Ultraviolet Imaging Telescope views
Debate on the Scale of the Universe
Infrared Space Observatory maps
The Messier Page
A Searchable Set of Astronomical Images
Galaxy course notes from William Keel
Galaxy images from William Keel
NASA Extragalactic Database
Spitzer View of Galaxy M81
Spitzer Spectrum of Organic Materials in a Galaxy
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)
Astronomers at the Space Telescope Science Institute today unveiled the deepest portrait of the visible universe ever achieved by humankind. Called the Hubble Ultra Deep Field (HUDF), the million-second-long exposure reveals the first galaxies to emerge from the so-called "dark ages," the time shortly after the big bang when the first stars reheated the cold, dark universe. The new image should offer new insights into what types of objects reheated the universe long ago.
This historic new view is actually two separate images taken by Hubble's Advanced Camera for Surveys (ACS) and the Near Infrared Camera and Multi-object Spectrometer (NICMOS). Both images reveal galaxies that are too faint to be seen by ground-based telescopes, or even in Hubble's previous faraway looks, called the Hubble Deep Fields (HDFs), taken in 1995 and 1998.
"Hubble takes us to within a stone's throw of the big bang itself," says Massimo Stiavelli of the Space Telescope Science Institute in Baltimore, Md., and the HUDF project lead. The combination of ACS and NICMOS images will be used to search for galaxies that existed between 400 and 800 million years (corresponding to a redshift range of 7 to 12) after the big bang. A key question for HUDF astronomers is whether the universe appears to be the same at this very early time as it did when the cosmos was between 1 and 2 billion years old.
The HUDF field contains an estimated 10,000 galaxies. In ground-based images, the patch of sky in which the galaxies reside (just one-tenth the diameter of the full Moon) is largely empty. Located in the constellation Fornax, the region is below the constellation Orion.
The final ACS image, assembled by Anton Koekemoer of the Space Telescope Science Institute, is studded with a wide range of galaxies of various sizes, shapes, and colors. In vibrant contrast to the image's rich harvest of classic spiral and elliptical galaxies, there is a zoo of oddball galaxies littering the field. Some look like toothpicks; others like links on a bracelet. A few appear to be interacting. Their strange shapes are a far cry from the majestic spiral and elliptical galaxies we see today. These oddball galaxies chronicle a period when the universe was more chaotic. Order and structure were just beginning to emerge.
Installed in 2002 during the last servicing mission to the Hubble telescope, the ACS has twice the field of view and a higher sensitivity than the older workhorse camera, the Wide Field Planetary Camera 2, installed during the 1993 servicing mission. "The large discovery efficiency of the ACS is now being exploited in sky surveys such as the HUDF," Stiavelli says.
The NICMOS sees even farther than the ACS. The NICMOS reveals the farthest galaxies ever seen, because the expanding universe has stretched their light into the near-infrared portion of the spectrum. "The NICMOS provides important additional scientific content to cosmological studies in the HUDF," says Rodger Thompson of the University of Arizona and the NICMOS Principal Investigator. The ACS uncovered galaxies that existed 800 million years after the big bang (at a redshift of 7). But the NICMOS may have spotted galaxies that lived just 400 million years after the birth of the cosmos (at a redshift of 12). Thompson must confirm the NICMOS discovery with follow-up research.
"The images will also help us prepare for the next step from NICMOS on the Hubble telescope to the James Webb Space Telescope (JWST)," Thompson explains. "The NICMOS images reach back to the distance and time that JWST is destined to explore at much greater sensitivity." In addition to distant galaxies, the longer infrared wavelengths are sensitive to galaxies that are intrinsically red, such as elliptical galaxies and galaxies that have red colors due to a high degree of dust absorption.
The entire HUDF also was observed with the advanced camera's "grism" spectrograph, a hybrid prism and diffraction grating. "The grism spectra have already yielded the identification of about a thousand objects. Included among them are some of the intensely faint and red points of light in the ACS image, prime candidates for distant galaxies," says Sangeeta Malhotra of the Space Telescope Science Institute and the Principal Investigator for the Ultra Deep Field's ACS grism follow-up study. "Based on those identifications, some of these objects are among the farthest and youngest galaxies ever seen. The grism spectra also distinguish among other types of very red objects, such as old and dusty red galaxies, quasars, and cool dwarf stars."
