of the Cosmic Distance Scale
Hubble Key Project on the Cosmic Distance Scale
2dF Galaxy RedShift Survey
Sloan Digital Sky Survey
High-Z Supernova Project
Supernova Cosmology Project
Introduction to the Cosmic Distance Scale
Compton Gamma-Ray Observatory
High-Energy Transient Explorer 2
For the Virgo Consortium showing models of galaxy formation with different assumptions on dark matter
One of the largest astronomy catalogs ever compiled was released to the public today by the Sloan Digital Sky Survey (SDSS).
With photometric and spectroscopic observations of the sky gathered during the last two years, this second data release (DR2) offers six terabytes of images and catalogs, including two terabytes in an easy to use searchable database.
This public data release provides digital images and measured properties of more than 88 million celestial objects, as well as spectra and redshifts of over 350,000 objects. The data are available from the SDSS Web site (http://www.sdss.org/DR2) or from the SkyServer Web site more attuned to the general public (http://skyserver.sdss.org/).
The SDSS is the most ambitious astronomical survey ever undertaken. A consortium of more than 200 astronomers at 13 institutions around the world, the SDSS will map in detail one-quarter of the entire sky, determining the positions and brightnesses of several hundred million celestial objects. It will also measure the distances to approximately one million galaxies and quasars.
"Getting DR2 out to the broader astronomical community and to the general public will allow these data to be analyzed for projects limited only by the imagination and ingenuity of the user," said Michael Strauss of Princeton University, scientific spokesperson for the SDSS.
Strauss explained that while members of the SDSS international collaboration have written more than 200 scientific papers with SDSS data, "we feel we've barely started. There is far more interesting science to be done and discoveries to be made with these data than we have time or people to do. This is why this data release is so important." Public searchable data in the survey have doubled from June 2003 to today.
"Many external researchers are already using the data from earlier public releases", explained Alex Szalay of the Johns Hopkins University, an architect of the SDSS's data mining tools. In fact, researchers from outside of the consortium wrote roughly half of the SDSS-related papers presented at recent American Astronomical Society meetings. "This is a clear indication that we've kept our promise to the scientific community of getting them uniformly high quality data in a timely manner and in a searchable format."
The first public data release from the SDSS in 2003 contained information on 50 million objects, including spectra and redshifts for almost 200,000 of these objects. The SDSS is an ongoing survey that recorded its first observations in May 1998 and is funded for operations through Summer 2005.
The 2.5-meter SDSS telescope is located at Apache Point Observatory in New Mexico and is operated by the Astrophysical Research Consortium. The telescope has two main instruments: an imaging camera, one of the largest ever built, and a spectrograph capable of recording data from 640 objects at a time. The camera creates images from digital scans through five filters: ultraviolet, green, red, and two infrared bands.
DR2 consists of images from 3,324 square degrees of the Northern sky and more than 88 million galaxies, stars, and quasars. The survey is complete for objects as faint as 22.2 magnitude, three million times fainter than the faintest star that can be seen with the naked eye on a dark night.
In addition to images from the SDSS telescope, the DR2 includes the spectra, and therefore redshifts, of 260,000 galaxies, 36,000 quasars, and 48,000 stars, according to consortium member Mark Subbarao of the University of Chicago. The galaxy and quasar catalogs are the largest ever produced.
"The SDSS is a BIG database with researchers making very complicated queries for spatial, color and space parameters," explained Gray, a distinguished engineer in Microsoft's Scaleable Servers Research Group and manager of Microsoft's Bay Area Research Center.
"It has been very rewarding working with the SDSS. The people are very creative, enthusiastic, and bright. The SDSS has shown that we database folks need to do a better job in many ways," Gray said. "For Microsoft, the SkyServer and Catalog Archive Server are an information-at-your- fingertips project we've helped develop for astronomers. I see them as archetypes of what all the sciences need."
Ani Thakar, an SDSS astronomer from the Johns Hopkins University's Center for Astrophysical Sciences, who has worked closely with Szalay and Gray on the SkyServer, said the DR2 database has a form-based Web page for imaging and spectroscopic queries.
