As of May 1996, the Hubble Space Telescope Key Project--Wendy Freedman and colleagues--reported that they have now studied a dozen galaxies and that the Hubble Constant is in the range 68-78 km/s/Mpc. That interval corresponds to ages of 9 billion to 12 billion years for the galaxy.
Simultaneously, Cepheid-variable distances to 5 galaxies in which supernovae have been detected allow the distance scale to be extended a thousand times farther out, and other scientists report a Hubble constant of 50 based on these data, which corresponds to ages of 11 to 15 billion years.
It is interesting that these ranges overlap, though the lower ages are still discrepant with the ages measured for globular clusters.
The southern-hemisphere set of radio telescopes used as an interferometer and known as the Australia Telescope contains 6 22-m dishes spaced a maximum of 6 km apart, used with the 64-m Parkes radio telescope and another 22-m radio telescope located at greater distance.
The existence of tails around colliding galaxies may mean that there is not much dark matter around them. Supercomputer simulations have been calculated for the galaxy pair known as the Antennae, shown in the text in an older simulation
Two international teams of astronomers, using NASA's Hubble Space Telescope, are reporting major progress in converging on an accurate measurement of the universe's rate of expansion -- a value which has been debated for over half a century.
The Hubble Space Telescope Key Project team, an international group of over 2 0 astronomers, is led by Wendy Freedman of Carnegie Observatories, Pasadena, CA, Robert Kennicutt, University of Arizona, Tucson, AZ, and Jeremy Mould, Mount Stromlo and Siding Springs Observatory, Australia. The group's interim results, announced May 9 at a meeting held at the Space Telescope Science Institute in Baltimore, Maryland, are consistent with their preliminary result, announced in 1994, of 80 kilometers per second per megaparsec (km/sec/Mpc), based on observations of a galaxy in the Virgo cluster. These new results yield ranges for the age of the Universe from 9-12 billion years, and 11-14 billion years, respectively. The goal of the project is to measure the Hubble Constant to 10% accuracy.
"We have five different ways of measuring the Hubble Constant with HST," saidDr. Freedman. "The results are coming in between 68 and 78 km/sec/Mpc". (For example, at an expansion rate of 75 km/sec/Mpc, galaxies appear to be receding from us at a rate of 162,000 miles per hour for every 3.26 million miles farther out we look).
Two months ago, a second team, led by Allan Sandage, also of the Carnegie Observatories, Abhijit Saha (Space Telescope Science Institute), Gustav Tammann and Lukas Labhardt (Astronomical Institute, University of Basel), Duccio Macchetto and Nino Panagia (Space Telescope Science Institute/European Space Agency) reported a slower expansion rate of 57 km/sec/Mpc.
The value of the Hubble Constant allows astronomers to calculate the expansio n age of the Universe, the time elapsed since the Big Bang. Astronomers have been arguing recently whether the time since the Big Bang is consistent with the ages of the oldest stars.
The ages are calculated from combining the expansion rate with an estimate of how much matter is in space. The younger age values from each team assume the Universe is at a critical density where it contains just enough matter to expand indefinitely. The higher age estimates are calculated based on a low density of matter in space.
"A point of great interest is whether the age of the Universe arrived at this way is really older than the independently derived ages of the oldest stars," said Saha, an investigator on both Hubble teams.
"The numbers lean on the side that the stellar ages are a little lower, or that the hypothesis that we live in a critical density universe needs to be questioned," said Saha. "As further results accumulate over the next few years, we hope to tighten the constraints on these issues."
THE OBSERVATIONS
The Key Project team is midway along in their three-year program to derive th e expansion rate of the Universe based on precise distance measurements to galaxies. They have now measured Cepheid distances to a dozen galaxies, and are about halfway through their overall program.
The Key Project team also presented a preliminary estimate of the distance to the Fornax cluster of galaxies. The estimate was obtained through the detection and measurement with the Hubble Space Telescope of pulsating stars known as Cepheid variables found in the Fornax cluster. The Fornax cluster is measured to be approximately as far away as the Virgo cluster of galaxies -- about 60 million light-years.
The Key Project team member who led this effort, Caltech astronomer Barry Madore said, "This cluster allows us to make independent estimates of the expansion rate of the Universe using a number of different techniques. All of these methods are now in excellent agreement. With Fornax we are now at turning point in this field."
The team is measuring Cepheid distances to the Virgo and Fornax clusters of galaxies as a complementary test. Their strategy is to compare and contrast expansion numbers from a variety of distance indicators.
