mountain profile Institute for Astronomy University of Hawaii

Cosmology

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There is now nearly universal agreement that the universe began in a hot Big Bang more than 13 billion years ago and has expanded from a nearly uniform early state of very hot gas and radiation. One of the most compelling problems of modern astrophysics is to understand the evolution from this early uniform state to our current structured universe of stars and galaxies. When and how did galaxies form? When and where did the first stars form? Will galaxy formation go on forever, or are we at a special time in the history of the universe?

To try to understand these issues, we study galaxies and gas clouds at various redshifts, where an object's redshift (the shift of its light to longer wavelengths, usually because the object is moving away from Earth) is a measure of its distance. With the telescopes on Mauna Kea, we can detect galaxies that are so distant that their light has been traveling through space for billions of years. Since photons are perfectly preserved in their passage through space, we can directly view the universe as it was billions of years ago. By studying objects at different redshifts, we hope to be able to understand how our galaxy formed and how the universe developed its structure.

Deep Surveys

Link to Hawaii  Deep Field projectLennox Cowie, Antoinette Songaila Cowie, and Esther Hu used the UH 2.2-meter and the spectrographs on the Canada-France-Hawaii and Keck Telescopes, to carry out one of the first extremely deep surveys of all the galaxies in a representative part of the sky. Their Hawaii Deep Surveys have found colors and redshifts for many hundreds of galaxies out to redshifts beyond 2 (two-thirds of the way across the universe). How galaxy colors change with redshift (and hence time) is a clue to galactic evolution, since a collection of newly formed stars has a bluer color than a collection of older stars. In this way, Drs. Cowie, Songaila, and Hu have been able to show that star formation in galaxies peaked some time ago, at redshifts greater than 1, and that the characteristic luminosity of a galaxy that contains many young stars has been decreasing steadily since then. They predict that as star formation gradually dies out, future galaxies will not be as luminous as those familiar to us now.

Link to SCUBA deep survey page

Drs. Cowie, Songaila, and Hu have also been involved in the spectroscopic identification of objects in the Hubble Deep Field-the deepest-ever image of the sky-taken by the HST in 1995, and they have used the HST to get detailed images of the galaxies in the Hawaii Deep Survey.

Dust in galaxies absorbs starlight and reradiates it at much longer wavelengths, and in very dusty galaxies most of the light emitted in the visible wavelengths may be reradiated into the far infrared at wavelengths of around 100 microns. For galaxies at large distances this light is further redshifted by the expansion of the Universe to the so-called submillimeter wavelengths, which may be observed from the ground. Postdoctoral fellow Amy Barger, together with Lennox Cowie, David Sanders and a team Japanese of astronomers have recently used  the SCUBA array of bolometers on the JCMT 15-meter telescope to search for such high redshift galaxies for the first time. They found several such galaxies that are forming stars at a rate 10 to 100 times faster than a conventionally selected galaxy. The discovery of these submillimeter sources hint that an era of intense star formation may have occurred at a time before most galaxies emitted much optical light. The figure shows a 2.7' diameter region of sky at 850 microns: the brightest objects are believed to be starburst galaxies at redshifts between 1.5 and 3.

Esther Hu and Lennox Cowie are studying galaxies in the very early universe. These objects, with redshifts z > 5, have been detected through hydrogen emission lines excited in the first outbursts of star formation. They are among the most distant known objects, and form one of the key subjects to be investigated with the Next Generation Space Telescope. Ground-based optical and infrared observations conducted with the 10-m Keck and 8.3-m Subaru telescopes offer a glimpse into the distant past and origins of galaxies.

 

Quasar Absorption Lines

Observationally, we are able to follow the conversion of gas into stars and galaxies in the early universe by looking at the absorption provided by the gas in the spectra of high-redshift quasar-the so-called quasar absorption lines. Drs. Songaila, Hu, and Cowie have been using the Keck HIRES spectrograph to concentrate on the evolution of the most tenuous gas (the "forest"). They were one of the first groups to show that a surprisingly large fraction of these clouds at redshifts greater than 3 have already been contaminated by carbon that could only have been made in stars. This is about the same amount of enrichment that is seen in denser clouds, leading to the idea that stars may have existed even before there were big galaxies.

