ESA Science & Technology - INFO 09-1998: New water and remote galaxies complete ISO's observations

Thanks to superb engineering by European industry, which built the spacecraft and its super-cool telescope, ISO has given astronomers almost a year longer than that. During the extra time the count of ISO's observations of cosmic objects has risen from 16,000 to about 26 000. Among the benefits of ISO's longevity has been the chance to examine an important region of the sky, in and around the constellation of Orion. This was not accessible in the nominal mission but has now been observed in two periods.

Four international teams, supported by national funding agencies, supplied the instruments to analyse the infrared rays received by ISO's telescope. The principal investigators leading the teams are Dietrich Lemke (Heidelberg, Germany) for the versatile photometer ISOPHOT, Catherine Cesarsky (Saclay, France) for the camera ISOCAM, Thijs de Graauw (Groningen, the Netherlands) for the Short Wavelength Spectrometer SWS, and Peter Clegg (London, UK) for the Long Wavelength Spectrometer LWS.

Water vapour on Titan

A big difference between ISO and the only previous infrared astronomy satellite (IRAS 1983) has been its ability to examine individual objects across a wide range of accurately defined infrared wavelengths. Many spectra showing patterns of intensities at the different wavelengths have enabled astronomers to deduce the presence of diverse materials in interstellar space, in the surroundings of stars, and in other galaxies.

As previously reported, ISO has identified stony materials, tarry compounds of carbon, and vapours and ices like water and carbon monoxide. Together they give the first clear picture of how Mother Nature prepares, from elements manufactured in stars, the ingredients needed for planets and for life itself.

Particularly striking for the human imagination are ISO's repeated discoveries of water in the deserts of space. They encourage expectations of life elsewhere in the Universe. Water has turned up around dying stars, newborn stars, in the general interstellar medium, in the atmospheres of the outer planets and in other galaxies too. A link to the Earth's oceans and the water we live by comes in the water- ice long known to be a major ingredient of comets, which are relics from the era of planet-building.

A further link to the investigation of the origin of life is the apparent detection of water vapour in the mysterious atmosphere of Saturn's largest moon, Titan. A preliminary announcement comes from an international team headed by Athena Coustenis of Paris Observatory and Alberto Salama of the ISO Science Operations Center at Villafranca.

The team used ISO's Short Wavelength Spectrometer during several hours of observations last December, when Titan was at its farthest from Saturn as seen by ISO. Emissions at wavelengths of 39 and 44 microns showed up, as an expected signature of water vapour. The news will excite the scientists involved in ESA's probe Huygens, launched last year aboard NASA's Cassini spacecraft. It will parachute into Titan's atmosphere to see what the chemistry of the Earth may have been like before life began.

"Water vapour makes Titan much richer," comments Athena Coustenis. "We knew there was carbon monoxide and carbon dioxide in Titan's atmosphere, so we expected water vapour too. Now that we believe we've found it, we can expect to better understand the organic chemistry taking place on Titan and also the sources of oxygen in the Saturnian System. After ISO, the Huygens probe will reveal the actual degree of complexity in a mixture of elaborate organic molecules closely resembling the chemical soup on the young Earth."

Balllet corps of Young Stars

Infrared images of the spectacular Orion star-forming regions, at a distance of approximately 1500 light years, are bonuses from ISO's extended life. In the Horsehead Nebula, visible light shows a large dark dust cloud from which a black wisp shaped like a horse's head protrudes into a luminous cloud of gas. When seen by ISO's camera ISOCAM, dense parts of the dusty region appear as shiny filaments and the horse's head almost disappears. Young stars are detected in the horse's forehead and in the nearby nebula NGC 2023.

Other well-known nebulae in the Orion region include NGC 2068 and NGC 2071. Emission by carbon compounds (polycyclic aromatic hydrocarbons or PAH) makes the infrared nebulae spectacular, as seen by ISOCAM. And thanks to ISOCAM's sensitivity and the ability of infrared rays to penetrate a dust cloud better than visible light, ballet corps of young stars appear on the stage, seen as spots in the centre of these two nebulae. This is not surprising, because the dense, dusty regions called molecular clouds are often the breeding grounds of new stars, but ISOCAM detects fainter and more obscured objects.