"This gives astronomers the ability to extract detailed information from the database without having to learn a query language. We've also added a batch service that lets users submit queries that are likely to take a long time. They can come back later and pick up the results," Thakar explained.
DR2 also offers enhanced querying and filtering options like image cutout and finding chart services. Users can cross-identify objects by uploading lists of object positions on the sky.
The SDSS anticipates releasing more data in its ongoing celestial census late this year.
NGC 5774 (right) and NGC 5775 (left), a pair of interacting galaxies in the
Virgo constellation, about 80 million light years from Earth. NGC 5775 is
seen edge-on, and shows a reddish color due to extensive dust in its disk.
NGC 5774 is seen nearly face-on; spiral arms are blue due to a large
number of young stars. (CREDIT: Robert Lupton, The Sloan Digital Sky Survey)
Messier 13, a globular cluster containing roughly one million stars in the halo of the Milky Way. It lies in the constellation Hercules, 25,000 light years from the Sun. The SDSS obtains images in five filters, allowing these stunning multi-color images to be made. In particular, the range of colors of stars, from red giants to so-called blue straggler stars, is apparent in this image. (CREDIT: Robert Lupton, The Sloan Digital Sky Survey)
An international team of astronomers may have set a new record in discovering what is the most distant known galaxy in the universe. Located an estimated 13 billion light-years away, the object is being viewed at a time only 750 million years after the big bang, when the universe was barely 5 percent of its current age.
The primeval galaxy was identified by combining the power of NASA's Hubble Space Telescope and CARA's W. M. Keck Telescopes on Mauna Kea in Hawaii. These great observatories got a boost from the added magnification of a natural "cosmic gravitational lens" in space that further amplifies the brightness of the distant object.
The Caltech team reporting on the discovery consists of Drs. Jean-Paul Kneib, Richard S. Ellis, Michael R. Santos and Johan Richard. Drs. Kneib and Richard also serve the Observatoire Midi-Pyrenees of Toulouse, France. Dr. Santos also represents the Institute of Astronomy, Cambridge, UK.To see and read more, please visit:
"From the outset of the project in the late 80's, one of our key goals has been a precision measurement of how galaxies cluster under the influence of gravity", explained Richard Kron, SDSS's director and a professor at The University of Chicago.SDSS Project spokesperson Michael Strauss from Princeton University and one of the lead authors on the new study elaborated that: "This clustering pattern encodes information about both invisible matter pulling on the galaxies and about the seed fluctuations that emerged from the Big Bang."
Images of these seed fluctuations were released from the Wilkinson Microwave Anisotropy Probe (WMAP) in February, which measured the fluctuations in the relic radiation from the early Universe.
"We have made the best three-dimensional map of the Universe to date, mapping over 200,000 galaxies up to two billion light years away over six percent of the sky", said another lead author of the study, Michael Blanton from New York University. The gravitational clustering patterns in this map reveal the makeup of the Universe from its gravitational effects and, by combining their measurements with that from WMAP, the SDSS team measured the cosmic matter to consist of 70 percent dark energy, 25 percent dark matter and five percent ordinary matter.
They found that neutrinos couldn't be a major constituent of the dark matter, putting the strongest constraints to date on their mass. Finally, the SDSS research found that the data are consistent with the detailed predictions of the inflation model.
"Different galaxies, different instruments, different people and different analysis - but the results agree", says Max Tegmark from the University of Pennsylvania, first author on the two papers. "Extraordinary claims require extraordinary evidence", Tegmark says, "but we now have extraordinary evidence for dark matter and dark energy and have to take them seriously no matter how disturbing they seem.""The real challenge is now to figure what these mysterious substances actually are", said another author, David Weinberg from Ohio State University.
"The SDSS is really two surveys in one", explained Project Scientist James Gunn of Princeton University. On the most pristine nights, the SDSS uses a wide-field CCD camera (built by Gunn and his team at Princeton University and Maki Sekiguchi of the Japan Participation Group) to take pictures of the night sky in five broad wavebands with the goal of determining the position and absolute brightness of more than 100 million celestial objects in one-quarter of the entire sky. When completed, the camera was the largest ever built for astronomical purposes, gathering data at the rate of 37 gigabytes per hour.
