Chapter 25:

Hubble's Ultra Deep Field Released

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.

Dark Energy, Supernovae Studied with Hubble

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.

Additional information:

"What is Host-Galaxy Extinction?"

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.

"What is Dark Energy?"

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.

GAIA, the Astronometry Satellite, Is Confirmed

At the May 2002 meeting of the European Space Agency's Scientific Programme Committee, at which the Venus Express mission was canclled, the GAIA mission was retained. It is to map the brightness, color, and relative positions of a billion celestial objects. Launch is hoped for by 2009. Many exoplanets, even thousands, could be discovered during the mission.

astro.estec.esa.nl/GAIA

references: Nature, vol. 417, 30 May 2002, p. 474,
Science, vol 296, 31 May 2002, p. 1585

FAME Astrometric Mission Is Cancelled

A New Mapping Astrometry Satellite: FAME

"FAME" WILL SIZE UP THE UNIVERSE AND SEARCH FOR DISTANT PLANETS

Harvard-Smithsonian Center for Astrophysics Press Release

A space experiment with major contributions from the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, MA, has been selected for NASA's Medium-Class Explorer, or MIDEX, program, and scheduled to be launched in 2004.

The Full-Sky Astrometric Mapping Explorer (FAME) is an Earth orbiting optical telescope that will gather information on 40 million stars in the Milky Way Galaxy with unprecedented measurement accuracy. For bright stars, positions will be determined to the equivalent of the width of a footprint on the Moon as seen from Earth (50 millionths of a second of arc). This exacting precision is central to the study of key issues of scientific and general interest including the existence of other "solar systems," the size and age of the universe, and an investigation of the mysterious "dark matter" in our portion of the Galaxy.

"FAME will increase by more than 1000-fold the volume of space in which we can determine the distances to stars. By using the parallax method, we will directly determine the lower rungs of the 'cosmic distance ladder,'" says Dr. Robert Reasenberg of the Harvard-Smithsonian Center for Astrophysics. "Further, the star coordinates determined by FAME will be more than 20 times more accurate than any available today, opening the way for a rich scientific yield from the mission and producing a resource for future researchers."

"By measuring the wobbling of star positions, FAME will discover companions, including 'brown dwarfs' and giant planets," says Dr. James Phillips of the CfA, who serves as deputy project scientist. "Because of the large number of stars FAME will observe, it will provide the first statistically useful survey of such companions and elucidate the transition region between brown dwarfs and giant planets."

In addition to determining the positions, motions, and distances of the stars, this satellite will measure the brightness of stars in each of several color bands, repeatedly during the mission, to achieve millimagnitude accuracy for bright stars. When combined with the distance measurements, this photometric information will permit a determination of stellar type and intrinsic brightness, and will contribute to an understanding of the evolution of stars. FAME will contribute to the accurate inertial reference frame needed both for studies of solar-system objects and by Gravity Probe B, which will test the "frame dragging" predicted by general relativity.

For more information on FAME, visit its website at http://aa.usno.navy.mil/FAME. The NASA press release about this announcement is available at http://spacescience.nasa.gov as an October 14, 1999, entry.

Defining the Astronomical Unit

Prof. Bruce Margon, Space Telescope Science Institute, asks the following question and, after discussion, asked the following answer:

How is the length of the AU determined? Page 458ff. defines the AU, and tells the reader that once you measure a parallax, simple trigonometry gives you the distance to a star. But this method works to give distances in linear units like kilometers only if you know the linear length of one of the other sides of the triangle.

A full explanation can get pretty long. You can state how the AU was first inferred (transits of Venus), accurately inferred (parallax of Eros), or done today (radar ranging).

Prof. Margon suggests the following addition to my text: "Of course you need to know the linear size of the AU to get this trigonometry to work, and this can be inferred accurately from observations of motions of solar system bodies plus knowledge of Kepler's Law's. Captain Cook used observations of Venus made from Tahiti for this purpose centuries ago, and today it can be done with great accuracy by direct radar ranging."

Let me add that transits of Venus are exceedingly rare, and come in pairs with the members of the pair separated by 8 years but then with over 100 years until the next pair. There were transits of Venus in 1761 and 1769 (which were observed by Captain Cook, who mapped Australia and New Zealand as spinoffs of this astronomical work), and in 1874 and 1882. The next transits of Venus will take place on June 8, 2004, and on June 5/6, 2012. Don't miss them. Venus takes several hours to cross the sun.

Star Charts

A web site with all sorts of star charts is available.

Hipparcos

The web site of the Hipparcos spacecraft, which measured parallaxes and proper motions of over 120,000 stars, includes an education page from which students can learn about variable star measurement and analysis by actually working with data. Information on the accuracy of these measurements and H-R diagrams resulting from them, are also on line.

The Eye is Not Logarithmic

Eric Schulman of the National Radio Astronomy Observatory and Caroline Cox of the University of Virginia have pointed out (American Journal of Physics 65 (10), October 1997, pp. 1003-1007) that the response of the eye is really a power law, not the logarithmic law that was believed when Pogson set down the magnitude scale numerically in 1856. Their Figure 1 nicely compares the logarithmic and power laws, which agree at only two points. The difference may lead to faulty estimates of magnitudes by visual observers using comparison stars.