"We have used ISOCAM to make a census of families of young stars," comment Lennart Nordh and Goran Olofsson of Stockholm University, who lead a team of astronomers from Sweden, France, Italy, the UK and ESA. "By comparing the intensities of the point-like objects at different infrared wavelengths we can efficiently identify the ensemble of young stars still embedded in its parental molecular clouds."

From their study of ISO's early observations of four star-forming clouds, the astronomers report the detection of small stars. "Almost 300 young stars have been identified to date, many of which were previously not recognized," Nordh and Olofsson say. "Most of the latter objects have luminosities 10-100 times lower than revealed by earlier observations. Our preliminary analysis indicates that at least ten per cent of the embedded young stars will become small brown dwarfs, or ownerless super-planets, less than one-tenth of the mass of the Sun."

Colliding galaxies

Some galaxies are unusually bright in the infrared because of cosmic traffic accidents that bring them into collision with other galaxies. The result is a frenzy of star formation called a starburst. The explosion of short-lived stars then creates a pall of warm dust which ISO observes in the infrared. The relative intensities of different wavelengths enable astronomers to distinguish starburst events from other sources of strong infrared rays, such as the environment of a black hole in the nucleus of a galaxy. Collisions and starbursts play an important part in the evolution of galaxies.

A famous pair of colliding galaxies called the Antennae was one of the first objects to be examined by ISO. Continuing study of the Antennae over the past two years has revealed a clear picture of a starburst occurring exactly where the dense disks of the galaxies intersect. The nuclei of the two galaxies are plainly distinguished too.

Centaurus A is a galaxy that first attracted the attention of astronomers by its strong of radio emissions. In its visible appearance, a large, round (elliptical) galaxy has a dark band across its face. This too turns out to be the result of a galactic collision. The dark band is a flat, disk-shaped galaxy seen almost edge-on. Centaurus A is the nearest case of a phenomenon seen elsewhere by ISO, in which a flat galaxy has merged with an elliptical galaxy while preserving its flat configuration.

ISOCAM gives an image of Centaurus A in which the disk galaxy is the more conspicuous object. The orientation of the disk becomes clear. It is at right angles to the axis of the radio-emitting regions, which are powered by jets of electrons driven by a black hole in the centre of the galaxy. Excited emissions detected by ISO's Short Wavelength Spectrometer also indicate the presence of an active black hole.

"Centaurus A is an example of ISO's magic," says Catherine Cesarsky of CEA Saclay in France, leader of the ISOCAM instrument team. "It transforms opaque clouds seen by visible light into glowing scenes in the infrared. The same thing happens in dust clouds hiding newborn stars, and on a huge scale in dusty starburst galaxies - which become infrared beacons lighting our way deep into the Universe."

Distant Galaxies seen through the Holes in the Sky

When ISO was launched, one of the hopes for the space observatory was that it would detect galaxies made luminous by starburst events, or by black-hole activity, very far away in space and therefore far back in time. Dust in our own Milky Way Galaxy usually obscures the remotest and faintest galaxies. But when they look northwards and southwards, at right-angles to the disk of the Milky Way, astronomers find holes in the dust clouds through which distant galaxies are discernible.

Both for ISO and the Hubble Space Telescope these holes have been special targets for observations with long exposures, to reveal faint galaxies. ISOCAM results through the northern hole, by a Japanese-led team, were reported last year in an ESA Information Note (25.97) and a picture release (ESA/ISO 97:8/1). They revealed many infrared-luminous galaxies billions of light- years away, from an era corresponding with about half the present age of the Universe. Even more distant and earlier galaxies may be present in ISO's observations, including some objects not yet seen by visible light.