On nights with moonshine or mild cloud cover, the imaging camera is replaced with a pair of spectrographs (built by Alan Uomoto and his team at The Johns Hopkins University). They use optical fibers to obtain spectra (and thus redhsifts) of 608 objects at a time. Unlike traditional telescopes in which nights are parceled out among many astronomers carrying out a range of scientific programs, the special-purpose 2.5m SDSS telescope at Apache Point Observatory in New Mexico is devoted solely to this survey, to operate every clear night for five years.
The first public data release from the SDSS, called DR1, contained about 15 million galaxies, with redshift distance measurements for more than 100,000 of them. All measurements used in the findings reported here would be part of the second data release, DR2, which will be made available to the astronomical community in early 2004.
Strauss said the SDSS is approaching the halfway point in its goal of measuring one million galaxy and quasar redshifts.
"The real excitement here is that disparate lines of evidence from the cosmic microwave background (CMB), large-scale structure and other cosmological observations are all giving us a consistent picture of a Universe dominated by dark energy and dark matter", said Kevork Abazajian of the Fermi National Accelerator Laboratory and the Los Alamos National Laboratory.(A complete list of authors and institutions can be found at www.sdss.org) ILLUSTRATIONS: http://www.hep.upenn.edu/~max/sdss/release.html
NASA Press Release 02-73, April 24, 2002
Pushing the limits of its powerful vision, NASA's Hubble Space Telescope has uncovered the oldest burned-out stars in our Milky Way Galaxy. These extremely old, dim "clockwork stars" provide a completely independent reading on the age of the universe from previous methods.
The ancient white dwarf stars, as seen by Hubble, turn out to be 12 to 13 billion years old. Because earlier Hubble observations show that the first stars formed less than one billion years after the universe's birth in the big bang, finding the oldest stars puts astronomers well within arm's reach of calculating the absolute age of the universe.
Though previous Hubble research sets the age of the universe at 13 to 14 billion years based on the rate of expansion of space, the universe's birthday is such a fundamental and profound value that astronomers have long sought other age-dating techniques to cross-check their conclusions. "This new observation short-circuits getting to the age question, and offers a completely independent way of pinning down that value," says Harvey Richer of the University of British Columbia, Vancouver, Canada.
The new age-dating observations were done by Richer and colleagues by using Hubble to hunt for elusive ancient stars hidden inside a globular star cluster 7,000 light-years away in the constellation Scorpius. The results will be published in the Astrophysical Journal Letters.
Conceptually, the new age-dating observation is as elegantly simple as estimating how long ago a campfire burned by measuring the temperature of the smoldering coals. For Hubble, the "coals" are white dwarf stars, the burned-out remnants of the earliest stars in our galaxy.
Hot, dense spheres of carbon "ash" left behind by the long-dead star's nuclear furnace, white dwarfs cool down at a predictable rate -- the older the dwarf, the cooler it is, making it a perfect "clock" that has been ticking for almost as long as the universe has existed.
This approach has been recognized as more reliable than age-dating the oldest stars still burning by nuclear fusion, which relies on complex models and calculations about how a star burns its nuclear fuel and ages. White dwarfs are easier to age-date because they are simply cooling, but the trick has been finding the dimmest and hence longest- running "clocks."
As white dwarfs cool they grow fainter, and this required that Hubble train a steady gaze on the ancient globular star cluster M4 for eight days over a 67-day period. This allowed for even fainter dwarfs to become visible, until at last the coolest -- and oldest -- dwarfs were seen. These stars are so feeble (30th magnitude -- considerably fainter than originally anticipated for any Hubble telescope imaging with the original cameras), they are less than one-billionth the apparent brightness of the faintest stars that can be seen by the naked eye.
Globular clusters are the first pioneer settlers of the Milky Way. Many coalesced to build the hub of our galaxy and formed billions of years before the appearance of the Milky Way's magnificent pinwheel disk (as further confirmed by Richer's observations). Today, 150 globular clusters survive in the galactic halo. The globular cluster M4 was selected because it is the nearest to Earth, so the intrinsically feeblest white dwarfs are still apparently bright enough to be picked out by Hubble.