Galaxies evolved so quickly in the universe that their most important changes happened within a billion years of the big bang. "Where the HDFs showed galaxies when they were youngsters, the HUDF reveals them as toddlers, enmeshed in a period of rapid developmental changes," Stiavelli says.
Hubble's ACS allows astronomers to see galaxies two to four times fainter than Hubble could view previously, and is also very sensitive to the near-infrared radiation that allows astronomers to pluck out some of the farthest observable galaxies in the universe. This will hold the record as the deepest-ever view of the universe until ESA, together with NASA, launches the James Webb Space Telescope in 2011.
Though ground-based telescopes have, to date, spied objects that existed just 500 million years after the big bang (at a redshift of 10), they need the help of a rare natural zoom lens in space, called a gravitational lens, to see them. However, the ACS can reveal typical galaxies at these great distances. Even much larger ground-based telescopes with adaptive optics cannot reproduce such a view. The ACS picture required a series of exposures taken over the course of 400 Hubble orbits around Earth. This is such a big chunk of the telescope's annual observing time that Institute Director Steven Beckwith used his own Director's Discretionary Time to provide the needed resources.
The HUDF observations began Sept. 24, 2003 and continued through Jan. 16, 2004. The telescope's ACS camera, the size of a phone booth, captured ancient photons of light that began traversing the universe even before Earth existed. Photons of light from the very faintest objects arrived at a trickle of one photon per minute, compared with millions of photons per minute from nearer galaxies.
Just like the previous HDFs, the new data are expected to galvanize the astronomical community and lead to dozens of research papers that will offer new insights into the birth and evolution of galaxies.Electronic images and additional information are available at:
A collision of two galaxies has left a merged star system with an unusual appearance as well as bizarre internal motions. Messier 64 (M64) has a spectacular dark band of absorbing dust in front of the galaxy's bright nucleus, giving rise to its nicknames of the "Black Eye" or "Evil Eye" galaxy.
This image of M64 was taken with Hubble's Wide Field Planetary Camera 2 (WFPC2) in 2001. The color image is a composite prepared by the Hubble Heritage Team from pictures taken through four different color filters. These filters isolate blue and near-infrared light, along with red light emitted by hydrogen atoms and green light from Strömgren y.
Image Credit: NASA and The Hubble Heritage Team (AURA/STScI) Acknowledgment: S. Smartt (Institute of Astronomy) and D. Richstone (U. Michigan)To see and read more, please visit:
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:
Using the ISAAC near-infrared instrument on ESO's Very Large Telescope, and the magnification effect of a gravitational lens, a team of French and Swiss astronomers  has found several faint galaxies believed to be the most remote known.
Further spectroscopic studies of one of these candidates has provided a strong case for what is now the new record holder - and by far - of the most distant galaxy known in the Universe.
Named Abell 1835 IR1916, the newly discovered galaxy has a redshift of 10  and is located about 13,230 million light-years away. It is therefore seen at a time when the Universe was merely 470 million years young, that is, barely 3 percent of its current age.
This primeval galaxy appears to be ten thousand times less massive than our Galaxy, the Milky Way. It might well be among the first class of objects which put an end to the Dark Ages of the Universe.
This remarkable discovery illustrates the potential of large ground-based telescopes in the near-infrared domain for the exploration of the very early Universe.The full text of this Press Release and the two associated ESO PR Photos 05a-b/04 are available.
More precisely, astronomers are trying to explore the last "unknown territories", the boundary between the "Dark Ages" and the "Cosmic Renaissance".
Rather shortly after the Big Bang, which is now believed to have taken place some 13,700 million years ago, the Universe plunged into darkness. The relic radiation from the primordial fireball had been stretched by the cosmic expansion towards longer wavelengths and neither stars nor quasars had yet been formed which could illuminate the vast space. The Universe was a cold and opaque place. This sombre era is therefore quite reasonably dubbed the "Dark Ages".
A few hundred million years later, the first generation of stars and, later still, the first galaxies and quasars, produced intense ultraviolet radiation, gradually lifting the fog over the Universe.
This was the end of the Dark Ages and, with a term again taken over from human history, is sometimes referred to as the "Cosmic Renaissance".
Astronomers are trying to pin down when - and how - exactly the Dark Ages finished. This requires looking for the remotest objects, a challenge that only the largest telescopes, combined with a very careful observing strategy, can take up.
