ASTRONOMY

Astronomers are reveling in new cosmic images, from close-ups of the moons of Jupiter to pictures of the very first galaxies, thanks to instruments that show them the universe at wavelengths from visible to the far-infrared. This composite image of the Jovian moon Io was the highest-resolution picture of that body when it was taken this spring by the Galileo spacecraft, then 294,000 km from Io. The image shows dozens of volcanic vents where lava flows hotter than any surface temperature recorded

Sep 1st, 1998

ASTRONOMY

Scientists image universe across the spectrum

Neil Savage

Astronomers are reveling in new cosmic images, from close-ups of the moons of Jupiter to pictures of the very first galaxies, thanks to instruments that show them the universe at wavelengths from visible to the far-infrared. This composite image of the Jovian moon Io was the highest-resolution picture of that body when it was taken this spring by the Galileo spacecraft, then 294,000 km from Io. The image shows dozens of volcanic vents where lava flows hotter than any surface temperature recorded on any other planetary body. Temperatures at one vent, Pillan Patera, may run as high as 3140°F.

Observations by the Voyager spacecraft in 1979 had put the highest temperature estimates at 710°F, and ground-based telescopes raised the estimates to 1160°F in 1986. While the lower temperature had led scientists to believe they were seeing evidence of sulfur volcanoes, the newer measurements suggest silicate rock, changing ideas about the moon`s geology.

The image at right--taken at 400 nm, 560 nm, and 1 µm--shows dark-green patches at the cores and margins of bright regions rich in sulfur dioxide. "It wouldn`t look that terribly different to your eye. There are a couple of small green spots that would look much more yellow," said Paul Geissler, a senior research associate at the Lunar and Planetary Laboratory of the University of Arizona (Tucson, AZ). The dark-green spots "are fairly exciting to us, because we haven`t ever seen them before."

Dark spots mark the sites of current volcanic activity, with the bright red representing deposits of material explosively ejected from the vents. Most of the white and yellow comes from sulfur compounds, but the dark material making up the flows and volcanic caldera are probably silicate rock. The blue background is the top of Jupiter`s clouds, which absorb in the infrared.

The image was taken with Galileo`s solid-state imaging (SSI) system, capable of imaging the Jovian satellites at spatial resolutions of less than 1 km. The SSI uses a Cassegrain telescope with a 176.5-mm aperture, a fixed focal ratio of f/8.5, and a focal length of 150 cm. It has a field of view of 8.1 mrad. The instrument was designed to have high resolution and a large field of view so it could study the motion of Jupiter`s atmosphere and the geologic formations of its moons, but it was also given a variety of filters to let it study the composition of Jupiter and satellite surfaces. A wheel has filters at 611, 404, 559, 671, 734, 756, 887, and 986 nm.

The telescope is accompanied by a virtual-phase charge-coupled-device (CCD) camera of 800 ¥ 800 pixels, designed by Texas Instruments (Dallas, TX) and NASA`s Jet Propulsion Laboratory (Pasadena, CA). To shield against radiation in the harsh environment of Jupiter`s magnetosphere, the CCD is surrounded by a 1-cm-thick layer of tantalum. A preflash system bathes the CCD in light at 930 nm several times to clear residual images after exposure.

Scientists also viewed the surface with the Near-Infrared Mapping Spectrometer (NIMS). The instrument has a 22.8-cm diameter, 80-cm-focal-length Ritchev-Chretien telescope with a spatial scanning secondary mirror and a spectrometer that uses a diffraction grating rather than a prism. Its 15 indium antimonide (InSb) detectors are cooled to 64 K. NIMS overlaps the wavelength range of the SSI, running from 700 nm to 5.2 µm. Geissler said that Galileo has not yet definitively determined what some of the material on Io`s surface is, because the NIMS observations were too distant to resolve small surface features, and the SSI spectra included only six data points. But he hopes those questions will be addressed when Galileo returns to Io in late 1999 for a flyby within 800 km.

Galileo arrived at Jupiter in December 1995 and is now on an extended mission to study the moons, particularly looking for signs of an ocean on Europa. Other recently released images suggest that an ancient subsurface ocean once existed on Jupiter`s largest moon, Ganymede.

Early images

Closer to home, but looking farther out, the Very Large Telescope (VLT) of the European Southern Observatory (Paranal Observatory in Northern Chile) is going through a commissioning phase to test its systems, but is already gathering images. The photograph above shows a polar ring galaxy, NGC 4650A, which lies 165 million light-years from Earth in the southern constellation Centaurus. The system consists of a lenticular-shaped galaxy surrounded by a knotty, extended ring-like collection of stars, dust, and gas. The two components are nearly perpendicular to each other. Scientists are unsure how NGC 4650A came to be this way, but believe it may have involved two galaxies passing close to each other.