Quasar absorption lines also provide clues to the deepest cosmological questions. The overall fate of the universe depends, in our understanding of the Big Bang, on the density of matter in it. Using high-redshift clouds observed in quasar spectra, Drs. Songaila and Cowie have measured the amount of deuterium at high-redshift, and have combined this information with what is known about the amount of ordinary "baryonic" matter in the universe. They have concluded that if the universe is going to recollapse, the bulk of the matter must be in an exotic non-baryonic form, the so-called "dark matter."

The Hubble constant

Within the framework of the big bang cosmology, the two cosmological parameters--the Hubble constant and the energy density of the universe--embody the past history and the predicted future of the universe as a whole. It is no wonder that attempts to measure these numbers underlie much of current extragalactic research.

The Hubble constant relates the apparent velocities at which galaxies are moving away from Earth (their redshifts) to their distances from us. It sets the measured size scale of the universe and is also related to its age--the time back to the big bang. The extragalactic distance scale results from a painstaking effort to relate well-understood measures of distance on ever-increasing scales.

Much of John Tonry's recent work has established his pioneering use of galaxy surface brightness fluctuations (SBF) as a distance measure in comparison with other estimators and has demonstrated that the calibration of SBF is universally valid. He has also extended the technique into infrared wavelengths using IfA's state-of-the-art instruments and taking advantage of the transparency of the atmosphere above Mauna Kea to infrared radiation. Recent work on Virgo cluster ellipticals has confirmed the previous distance calibration for the Local Group of galaxies based on visible light fluctuations. It has also shown that working in the infrared, where the fluctuations are brighter relative to contaminating nonfluctuating point sources, and where the seeing is better, is a way of extending the technique to greater distances.

It is equally important to establish the first links in the distance ladder. Dr. Tonry and collaborators also participate in project DIRECT, which aims to determine direct distances to two important nearby galaxies, M31 (Andromeda) and M33 (Triangulum), by a method free of any intermediate steps.

High-z supernova search

John Tonry is a leading member of a multi-university collaboration that is trying to measure the expansion of the universe by using Type Ia supernovae.  These stellar explosions are so bright that they can be pinpointed in galaxies with extremely large redshifts. Supernovae are discovered by comparing images of distant galaxies taken a month apart with the CFHT or other large telescopes. Once a supernova is discovered its light variations are studied intensely in follow-up observations on a number of telescopes.    Since the total amount of light given out by a Type Ia supernova is fairly accurately known, Tonry and his collaborators, who include graduate students Brian Barris and Megan Novicki, can estimate the distance to the galaxy in which the supernova occurs by measuring how bright the supernova appears. Because this calculated distance is independent of the redshift it is possible to compare the universe's expansion rate at different epochs in the past.  The unexpected result of this program is the discovery of evidence that the expansion of the Universe is accelerating - a result that can only be understood by the existence of a mysterious dark energy that fills the universe. 

Local Velocity Field

One of the complications that arises when measuring the Hubble constant is that of separating the galaxies' peculiar velocities, which presumably arise as a response to local gravity, from their pure Hubble flow recession. Conversely, knowledge of the peculiar velocities is a powerful way of constraining models of large-scale structure formation in the universe. Brent Tully's recent work has in part been an investigation of the local velocity field. Using the luminosity--line-width (Tully-Fisher) relation to estimate distances to a nearby unbiased sample of spiral and irregular galaxies, Dr. Tully finds a significant increase in the local value of the Hubble constant with distance that appears to arise from peculiar velocities associated with the Coma-Sculptor Cloud and the Leo Spur. In turn, he has been able to model the local density enhancements that might produce such velocities. A major focus has been the study of perturbations in motion caused by the Virgo cluster of galaxies and the region called the "Great Attractor" in Hydra-Centaurus.

Another crucial cosmological parameter is the matter density of the universe. Using a knowledge of the galaxies' present redshifts and angular positions, and the condition that the peculiar velocities are small at high redshift, Dr. Tully has been involved in an ingenious attempt to find the global ratio of mass to light by reconstructing the orbits that many galaxies followed to reach their present positions. Results indicate that the overall matter density is relatively low, but that there are special places where the ration of mass to light is especially high..