Further in the past, however, observations of galaxies and quasars become scarce. Currently, only a handful of very faint galaxies are seen approximately 1,200 to 750 million years after the Big Bang (redshift 5-7). Beyond that, the faintness of these sources and the fact their light is shifted from the optical to the near infrared has so far severely limited the studies.
An important breakthrough in this quest for the earliest formed galaxy has now been achieved by a team of French and Swiss astronomers  using ESO's Very Large Telescope (VLT) equipped with the near-infrared sensitive instrument ISAAC. To accomplish this, they had to combine the light amplification effect of a cluster of galaxies - a Gravitational Telescope - with the light gathering power of the VLT and the excellent sky conditions prevailing at Paranal.
First of all, very deep images of a cluster of galaxies named Abell 1835 were taken using the ISAAC near-infrared instrument on the VLT. Such relatively nearby massive clusters are able to bend and amplify the light of background sources - a phenomenon called Gravitational Lensing and predicted by Einstein's theory of General Relativity.
This natural amplification allows the astronomers to peer at galaxies which would otherwise be too faint to be seen. In the case of the newly discovered galaxy, the light is amplified approximately 25 to 100 times! Combined with the power of the VLT it has thereby been possible to image and even to take a spectrum of this galaxy. Indeed, the natural amplification effectively increases the aperture of the VLT from 8.2-m to 40-80 m.
The deep near-IR images taken at different wavelengths have allowed the astronomers to characterise the properties of a few thousand galaxies in the image and to select a handful of them as potentially very distant galaxies. Using previously obtained images taken at the Canada-France- Hawaii Telescope (CFHT) on Mauna Kea and images from the Hubble Space Telescope, it has then been verified that these galaxies are indeed not seen in the optical. In this way, six candidate high redshift galaxies were recognised whose light may have been emitted when the Universe was less than 700 million years old.
To confirm and obtain a more precise determination of the distance of one of these galaxies, the astronomers used again ISAAC on the VLT, but this time in its spectroscopic mode. After several months of careful analysis of the data, the astronomers are convinced to have detected a weak but clear spectral feature in the near-infrared domain. The astronomers have made a strong case that this feature is most certainly the Lyman-alpha emission line typical of these objects. This line, which occurs in the laboratory at a wavelength of 0.1216 micron, that is, in the ultraviolet, has been stretched to the near infrared at 1.34 micron, making Abell 1835 IR1916 the first galaxy known to have a redshift as large as 10.
From the images of this galaxy obtained in the various wavebands, the astronomers deduce that it is undergoing a period of intense star formation. But the amount of stars formed is estimated to be "only" 10 million times the mass of the sun, approximately ten thousand times smaller than the mass of our Galaxy, the Milky Way.
In other words, what the astronomers see is the first building block of the present-day large galaxies. This finding agrees well with our current understanding of the process of galaxy formation corresponding to a successive build-up of the large galaxies seen today through numerous mergers of "building blocks", smaller and younger galaxies formed in the past.
It is these building blocks which may have provided the first light sources that lifted the fog over the Universe and put an end to the Dark Ages.
For Roser Pello, from the Observatoire Midi-Pyrenees (France) and co-leader of the team, "these observations show that under excellent sky conditions like those at ESO's Paranal Observatory, and using strong gravitational lensing, direct observations of distant galaxies close to the Dark Ages are feasible with the best ground-based telescopes."
The other co-leader of the team, Daniel Schaerer from the Geneva Observatory and University (Switzerland), is excited: "This discovery opens the way to future explorations of the first stars and galaxies in the early Universe."
Additional explanations and images are available on the authors' web page.Notes  This press release is issued in coordination between ESO, the Swiss National Science Foundation, the French Centre National de la Recherche Scientifique, and the European Journal Astronomy and Astrophysics.
"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
An illustrated version of this release is on the web at http://www.lbl.gov/Science-Articles/Archive/Phys-HST-supernovae.html
BERKELEY, CA -- A unique set of 11 distant Type Ia supernovae studied with the Hubble Space Telescope sheds new light on dark energy, according to the latest findings of the Supernova Cosmology Project, recently posted at http://www.arxiv.org/abs/astro-ph/0309368, and soon to appear in the Astrophysical Journal.