In 1928, Edwin Hubble's measurements of galaxies made him realize that the universe was uniformly expanding, which meant the universe had a finite age that could be estimated by mathematically "running the expansion backward." Edwin Hubble first estimated the universe was only two billion years old. Uncertainties over the true expansion rate led to a spirited debate in the late 1970s, with estimates ranging from 8 billion to 18 billion years. Estimates of the ages of the oldest normal "main-sequence" stars were at odds with the lower value, since stars could not be older than the universe itself.
In 1997 Hubble Space Telescope astronomers broke this impasse by announcing a reliable age for the universe, calculated from a very precise measurement of the expansion rate. The picture soon got more complicated when astronomers using Hubble and ground-based observatories discovered the universe was not expanding at a constant rate, but accelerating due to an unknown repulsive force termed "dark energy." When dark energy is factored into the universe's expansion history, astronomers arrive at an age for the universe of 13-14 billion years. This age is now independently verified by the ages of the "clockwork" white dwarfs measured by Hubble.
PPARK Press Release (PPARK is the UK equivalent of the NSF), April 4, 2002
The cause of gamma ray bursts, the most violent and explosive events in the Universe, has remained a mystery since they were first discovered in 1967. Now a team of scientists, led by astronomers from the University of Leicester, believes they have found an answer to the puzzle. Their research results [published in 'Nature' on 4th April 2002] indicate that gamma ray bursts are caused by the death of a star so huge that when it dies in a supernova, its core collapses to form a black hole, resulting in an intense outburst of gamma rays.
Dr. Julian Osborne of the University of Leicester explained,' Until now it was unclear whether gamma ray bursts were caused by a supernova explosion of a giant star collapsing into a black hole, or by the coalescence of two neutron stars. Each event could result in an intense outburst of gamma rays, followed by an X-ray afterglow. By analysing this X-ray afterglow we believe we have determined the gamma ray burst origin.'
The scientists used the EPIC (European Photon Imaging Camera) instrument on the European Space Agency's [ESA] XMM-Newton space telescope to capture the X-ray afterglow of a recent gamma ray explosion in a galaxy 10 billion light years from Earth, and then conducted a detailed spectral analysis of the data. The results were a great surprise.
Dr. James Reeves of the University of Leicester added, 'For the first time ever traces of light chemical elements were detected - including magnesium, silicon, sulphur, argon, and calcium - neutron star collisions are not expected to make these. Also, the hot cloud containing these elements is moving towards us at one tenth of the speed of light. This suggests that the gamma ray burst resulted from the collapse of the core of a giant star following a supernova explosion. This is the only way the light elements seen by XMM-Newton, speeding away from the core, could be produced. So the source of the gamma ray burst is a supernova and not a neutron star collision.' He added, 'The confirmation by XMM-Newton that gamma ray bursts are associated with supernovae therefore brings scientists closer to understanding the process that leads to the burst itself'.
The UK has taken a leading role in XMM-Newton, as Prof. Ian Halliday, Chief Executive of the Particle Physics and Astronomy Research Council explains "From the development of the XMM-Newton concept, the UK has been taking a strong role, with the University of Leicester leading the way on the EPIC instrument and on the Survey Science Centre consortium which processes all the data recorded. The Mullard Space Science Laboratory built the Optical Monitor telescope on XMM-Newton, which also captured the fading optical signature of the gamma ray burst."
Prof. Ian Halliday, adds "These latest findings will be tested by SWIFT; a NASA led space mission that scientists from the University of Leicester and Mullard Space Science Laboratory are helping to build. Once it is launched in autumn next year , SWIFT will study over 1000 gamma ray bursts, spotting them rapidly then automatically turning two much more sensitive telescopes to study these events. This means that the resulting X-rays can be studied mere seconds after the event, instead of the hours presently required to manoeuvre existing space-based telescopes with instructions from the ground."