The Key Project team is systematically looking into a variety of methods for measuring distances. They are using Cepheids in a large sample to tie into five or six "secondary methods". One such secondary method relates the total luminosity of a galaxy to the rate at which the galaxy is spinning, the Tully-Fisher relation. Another secondary method makes use of a special class of exploding star known as a type Ia supernova. This phase of the Hubble Constant research will be completed within another two years.
In contrast, the Sandage team focused on a single secondary distance indicator, one of the same indicators also used by the Key Project team, the type Ia supernova. Sandage maintains that these stars are "standard bombs" according to theory. He suggests that when they explode they all reach exactly the same intrinsic brightness. This would make them extremely reliable "standard candles," (objects with a well-known intrinsic brightness) visible 1,000 times farther away than Cepheids. Since they are intrinsically brighter than any other standard candle, they offer the opportunity for an accurate measurement of the Universe's overall expansion by looking out the farthest.
Although both teams are still in disagreement over the precise rate at which the Universe is expanding and on how old it is, they are optimistic that their estimates will continue to converge with further observations and analysis.
A review entitled "Measuring the Hubble Constant with the Hubble Space Telescope," by Robert Kennicutt of the University of Arizona, Wendy Freedman of the Carnegie Observatories, and Jeremy Mould of the Australian National University appeared in the Astronomical Journal for October 1995, vol. 110, pp. 1476-1491. It summarizes the Extragalactic Distance Scale Key Project, one of three "key projects" on the Hubble Space Telescope. It is based on talks given before the American Astronomical Society. It shows, among many other things, the importance of Hubble for Cepheid measurements not only because it is able to see faint objects but also because observing can be scheduled at times not linked to the phase of the Moon. Figures 5 and 6 show the range of values for Hubble's Constant deduced over a period of years, which "now cover a nearly continuous range of 45-90 km s-1 Mpc-1."
On his Web site, Prof. William Keel of the University of Alabama has galaxy pairs and mergers, an empirical time sequence of mergers, a rotation curve and optical image of NGC 5746, and most of the M81 sequence (X-ray, UV color, optical color, IRAS deconvolved color, and 20-cm radio image) from the ASP slide set. (The globular cluster 47 Tuc also appears, since it was conveniently in the sky during a break in an observing sequence at the Cerro Tololo Inter-American Observatory.)
The Ultraviolet Imaging Telescope that flew aboard the Astro-2 mission of the Space Shuttle Endeavour in 1995 photographed several galaxies, including the spiral galaxy M101, the Large Magellanic Cloud, and a starburst galaxy. The effective wavelength was about 162 nm, far shorter than the minimum optical wavelength of 300 nm.
In September 1996, the Japanese Space Agency will launch an antenna that is to be used with ground-based radio telescopes in very-long-baseline interferometry.
A debate about "The Scale of the Universe" was held on April 21, 1996, at the National Academy of Sciences, Washington. Debaters were Sidney van den Bergh of Canada, speaking for a high Hubble constant (about 80) and Gustav Tammann of Switzerland, Alan Sandage's chief collaborator, for the low Hubble constant (about 50). John Bahcall of the Institute for Advanced Study moderated. Owen Gingerich at Harvard and Virginia Trimble of the University of California at Irvine and the University of Maryland gave introductory lectures.
The optical counterpart--known as FSC10214+4728--of the radio source 4C41.17 is the most distant galaxy known. It was also thought to be the intrinsically brightest object in the Universe, given its observed brightness and its extreme distance. An optical view is shown in Figure 31-38a, p. 565, of ETU1995, and an infrared view--the first scientific image taken with the Keck Telescope on Mauna Kea--is shown as Figure 31-38b, p. 566.
Observations of this object taken with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope in December 1994 and revealed in June 1995 show that the object is a gravitational lens. Its arc-like shape was already revealed by the Keck image. But the Hubble view shows three objects in an arrangement that shows even the "lensing galaxy," the galaxy that magnifies the source by a factor of about 100.
Thus the image is a celestial mirage, and the object is not as intrinsically bright as had been thought.
Hundreds of uncatalogued, faint galaxies have been detected in the sky. Christopher Impey of the University of Arizona, David Sprayberry of the Netherlands, Michael Irwin of the Royal Greenwich Observatory in England, and Gregory Bothun of the University of Arizona have discovered so many dim galaxies that they could double the number of galaxies we think are in the Universe. These new galaxies make a major contribution to the "missing mass," especially given the dark matter that we think is associated with them. They appear as blue, since the original ultraviolet light from hot stars in these galaxies has been redshifted to that spectral region because of the expansion of the universe.