Light curves and spectra from the 11 distant supernovae constitute "a strikingly beautiful data set, the largest such set collected solely from space," says Saul Perlmutter, an astrophysicist at Lawrence Berkeley National Laboratory and leader of the Supernova Cosmology Project (SCP). The SCP is an international collaboration of researchers from the United States, Sweden, France, the United Kingdom, Chile, Japan, and Spain.
Type Ia supernovae are among astronomy's best "standard candles," so similar that their brightness provides a dependable gauge of their distance, and so bright they are visible billions of light years away.
The new study reinforces the remarkable discovery, announced by the Supernova Cosmology Project early in 1998, that the expansion of the universe is accelerating due to a mysterious energy that pervades all space. That finding was based on data from over three dozen Type Ia supernovae, all but one of them observed from the ground. A competing group, the High-Z Supernova Search Team, independently announced strikingly consistent results, based on an additional 14 supernovae, also predominantly observed from the ground.
Because the Hubble Space Telescope (HST) is unaffected by the atmosphere, its images of supernovae are much sharper and stronger and provide much better measurements of brightness than are possible from the ground. Robert A. Knop, assistant professor of physics and astronomy at Vanderbilt University in Nashville, Tenn., led the Supernova Cosmology Project's data analysis of the 11 supernovae studied with the HST and coauthored the Astrophysical Journal report with the 47 other members of the SCP.
"The HST data also provide a strong test of host-galaxy extinction," Knop says, referring to concerns that measurements of the true brightness of supernovae could be thrown off by dust in distant galaxies, which might absorb and scatter their light. But dust would also make a supernova's light redder, much as our sun looks redder at sunset because of dust in the atmosphere. Because the data from space show no anomalous reddening with distance, Knop says, the supernovae "pass the test with flying colors."
"Limiting such uncertainties is crucial for using supernovae - -- or any other astronomical observations -- to explore the nature of the universe," says Ariel Goobar, a member of SCP and a professor of particle astrophysics at Stockholm University in Sweden. The extinction test, says Goobar, "eliminates any concern that ordinary host-galaxy dust could be a source of bias for these cosmological results at high-redshifts." (See "What is Host-Galaxy Extinction?" under additional information, below.)
The term for the mysterious "repulsive gravity" that drives the universe to expand ever faster is dark energy. The new data are able to provide much tighter estimates of the relative density of matter and dark energy in the universe: under straightforward assumptions, 25 percent of the composition of the universe is matter of all types and 75 percent is dark energy. Moreover, the new data provides a more precise measure of the "springiness" of the dark energy, the pressure that it applies to the universe's expansion per unit of density.
Among the numerous attempts to explain the nature of dark energy, some are allowed by these new measurements -- including the cosmological constant originally proposed by Albert Einstein -- but others are ruled out, including some of the simplest models of the theories known as quintessence. (See "What is Dark Energy?" under additional information, below.)
High-redshift supernovae are the best single tool for measuring the properties of dark energy -- and eventually determining what dark energy is. As supernova studies with the HST demonstrate, the best place to study high-redshift supernovae is with a telescope in space, unaffected by the atmosphere.
Nevertheless, "to make the best use of a telescope in space, it's essential to make the best use of the finest telescopes on the ground," says SCP member Chris Lidman of the European Southern Observatory.
For the supernovae in the present study, the SCP team invented a strategy whereby the Hubble Space Telescope could quickly respond to discoveries made from the ground, despite the need to schedule HST time long in advance. Working together, the SCP and the Space Telescope Science Institute implemented the strategy to superb effect.
The current study, based on HST observations of 11 supernovae, points the way to the next generation of supernova research: in the future, the SuperNova/Acceleration Probe, or SNAP satellite, will discover thousands of Type Ia supernovae and measure their spectra and their light curves from the earliest moments, through maximum brightness, until their light has died away.
SCP's Perlmutter is now leading an international group of collaborators based at Berkeley Lab who are developing SNAP with the support of the U.S. Department of Energy's Office of Science. It may be that the best candidate for a correct theory of dark energy will be identified soon after SNAP begins operating. A world of new physics will open as a result.
"New constraints on omega-m, omega-lambda, and w from an independent set of eleven high-redshift supernovae observed with the HST," by Robert A. Knop and 47 others (the Supernova Cosmology Project), will appear in the Astrophysical Journal and is currently available online.