Hypernova, XMM, and Swift images are available from the PPARC web site:
At 3.9 tonnes and 10 metres long, the X-ray Multi-Mirror [XMM]-Newton is the biggest and most sensitive X-ray telescope ever to be placed in to orbit. It is able to study extremely faint X-ray sources from stars and galaxies in the most distant parts of the Universe.
EPIC the European Photon Imaging Camera
This instrument, built by an international team of European scientists led by Dr. Martin Turner of the University of Leicester uses silicon Charge Coupled Devices to provide simultaneous X-ray images and spectra so that the chemical composition of distant X-ray sources can be studied.
Gamma ray bursts
Every day, somewhere in the Universe, there is a gamma ray burst; they are the most powerful and violent phenomena in the Universe. Gamma ray bursts last only a minute or so. But in that time the energy released is equivalent to the instantaneous conversion of up to the entire Sun's mass into energy, following Einstein's famous equation E=mc2, making gamma ray bursts second only to the Big Bang in total power. The intense burst of gamma rays is followed by an X-ray glow that lasts a few days; it is this X-ray afterglow that was observed by XMM-Newton about 11 hours after the gamma ray burst.
Gamma ray bursts were discovered in 1967 by the US Military VELA satellites where they mimicked the signatures of terrestrial nuclear tests. Several thousand have been detected since, although it was only five years ago that they were proven to come from distant galaxies.
A supernova is the explosion produced when a massive star ends its life; one occurs in our own Galaxy approximately every hundred years. Seen from the Earth, it would be initially visible in daylight and would be the brightest star in the sky for about six months. Supernovae in distant galaxies appear much fainter, but briefly outshine the host galaxy itself. The chemical elements that make up the Earth, and ultimately ourselves, were formed in supernovae.
Elaboration by Rudolph Schild, Harvard-Smithsonian Center for Astrophysics, october 4, 2001. [to accompany the textbook, 6th ed., pp. 755-756]
Since spectra of two quasars at the same redshift can be very similar, and since the observed spectra of multiple images of gravitationally lensed can differ somewhat because of reddening effects or radio frequency scintillations along the path to the earth, the best way to be sure of gravitational lensing is to see the pattern of random quasar brightness fluctuations in the multiple images. This has been seen now in 7 of the 60 gravitational lenses, after the first was found by Schild in 1986. He found that in Q0957+561, the northern image arrived 1.1 years before the tardier southern image, so astrnomers have an astronomical curiosity where events to be seen in the southern image can be, and have been, predicted from observation of the prompter northern image.
[Brightness curves appear in Astronomical Journal, vol. 113, pp. 130ff (1997).]
More than a curiosity, the measurement of such time delays allows measurement of the Hubble constant by providing a key datum to close the system of equations that mathematically describe the lensing. This in turn provides a precise distance of the quasar and lens galaxy, and since their redshifts are known, a Hubble constant value results that is independent of assumptions about Cepheids and the distance of the Magellanic clouds. These determinations give 10% smaller Hubble constants than the value of 72 from the Hubble Key Project, indicating that the Hubble constant is 65 and that the universe is older than it would be with the larger value of the Hubble constant.
An important application of time delay determination results when the correctly phased brightness records are compared to look for small differences that might arise from microlensing by the stars in the lens galaxy. Such microlensing is recognized as a cuspy pattern of brightness spikes caused by luminous or dark objects along the light path to the quasar, predominantly in the massive lens galaxy. In this way, the presence of individual solar type stars has been inferred in the Q0956+561 lens galaxy at a redshift of z=0.37, and even the suggestion of a cosmologically significant population of dark matter "rogue planets."
Press Release 01-11 from the National Optical Astronomy Observatory
Astronomers using the National Science Foundation's Blanco Telescope in Chile have used the distorting effects of a weak gravitational lens to discover and locate a dim cluster of at least 15 galaxies at a significant distance from Earth, using only the mass properties of the cluster, not its visible light.
This first-time accomplishment raises clear prospects for a powerful technique called 3-D mass tomography to conduct large-scale searches for dark matter. This approach may provide a valuable independent check of current theories about the accelerating expansion of the Universe.
