ESA Science & Technology - INFO 02-1996: ISO unmasks colliding galaxies

Collisions between galaxies, each containing billions of stars, are the biggest events since the origin of the Universe. They may occur often enough to play a major role in the evolution of galaxies, and they provide a major theme for ISO's research teams. Collisions trigger star formation within dense dust clouds. These are opaque to visible light, so that even the Hubble Space Telescope is blind to the events within them. One target for ISO's Camera was a pair of galaxies known as the Antennae, 60 million light- years away. The name comes from antenna-like streamers of stars torn from the galaxies by a collision. The galaxies look quite similar by visible light. With its unprecedented ability to harvest and analyse infrared rays, ISO distinguishes different kinds of commotion provoked by the encounter.

Credit: ESA/ISO, CEA Saclay and ISOCAM Consortium

To ISO's penetrating infrared eye, one of the Antennae galaxies shows a large ring of intense starmaking around the central nucleus. This feature is absent in the other galaxy. Another region of star formation extends along a line marking the overlap of the disks of the two galaxies, where the collision is fiercest. ISO's Camera has also observed merging galaxies 230 million light-years away, known as Arp 220. Here the intense infrared emission is concentrated in such a small region that astronomers suspect a possible interaction with a giant black hole.

At a long infrared wavelength, ISO's Photometer has measured the temperature of the dust in a pair of colliding galaxies, NGC 6090. The result is minus 250 degrees Celsius. Astronomers estimate the rate of starmaking in NGC 6090 at 25 sun-like stars created every year in NGC 6090, compared with two or three a year in the Milky Way.

Credit: ESA/ISO, CEA Saclay and ISOCAM Consortium

Prof. Catherine Cesarsky, from Saclay in France, is Principal Investigator for ISO's Camera. She comments: "What excites me most about ISO is that, with our sensitivity, we can look for starbursts in very young galaxies and trace the history of galaxies right through to the present era. Only by studying other galaxies can we fully understand our own Galaxy, the Milky Way, and how it created the conditions for life."

The Birth of Stars

ISO also pursues starmaking processes within the Milky Way Galaxy. Penetrating the dusty clouds where stars are born, infrared rays can reveal protostars. These are strongly radiating cocoons of dust that surround new stars created by gravity squeezing the cloud. The spectrometers in ISO detect fingerprints of constituents of the clouds, by their emission or absorption of infrared rays at precisely defined wavelengths.

ISO's Short-Wavelength Spectrometer team tested their instrument on a dusty cloud called GL2591, which envelops a newly forming massive star. The astronomers have identified molecules in the solid state as ices, some of them never seen in space before, including hydrogen cyanide ice. Dr. Thijs de Graauw of Groningen in the Netherlands is Principal Investigator for ISO's Short-Wavelength Spectrometer. "We want to know how the Sun and the Earth came into existence," Dr. de Graauw says, "and ISO is giving us wonderful new information. Our task with our high-resolution spectrometer is to analyse temperatures, pressures and motions within the nests where the stars hatch. Molecules and dust observations provide us with diagnostic tools."

The death of stars

The ability of ESA's new space telescope to see invisible objects is graphically illustrated by an exploded star observed by ISO's Photometer. In a part of the sky where ordinary telescopes see only a few normal stars, this versatile instrument has mapped an object one quarter of the width of the Full Moon, glowing brightly in the infrared. The radiation comes from debris travelling outwards at 3000 kilometres per second from the scene of the explosion, which occurred about 1000 years ago. Called MSH 11-54, the object has also been imaged by other space astronomers, using X-ray telescopes such as Exosat. It lies 10 00 light-years from the Earth in the constellation of the Southern Cross.

Credit: ESA/ISO, MPIA Heidelberg and ISOPHOT Consortium

"We owe our existence to the corpses of stars," remarks Prof. Dietrich Lemke of Heidelberg, Germany, who is Principal Investigator for ISO's Photometer. "Our bones, flesh and the iron that colours blood are built from atoms made in stars that expired before the Sun and the Earth were formed. In our studies of supernova remnants we expect to identify many atoms, molecules and dust particles. And we are looking out for diamonds, which may be scattered into space by the stellar explosions."

Credits: ESA/ISO, CEA Saclay and ISOCAM Consortium

A galactic refrigerator

The Long-Wavelength Spectrometer operates over a range of the infrared spectrum that is entirely blotted out by the Earth's atmosphere. Instruments in high-flying aircraft and balloons have glimpsed some features in the sky at long infrared wavelengths, but only ISO offers a clear view. As a result the Long-Wavelength Spectrometer has been able to observe, in a wide variety of cosmic settings, radiation that plays an important part in cooling the Milky Way Galaxy.

Emissions at a wavelength of 158 microns (about a sixth of millimetre) come from ionized carbon atoms in the clouds that fill the spaces between stars. The Long-Wavelength Spectrometer has picked up the emissions from a region where stars are being born, from a dying star, from warm and cool clouds of dust and gas, and even from extremely cold clouds of hydrocarbons called infrared cirrus. Although completely invisible to the eye, the energy shed in this way may be equivalent to the light of a billion stars. It affects the energy budget - and therefore the temperature - of the Milky Way, and of other galaxies too.

"Spectroscopy is an essential tool for investigating the physical and chemical conditions in astronomical objects," says Prof. Peter Clegg from London, Principal Investigator of ISO's Long-Wavelength Spectrometer. "ISO has now opened a new spectroscopic window on the cosmos that has already shed new light on the physics of planets, young and old stars, and galaxies. The next eighteen months are likely to transform our view of the universe."

An extended lifetime

The reason why ISO now enjoys an undisputed command of the infrared sky is that an infrared space telescope is difficult to build. It has to be colder than the cosmic objects it observes, and that means operating within a couple of degrees of absolute zero, below minus 271 degree Celsius. After the IRAS mission sketched an infrared view of the Universe in 1983, ESA was the first space agency to decide, in that same year, that infrared astronomy merited the heroic engineering required to build such a cold observatory and fly it in space. Instruments were conceived, and then industrial work began in 1986.

A large tank of superfluid helium surrounds the telescope and its instruments. To maintain the low temperature, the helium slowly evaporates. Eventually it will run out. The specification required ISO to operate for 18 months. The latest estimate its final operating temperature, is that this will be exceeded by about 6 months and astronomical work should therefore continue until November 1997. All systems in the spacecraft are working well, and ISO is a triumph for the European industrial teams that created it.

Once a day, ISO follows an elongated orbit around the Earth. It makes astronomical observations for 16 hours out of the 24, when it is clear of the radiation belts surrounding the planet. ESA's ground station at Villafranca is in direct communication with ISO during much of that time. NASA's station at Goldstone, California, covers the time when the satellite is below the Spanish horizon.

"From both the scientific and the engineering points of view, ISO is a big success for ESA," says Dr Martin Kessler, the mission's Project Scientist, "Until now, we have been verifying the instruments. The astronomy has started in earnest only this month. It involves about 500 teams from Europe, the USA and Japan, and more than a thousand astronomers. We will have much more to report at a meeting of astronomers at ESTEC in the Netherlands at the end of May."