For more about the Supernova Cosmology Project visit http://supernova.lbl.gov/. For more about the Hubble Space Telescope and the Space Telescope Science Institute visit http://www.stsci.edu/resources/. For more about the SNAP satellite visit http://snap.lbl.gov/.
The Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.
Determining the expansion rate of the universe by comparing the brightness and redshift of far-off Type Ia supernovae therefore critically depends on accurate measurements of both.
One worrisome possible source of error in measuring distant supernovae has been host-galaxy extinction, the filtering effect of dust peculiar to the galaxy in which the supernova occurs. Dust occurs in our own galaxy too, but has been so extensively studied that it is of less concern in supernova distance measurements.
The concern is that distant supernovae appear dimmer not because of the accelerating effects of dark energy but, more prosaically, because of dust. There is a straightforward way to distinguish these effects, however, since dust normally reddens the light passing through it. Shorter, bluer wavelengths are absorbed and scattered more readily than longer, redder wavelengths.
"When you want to measure a supernova's brightness you can measure the light that was blue when it left, or the light that was red," says Greg Aldering, a member of the Supernova Cosmology Project and leader of the Nearby Supernova Factory program, which concentrates on studying the intrinsic properties of Type Ia supernovae. "Both measurements are valid, but what you want is to make sure you get the same answer on both sides of the spectrum. If the blue is fainter, you've got a dust problem."
The extraordinarily high quality of photometric data from the 11 distant supernovae studied by the Hubble Space Telescope in this study allowed their intrinsic brightness to be determined and compared in both bands.
The study determined that no anomalous effects of host-galaxy extinction occur at great distance; distant supernovae are comparable to nearby supernovae in this respect, underlining their utility as standard candles.
Not only did this discovery mean that the universe would never come to an end, more fundamentally it implied that a large part of the universe is made of something we know nothing about -- the mysterious whatever-it-is that goes by the name "dark energy."
Later, new measurements of cosmic microwave background (CMB) radiation provided strong evidence that the universe is flat (having an overall geometry of space like Euclid's, in which parallel lines never meet or diverge) -- and because there is not enough matter in the universe, whether visible or dark, to produce flatness, the difference can be attributed to dark energy, providing a strong confirmation of the supernova measurements.
The first attempt to explain the nature of dark energy was by invoking Albert Einstein's notorious "cosmological constant," an extra term he introduced early in the the equations of the theory of general relativity in the 20th century under the mistaken impression, shared by astronomers and cosmologists of the time, that the universe was static. The cosmological constant, which Einstein signified by the Greek letter lambda, made it so.
Einstein happily abandoned the cosmological constant when, in 1929, Edwin Hubble found the universe was not static but expanding. However, lambda came back strong -- albeit 70 years later! -- when supernova studies led to the discovery that expansion was accelerating.
"For the cosmological constant, the vacuum -- space itself - -- possesses a certain springiness," says Eric Linder, a cosmologist at Berkeley Lab and director of the Center for Cosmology and Spacetime Physics at Florida Atlantic University. "As you stretch it, you don't lose energy, you store extra energy in it just like a rubber band."
Such springiness, whether of matter, energy, or space itself, is described mathematically by a term called the equation-of-state parameter (w). For lambda, the value of this parameter is minus one, corresponding to a component of the universe that has "negative pressure" -- unlike matter or radiation, which have zero or positive pressure. True to its name, the cosmological constant doesn't change over time: the energy stored by lambda scales uniformly, increasing exactly as the volume of the universe increases.
The problem is that the most obvious source for lambda's stored energy is what quantum theory calls the energy of the vacuum ?? so much more powerful (10 to the 120th power!) than what's been observed for lambda, Linder says, that if this were the dark energy "it would overwhelm the expansion of the universe. It would have brought the universe to a swift end a miniscule fraction of a second after it was created in the big bang."
Other explanations of dark energy, called "quintessence," originate from theoretical high-energy physics. In addition to baryons, photons, neutrinos, and cold dark matter, quintessence posits a fifth kind of matter (hence the name), a sort of universe-filling fluid that acts like it has negative gravitational mass. The new constraints on cosmological parameters imposed by the HST supernova data, however, strongly discourage at least the simplest models of quintessence.
Quite different "topological defect" models attribute dark energy to defects created as the early universe cooled, during the phase changes that precipitated different forces and particles from a highly symmetrical early state.