 

Hot gas in galaxy clusters

All estimates of Ω must take into account the existence of dark matter, for which there is evidence on all scales from galaxies to clusters of galaxies. For this reason there is considerable interest in trying to measure the mass-to-light ratio on the largest scales. This measurement is the most likely to give an unbiased estimate of the ratio of ordinary to dark matter. Patrick Henry, Isabella Gioia and Harald Ebeling have for many years been involved in the effort to discover and characterize X-rays from galaxy groups, clusters amd superclusters, first with the Einstein X-ray orbiting telescope and now with the Advanced Satellite for Cosmology and Astrophysics (ASCA) and the ROSAT X-ray satellite.

Gravitational Lensing

wpe42.jpg (5520 bytes)In As crucial as cluster X-ray mass estimates are, they still leave open the question of the biasing of light and dark matter on these scales. The relatively new technique of weak gravitational lensing--the slight systematic shear in the images of background galaxies by the gravitational bending of their light as it passes by a foreground galaxy or cluster --- was invented by Nick Kaiser and collaborators. It probes the dark matter distribution on many scales, including those of galaxies, clusters, and superclusters, and so potentially can give an unbiased measure of the mass density in the universe. recent years, Dr. Kaiser and collaborators have focused on measuring weak lensing around massive clusters of galaxies, and they have obtained results for a number of clusters. Dr. Kaiser and Gerard Luppino have used the very large format UH 8K CCD camera to very efficiently image the large spatial scales associated with galaxy clusters. They have obtained the first estimate of a cluster mass profile on a very large scale have also for the first time detected weak lensing shear associated with a high-redshift cluster (redshift = 0.8). The very large mass they measured for this cluster makes it clear that large mass concentrations existed even in very early times, an important constraint on theories of the formation of structure in the universe. The measurement also constrains the distribution of the background galaxies whose light is being lensed, making it possible to derive conclusions about the redshift distribution of galaxies that will always be too faint for direct spectroscopic observation.

Weak lensing studies on very large scales of about one degree may also probe the parameters of large-scale structure. With Lev Kofman, Dr. Kaiser has recently observed what might be a ``bridge'' connecting two clusters in a supercluster at a redshift of 0.4, a direct determination of the weblike structure of galaxies. Dr. Kaiser has also investigated theoretically the impact of the cosmological model on mass estimates derived from weak lensing. He finds, for example, that the very large cluster mass inferred at redshift 0.8 would be greatly reduced if Ω were dominated not by the density of ordinary matter, light and dark, but instead by vacuum energy density, Einstein's "cosmological constant."

Theoretical Cosmology

Simulated CMB mapIstvan Szapudi and his post-doctoral colleagues Jun Pan and Pablo Fosalba (the "Cosmowave" group) study the history of the Universe by looking for patterns in the way that cosmic matter was distributed in space when the Universe was young. The data they use includes large scale surveys of high redshift galaxies, such as the Sloan Digital Sky Survey (SDSS) and maps of the fluctuations in the 3 K background radiation, such as WMAP.

The patterns that we see now seen are the result of the gravitationally induced motions of matter in the era soon after the Big Bang. Szapudi and his colleagues use computers to simulate these motions in the early Universe under different physical assumptions, and then statistically compare their models with the observed data using powerful mathematical tools that they have developed. Since gravitational motions are caused as much by dark matter as by ordinary matter, the analysis of these cosmic patterns provide one of our most powerful ways of studying how dark matter and dark energy are distributed in the Universe. Contemporary data analysis tools are inadequate for future large data sets, even with the most powerful supercomputers existing or projected. The new approach consists of a powerful mixture of advanced computer science, statistics, and group theory. As an example, standard analysis of megapixel cosmic microwave background maps from the upcoming Planck survey would take a million years on hypothetical computers equipped with TeraBytes of memory. The new analysis suite created by Szapudi and collaborators, SpICE (Spatially Inhomogeneous Correlation Estimator) brings an order of billionfold improvement, enabling the analysis of WMAP and Planck in less then 5 and 40 minutes, respectively.