The colors in this image show how different the two components are. Most stars in the lenticular galaxy are older and therefore reddish, while the polar ring is made up of young, blue stars and gaseous nebulae. The way the material in this galaxy moves shows that there is a significant dark-matter halo surrounding it, although astronomers are not sure which component the halo belongs to. NGC 4650A is a popular subject for studying such halos, however, because its unusual shape lets scientists measure velocities of matter in two planes.

This picture was taken under mediocre observing conditions during a break in the commissioning work. It is made up of seven exposures, slightly processed to balance them. Astronomers took six 10-minute exposures, three at 430 nm, two at 530 nm, and one at 590 nm, as well as a four-minute exposure at 590 nm. The field of view is 1.5 ¥ 1.5 arcmin.

It was taken with UT1, the first of four planned 8.2-m telescopes that will comprise the VLT and will be able to work independently or together. When combined, they will provide the light-collecting power of a 16-m telescope. The useful wavelength range runs from about 300 nm to 25 µm. Each telescope has an 8.2-m monolithic Zerodur main mirror, a 1.1-m beryllium mirror, and a 1242 ¥ 866-mm elliptical Zerodur mirror. They can use a Cassegrain, Nasmyth, or Coude focus. The telescopes can also be used for interferometry.

The VLT is equipped with an active optics system that adjusts the main mirror to make up for static, thermal effects, and wind-buffeting, for example. A wavefront sensor CCD uses an offset star as a reference to monitor optical quality and 150 computer-controlled axial actuators apply force to the back of the mirror to make adjustments. There is also an adaptive optics system for the Nasmyth and Coude foci. A 4-W CW sodium laser emitting at 589 nm will act as an artificial guide star, focusing at an altitude of 90 km, where it can reflect off a layer of atomic sodium in the atmosphere (see Laser Focus World, June 1998, p. 90).

The UT1 saw first light in May and is expected to begin regular observations in April 1999, when testing is completed. All four units of the VLT are expected to be in service in seven years.

Long wavelengths

A new instrument in Hawaii is helping astronomers see even further out than possible with the VLT. The Submillimeter Common User Bolometer Array (SCUBA) at the James Clerk Maxwell Telescope on Mauna Kea in Hawaii recently found a group of galaxies that are radiating in the far-infrared as much as or more than the whole optical universe. The image on p. 24, obtained in 51 hours observing over several nights, is circular because of the instrument`s shape. The orange areas represent background noise, and the white objects are the galaxies emitting the radiation. "It`s not a very-high-resolution instrument, which is why they look like blobs," said Amy Barger, an astronomer at the University of Hawaii who was part of the team that took the images.

These galaxies are very distant--although at present astronomers do not know exactly how far away they are. They formed early in the history of the universe, perhaps as few as 2 billion years after the Big Bang. High amounts of dust absorbed the light from the hot, young stars in these galaxies and reradiated it in the far-infrared. The expansion of the universe red-shifted the light even further, to wavelengths just less than a millimeter.

The SCUBA consists of two arrays of bolometers--supercooled devices that detect changes in electrical resistance caused by radiation. The Long-Wave array has 37 pixels operating in the 750- and 850-µm atmospheric transmission windows. It also has three additional pixels around the edge, operating at 1.1, 1.35, and 2 mm. The Short-Wave array has 91 pixels observing at 350 and 450 µm. Each has diffraction-limited resolution of 7.5 arcsec at 450 µm and 14 arcsec at 850 µm. A dilution refrigerator cools the detectors to about 100 mK. Single-mode conical feedhorns and narrow-band filters limit the background power.

"No one has ever been able to do major surveys in this particular wavelength band," Barger said. While her team surveyed a larger area, a British group used the same instrument to look at the Hubble Deep Field, a distant area imaged at optical wavelengths by the Hubble Space Telescope. Both groups reported their findings in Nature (July 16, pp. 241 and 248). Barger said the advantage of SCUBA is that, with its greater imaging area and high sensitivity, it can be used to map a typical area of the sky. "In the past, submillimeter detectors were all just single-element bolometers, so you could only target an object if you knew where it was," she said.

For more information, try these websites: www.gifso.org; www.jpl.nasa.gov/galileo/; www.jach.hawaii.gifdu/JCMT/home.html

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