Giant hydrogen halos have been discovered around distant galaxies, using the Faint Object Spectrograph and the Goddard High Resolution Spectrograph aboard the Hubble Space Telescope. Ken Lanzetta of SUNY-Stony Brook, David Bowen of STScI, David Tyler of UC-San Diego, and John Webb of U. New South Wales, Australia, more recently looked at archival Hubble data for six quasars. Then they used ground based telescopes to identify galaxies near the clouds and to measure the distances to those galaxies. In most cases, the galaxies were within about 500,000 light-years of the clouds.
Reference: Astrophysical Journal, April 1st, 1995
Ring galaxies occur when one galaxy plows through the center of another, hollowing it out (ETU p. 550). The Hubble Space Telescope recently imaged the Cartwheel Galaxy, 500 million light years from us. These high resolution images (available from the STScI's public image server show hundreds of bright blue knots generated by a spreading wave that followed the collision. These knots are presumably clusters of new stars. The image that you can see by clicking on the word "images" at STScI shows that the center of the Cartwheel has been so cleaned out that you can see a distant galaxy beyond. Thus little dust is left in the Cartwheel's center, which may mean that the Cartwheel was mainly hydrogen gas, perhaps with a low stellar density, before the collision.
The extremely high resolution obtainable with the Very High Baseline Array allowed a team of astronomers to study the velocities of water-molecular masers near the center of the galaxy NGC 4258. Since we are looking edge-on at the galaxy, these studies reveal the circulation close to the center of that galaxy. From the 900-km/s velocities they measure, they deduce that the core of the galaxy has a density of at least 100 million suns per cubic light year. Thus, they conclude, it must have a black hole there.
The diamond jubilee of the Great Debate of 1920 on whether "spiral nebulae" were independent galaxies or not, linked with the Gamma-Ray Debate of 1995, about the distance to the gamma-ray bursts that are being discovered by the hundred with the Gamma Ray Observatory, is being celebrated with a homepage at Goddard Space Flight Center.
A debate was held at the National Academy of Sciences in Washington in April, 1995. In the gamma-ray debate, one side argued that the gamma-ray bursts that are detected at a rate of about 1 per day come from "cosmological," that is, huge, distances beyond nearby galaxies, or at least that the matter is undecided. The other side argued that the gamma-ray bursts come from a halo around our own galaxy. No vote was taken.
The Hubble Space Telescope consortium of Wendy Freedman of the Carnegie Observatories, et al., measured Cepheids in two more galaxies in the Virgo Cluster. At the American Astronomical Society meeting in Tucson in January 1995, they reported that their result was consistent with their early result of 80 km/s/Mpc for the Hubble constant. They said that they would not be reporting all their intermediate results over the next three years, as they carry out this Hubble key project, since the public wouldn't know how to properly interpret continual reports with the value for the Hubble constant (and thus for the age of the Universe) changing all the time.
A group that studied type Ia supernova reported that these supernovae are not standard candles but that they have found a relation between the speed of the decline of their light curves and their brightness. Using this slope-luminosity relation (similarly to the way the period-luminosity relation is used for Cepheid variables), they could make a tight relation that gave a Hubble constant of 60-70, with an uncertainly of about 5. They think that including the effect of interstellar dust will raise this range by about 10%.
A cluster of galaxies mostly hidden behind the Milky Way turns out to be richer than expected. It is the most prominent nearby concentration of mass in the southern sky. It is within 9 degrees of the position reported for the Great Attractor and of the same redshift as predicted for it, and the scientists who discussed it hold that it marks the Great Attractor's core.
Reference: R. C. Kraan-Korteweg, P. A. Woudt, V. Cayatte, A. P. Fairall, C. Balkowski, and P. A. Henning, Nature, 379, 8 February 1996, 519-21.
The Infrared Space Observatory (ISO), launched in November 1995, observed several galaxies in the infrared. The Antennae, discussed in the textbook as an example of colliding galaxies, shows a ring around the nucleus in which starmaking is going on at a feverish pace. Another region of starmaking is along the line where the disks of the two galaxies overlap, and therefore where the collision is most intense. ISO has also made several images of the spiral galaxy M51, also revealing regions of star formation.