Certain of these theoretical defects, known as domain walls, could have partitioned space into distinct cells whose boundaries would have repulsive gravity, thus filling the role of dark energy. But the new HST supernovae study rules out -- with 99 percent certainty -- domain walls as the source of dark energy.
While the case for the cosmological constant looks strong by comparison to these alternatives, many other exciting possibilities remain. Some even propose a cosmos in which our universe, having three dimensions of space, is afloat in a higher-dimensional world, with gravity free to interact among the dimensions.
Or there could be a time-varying form of dark energy that only temporarily mimics lambda. If it becomes less gravitationally repulsive in the future, it could bring acceleration to a halt, perhaps even causing expansion to reverse and triggering the collapse of the universe.
The opposite is also possible: superaccelerating dark energy. These models have w, the equation-of-state parameter, less than minus one -- unlike lambda, stored energy would not scale uniformly as the universe expands but increase faster than the increase in volume."One of the goals of the SuperNova/Acceleration Probe satellite is to determine whether w may be changing with time," says Saul Perlmutter, coprincipal investigator of the SNAP satellite now under development. "This will help us narrow the possibilities for the nature of dark energy. That's an exciting prospect for physicists, because understanding dark energy will be crucial to finding a final, unified picture of physics."
Shining as brightly as a million trillion suns yet seldom lasting even one minute, gamma-ray bursts (GRBs) were a great astronomical mystery only recently solved when they were conclusively shown to be linked to cataclysmic explosions called supernovae that mark the deaths of very massive stars.
Gamma-ray bursts come to us--across billions of light years of space and hence billions of years of time--from wholly random directions of the sky about once a day, but astronomers have long suspected they see only a small portion of the total number actually occurring. Until the recent gamma-ray burst/supernova link was made, proving this or even deriving a number of "actual-to-observed" bursts based on observations was exceedingly difficult.
With this link established, two scientists at MIT's Laser Interferometer Gravitational Wave Observatory (LIGO) have just derived an "actual-to-observed" ratio of 450-1. That is, there are roughly 450 GRBs occurring in the observable universe for every one that's detectable by orbiting satellites designed to look for them. This is a figure that not only utilizes the GRB-supernova link but also agrees well with a previous, independently derived ratio that did not use such a link.
These findings could have important consequences in the long hunt for elusive gravitational waves--tiny ripples in space-time predicted by Einstein's theory of general relativity but as yet never directly observed.
"Our 450-1 figure closely agrees with a 500-1 ratio derived in 2001 by other scientists, which makes us more confident in these results," said Maurice van Putten, assistant professor of applied mathematics. Van Putten collaborated with post-doctoral researcher Tania Regimbau in the study. "The earlier figure was based on a method totally independent of the supernova association involving spectral characteristics of the gamma-ray emissions themselves."
To derive their figure, van Putten and Regimbau assumed the now-standard "collapsar" model of GRBs. In that model, the core of an especially massive star undergoes a gravitational collapse (likely resulting in a black hole), producing a massive pressure wave that blasts out of the star in a particular direction. The blast wave collides with dust and gas in the surrounding interstellar medium at velocities near that of light, producing gamma-ray emissions. The type of star used in the collapsar model is also the type of star that ends its life in a supernova.
What astronomers saw in the spectral analysis of the light curves was the unmistakable signature of a supernova, including the presence of oxygen emission lines excited in the blast. This information provided powerful support to a previous, even closer blast on April 25, 1998 that had provided a less conclusive link between supernovae and GRBs.
Once the GRB-supernovae link was established, van Putten and Regimbau used a "very precious" sample of 33 GRBs whose distances (unlike most) are well known, to establish a mathematical relationship between how bright a given burst is and the rate at which the massive stars form and die.
They could do this because the massive stars involved in GRBs and supernovae live for only a few tens of millions of years, as opposed to billions of years. This fact, van Putten said, means such "massive stars essentially die at their place of birth."
One aspect of the collapsar model is that the burst (which precedes the actual supernova explosion) occurs along a particular axis in both directions, as opposed to a symmetric, radial one. Since axes of stars are oriented randomly throughout the universe, we detect only those bursts along or near whose axis the Earth happens to lie.
This effect is known as "beaming" and it means the angle through which the blast of energy is seen is relatively small for most observed blasts--no more than a few degrees of sky. Van Putten said this "beaming" effect is factored into their figure because the relationship is based on peak brightness.