Results released at the London press briefing on ISO include "deep field" examinations by groups of astronomers led by Catherine Cesarsky of CEA Saclay and Michael Rowan Robinson of Imperial College, London, analysing the northern and southern images respectively. In the northern deep field, when ISOCAM observations are superimposed on a Hubble picture of the same region, they pick out spiral galaxies experiencing starbursts. A different signature comes from large elliptical galaxies whose visible light has been shifted into the infrared by the expansion of the Universe. The astronomers estimate that some of the objects seen by ISOCAM are so far away that the Universe was only one-third of its present age when they emitted the radiation seen today.

The first ISO images from the opposite direction in the sky, in the southern deep field, show similar objects, again at great distances. A preliminary analysis indicates the presence of of 30-40 remote galaxies seen at a wavelength of 7 microns and 22-30 at 15 microns. One interesting source, bright in the infrared, is not seen by visible light even in a prolonged examination by the CTIO 4-metre telescope in Chile (A. Walker). Astronomers suspect that this object is undergoing an especially violent period of star formation. The interpretation can be checked when Hubble and other telescopes have a chance to examine this scene.

Besides illuminating the evolution of the galaxies, ISO's deep field results are encouraging for scientists planning another of ESA's astronomical space projects, FIRST. Its longer wavelengths will penetrate even deeper into the unknown.

Non-stop discoveries

The extended life was not the only outcome that made ISO a triumph for ESA, European industry and those responsible for its operations. The pointing accuracy of the telescope turned out to be ten times better than required in the specification and its jitter was one-fifth of what was considered tolerable. Stray light in the optical system was too small to measure. The scheduling systems achieved science observations for 90-95 per cent of the available time. Much of the rest of the time, when ISO was turning to new targets, was spent in mapping parts of the sky at a wavelength of 200 microns.

Activity concerning ISO will continue at the Villafranca ground station until the year 2001, long after the completion of the observational phase of the mission. During the space operations, the main objective was to make as many observations as possible. Thorough analysis and interpretation of the results will take several years.

"We still have plenty to do," says Martin Kessler, ESA's project scientist for ISO, "Our team at Villafranca is preparing a complete archive of ISO data on 500-1000 compact disks, after reprocessing with improved software. We'll release part of this archive to the world-wide astronomical community in the autumn of this year, and the rest in 1999. We shall also advise the astronomers who have used ISO, about the particular requirements for handling the data from each instrument, and we'll be doing some astronomy ourselves. There are far more results still to come from ISO."

Europe's infrared astronomers are already busy preparing ESA's FIRST and Planck missions, due for launch early in the new century. FIRST will observe long infrared wavelengths in the sub-millimetre range, while Planck will map the cosmic microwave background far more accurately than NASA's COBE mission did, to reveal the clumps of matter from which galaxies evolved. Also under study by ESA is a possible interferometer mission using a combination of infrared telescopes. In principle it might observe and characterize planets in orbit around other stars.

Meanwhile, Europe's space astronomy programme continues apace in other directions. ESA's participation in the Hubble Space Telescope and its eventual successor assures access to those important instruments for Europe's astronomers. The release in 1997 of the catalogues from ESA's unique star-mapping mission Hipparcos provided all astronomer with amazingly precise data for sizing up the stars and the wider Universe. Next year will see the launch of ESA's XMM satellite to observe X-rays from the Universe with the most ingenious and sensitive X-ray telescopes ever made. It will be followed by Integral in 2001, which will investigate cosmic gamma-rays with clever imaging devices called coded masks, and ultra-sensitive detectors.

"Our aim in space astronomy is that every ESA mission should be the best in the world at the time of its launch," says Roger Bonnet, ESA's director of science, "ISO is a shining example. It has revolutionized infrared astronomy. It has given us wonderful insights into cool and hidden places in the Universe, and into the origins of water and other materials to which we owe our very existence. A mission of this scale and complexity was feasible for Europe only through the multinational collaboration coordinated by ESA."