the qua sars. In our search for tical telescopes in the world today: ten the most distant quasars in the universe, meters across (and three signi½cantly larg-

100 Dædalus, the Journal ofthe American Academy of Arts & Sciences er telescopes are being planned for the next of the sky: “celestial cinematography,” as Michael A. decade, with diameters of twenty-two, lsst Chief Scientist Tony Tyson likes to Strauss thirty, and thirty-nine meters). Larger tele- phrase it. Indeed, surveys on much smaller scopes are capable of seeing much fainter, telescopes that repeatedly image the sky and therefore more distant, galaxies. As have discovered a wide range of variable we have already seen, this means that they phenomena, including the motions of as - can probe back to times when the galax- teroids in our own solar system, pulsating ies, and the universe as a whole, were much and exploding stars of all sorts, the subtle younger than they are today. motions of distant stars that reflect the gravitational potential of the Milky Way, As Pieter van Dokkum’s article in this and the flickering of quasars as parcels of gas swirl around the black holes that power volume describes, the early universe was Downloaded from http://direct.mit.edu/daed/article-pdf/143/4/93/1830845/daed_a_00309.pdf by guest on 25 September 2021 very different from the universe today. The them. The lsst will extend such studies to ½rst galaxies are thought to have formed much fainter astronomical objects, and is several hundred million years after the bound to ½nd new kinds of variable and Big Bang, and one of the most important transient phenomena that have not been areas of research today is to determine how anticipated to date. they grew and evolved from their initial Telescope observations from the surface formation. The next generation of planned of the Earth are affected by the atmo- surveys is designed to address this ques- sphere, which both blurs images and adds tion. The 8.2-meter op - a substantial amount of background light, erated by the National Astronomical Ob- especially at near-infrared wavelength. servatory of Japan has the largest ½eld of As the has dra- view of any existing telescope of its size, matically demonstrated, placing an obser- and is thus particularly well-suited for car - vatory in space allows much sharper im - rying out surveys. It has a wide-½eld imag- ages and the ability to observe much ing camera that has just started a major fainter objects. The Hubble Telescope it- survey covering over one thousand square self has a tiny ½eld of view, and is thus not degrees of sky, sensitive to objects twenty- well-suited for survey work, but the U.S. ½ve times fainter than the sdss was able National Reconnaissance Of½ce has re- to reach. In 2018, this will be followed up cently given nasa two telescopes that have with a spectroscopic survey of hundreds mirrors as large as Hubble’s (2.4 meters of thousands of galaxies from the ½rst few in diameter, similar to that of the sdss) bil lion years after the Big Bang. but are designed with a much larger ½eld The era of surveys will continue into the of view. There are active plans to use one next decade. The Large Synoptic Survey of them to map the large-scale distribu- Telescope (lsst) will spend ten years in a tion of dark matter by measuring the dis- dedicated wide-½eld imaging survey of the tortions it causes on the shapes of galax- sky.4 Polishing of its 8.4-meter primary ies. The combination of this space-based mirror is nearing completion; the tele- telescope with the lsst will be particu- scope will be constructed in the Chilean larly powerful; the coarser images of the Andes (famed for their clear and steady lsst will be balanced by its much larger skies) and will start a ten-year survey of sky coverage and measurements over a the sky in 2022, covering half the celestial wide range of wavelengths. The quantity sphere four times deeper than the Subaru of data these sur veys will produce will be survey will go. In doing so, it will map the measured in petabytes (one petabyte is a heavens multiple times, making a movie million gigabytes); analyzing and inter-

143 (4) Fall 2014 101 Surveys of preting these data will drive new technol - omers. With these surveys well underway the Sky as ogies in computer processing and analysis. a decade from now, astronomical discov- Discovery Engines in These data will be made public, allowing ery will continue to be driven by the Astronomy schoolchil dren to search for supernovae amazing datasets that they produce. at the same time as professional astron -

endnotes 1 The Sloan Digital Sky Survey is described in detail at http://www.sdss.org. See also the pop- ular book describing the building of the survey: Ann Finkbeiner, A Grand and Bold Thing: An Extraordinary New Map of the Universe Ushering in a New Era of Discovery (New York: Free Press, 2010). Downloaded from http://direct.mit.edu/daed/article-pdf/143/4/93/1830845/daed_a_00309.pdf by guest on 25 September 2021 2 The discovery of distant quasars in the sdss is described in Xiaohui Fan et al., “A Survey of z > 5.7 Quasars in the Sloan Digital Sky Survey,” The Astronomical Journal 125 (2003): 1649. 3 The structure of the halo of the Milky Way as seen in the sdss is described in V. Belokurov et al., “The Field of Streams: Sagittarius and Its Siblings,” The Astrophysical Journal 642 (2006): L147. 4 The Large Synoptic Survey Telescope is described at http://www.lsst.org. A comprehensive description of its science opportunities may be found in the LSST Science Book v. 2.0 (2009) at http://www.lsst.org/lsst/scibook.

102 Dædalus, the Journal ofthe American Academy of Arts & Sciences