Van Putten said knowing a ratio of actual to observed GRBs will help LIGO precisely because most GRBs are so distant that their gravitational waves won't be detectable by this array. Instead, the findings will give astronomers a sense of how often to expect a detectable gravitational wave produced by a sufficiently close GRB.
"This is an important finding for LIGO because these findings can give us a good handle on the local gravitational-wave event rate. It doesn't matter if the burst is beamed toward us or not because the gravitational wave energy is not beamed," van Putten said.
"Given what LIGO is capable of seeing, and using our results, we would expect an event rate of perhaps one per year, as opposed to one in 450 years, which would be hopeless," he said.
George R. Ricker, principal investigator for the MIT-run High Energy Transient Explorer (HETE2) satellite, hailed the findings as just the kind of success for which he and his colleagues had hoped."The HETE mission is rapidly transforming GRBs from vague cosmic mysteries into incisive cosmological probes. This new work is exactly the kind of stimulating research which we dreamed HETE's success would bring about," Ricker said.
Space Telescope Science Institute Press Release, May 9, 2002
Someday our Milky Way Galaxy and the neighboring Andromeda Galaxy may come crashing together in a horrendous collision that will twist and distort their shapes beyond recognition. Of course, to see that, you'll have to wait several billion years. But thanks to a combination of research science, Hollywood computer graphics, and large-scale, "immersive" visualization, visitors to the Smithsonian Institution's National Air and Space Museum in Washington, DC, can witness such an event today.
The Space Telescope Science Institute (STScI) in Baltimore, MD, the scientific home of NASA's Hubble Space Telescope, is extending its tradition of stunning imagery by creating a spectacular scientific visualization of two galaxies colliding. This incredibly detailed and immersive, full-dome video sequence will be a highlight of "Infinity Express: A 20-Minute Tour of the Universe," the inaugural show in the National Air and Space Museum's newly renovated Einstein Planetarium, opening Saturday, April 13, 2002.
The scientific visualization by Dr. Frank Summers, an astrophysicist in STScI's Office of Public Outreach, depicts a tremendous collision of two spiral galaxies. Because such events take hundreds of millions of years to occur, researchers use supercomputer simulations to study how galaxies are transformed and merge together. Dr. Summers has taken research data provided by Dr. Chris Mihos (Case Western Reserve University) and Dr. Lars Hernquist (Harvard University), and visualized it using the same software that Hollywood uses to produce blockbuster visual effects.
The result brings astrophysics out of the academic setting and presents a scientifically correct, yet compellingly beautiful animation directly to the planetarium audience. "By combining research simulations with Hollywood visualization techniques, we can create animations that are both accurate and artistic, while visually communicating complex astronomical events and ideas to the public," says Dr. Summers.
This contribution to the National Air and Space Museum marks the first release of scientific visualizations for full-dome video planetariums from the Informal Science Education Group at STScI. While Hubble images are a mainstay of planetarium shows, full-dome scientific visualizations represent a new level of astronomy outreach.
"NASA imagery will greatly benefit this emerging planetarium technology, and we can provide high-quality, dynamic content backed by the expertise of Hubble astronomers," says John Stoke, manager of Informal Science Education at STScI. Going forward, his group will distribute this galaxy collision sequence and other full-dome scientific visualizations, free of charge, to planetariums and show producers across the country and around the world.
Planetariums have entered a new era of full-dome digital video that immerses the viewer in the dynamic wonders of the universe. The video, projected across the entire hemisphere of a planetarium dome, has up to 23 times the resolution of a standard television and is wrapped 360 degrees around the audience, surrounding them in the experience.
While such systems are generally only in the larger planetariums today, technological advances are bringing the capability for full-dome video to thousands of smaller planetariums in the next couple of years. Worldwide, 100 million people visit planetariums every year.
Additional information is available on the Internet at:
http://oposite.stsci.edu/pubinfo/pr/2002/09 and via links in
The Space Telescope Science Institute (STScI) is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA). This work is partially supported by the National Science Foundation through the National Computational Science Alliance and the Partnerships for Advanced Computational Infrastructure. The National Air and Space Museum is owned and operated by the Smithsonian Institution.
Sets of images of galaxies and other objects (comets, etc.), including a rotation curve (under his "educational materials" catalogue), taken by Prof. William Keel of the University of Alabama at Tuscaloosa, are available on the Web.
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