"Most of the 'big action' in the Universe is governed by the mass of objects, not by their visible light or other electromagnetic radiation," explains Anthony Tyson of Lucent Technologies' Bell Labs, Murray Hill, NJ. "This fundamentally new approach lets us measure mass instead of light, based on simple physics that circumvents much of the unavoidable bias inherent in trying to use tiny variations in brightness to derive critical properties of distant objects."
Several recent astronomical discoveries have taken advantage of the fact that massive objects bend the path of light from more distant objects located behind them, as seen from Earth. This property, called gravitational lensing, was predicted by Albert Einstein.
In this case, the large mass of the newly found galaxy cluster forms a weak intervening lens that shears, or unevenly bends, the light from more distant galaxies behind it. The distorted, elongated images of the background galaxies were identified and processed by a computer program. More importantly, this program generated an estimate for the unknown cluster's redshift, or its velocity of recession from Earth. This redshift matched precisely with more traditional spectroscopic redshift measurements taken as an independent check by the research team.
"We have shown that we can measure the distance to the cluster by charting the amount of shear versus the distance to the background galaxies," says lead author David Wittman of Bell Labs. "This lets us locate the cluster in its proper place in three-dimensional space, all without resorting to studying its light. This is important because most of the mass in the Universe is dark."
The team used the National Science Foundation's Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory (CTIO), outfitted with an instrument called Big Throughput Camera, to take numerous images in four different filters of a relatively barren portion of the visible sky, in the direction of the constellation Pisces.
This effort yielded rough color-based (or photometric) redshifts for various objects in the images. A computer program then sifted through the images, looking for telltale distortions in the shape of the objects by intervening clumps of matter. The program sorts the distortions into different "bins" based on their locations and shapes, and then constructs a full three- dimensional map of the region of space.
The predicted astronomical redshift of the galaxy cluster and its mass properties match precisely with spectroscopic measurements made using the W. M. Keck telescopes on Mauna Kea, HI, at almost exactly the predicted redshift of z = 0.276. This number is equivalent to looking back in time to how the cluster appeared about three billion years ago.
"With a bit more experience we will be able to do a blind search and effectively become 'intergalactic prospectors' for extremely faint clumps of mass in any direction, out to about one-half the estimated age of the Universe," Tyson says. "This will offer a test of the idea of an accelerating Universe that is completely independent of the supernovae observations which have formed the basis of this theory."
This research result is a precursor to a larger project led by Tyson called the Deep Lens Survey, one of more than a dozen major sky surveys supported by the National Optical Astronomy Observatory (NOAO). Further information about the Deep Lens Survey, which should reveal scores of such clusters using the weak lens effect, is available on the Internet at:
Through Lucent Technologies' Bell Labs, Tyson and Wittman are partners with NOAO and the University of Arizona's Steward Observatory in developing a detailed concept for a future ground-based facility called the Large Synoptic Survey Telescope (LSST). A top priority of the most recent decadal survey of astronomy, the LSST features an 8-meter class mirror with an extraordinarily large, three-degree field-of-view that promises to image the entire sky to a faint stellar magnitude (R = 23.9) every few nights.
"If we can do this kind of analysis over a large-enough area, we can use statistics to tell us how mass has clumped together over cosmic time, and see how well the results match with the idea of accelerating expansion," Tyson explains. "We'll need billions of background galaxies to do a full-up investigation, and LSST is the only thing on the horizon that can do the job."
Other co-authors of the upcoming paper are Vera Margoniner of Bell Labs, Judy Cohen of the California Institute of Technology, Pasadena, CA, and Ian Dell'Antonio of Brown University, Providence, RI.
A camera image and a related mass map used to locate the galaxy cluster are available on the Internet at:
Located east of La Serena, Chile,
CTIO is the southern hemisphere site of the National Optical Astronomy Observatory,
which is based in Tucson, AZ. NOAO is operated by the Association of Universities
for Research in Astronomy (AURA), Inc., under a cooperative agreement with the
National Science Foundation.
Information not only about the Hubble constant but also about gamma ray bursters, gravitational lenses, Chandra X-ray Observatory, and other interesting topics.