Dust Amid The Galaxies
ESA's Infrared Space Observatory ISO has detected dust for the first time in the apparently empty space between the galaxies. German and Finnish astronomers made the discovery in the northern constellation of Coma Berenices, where more than 500 galaxies swarm together in the Coma Cluster. The intergalactic dust is concentrated towards the centre of the cluster.
In the past, astronomers have considered intergalactic space to be extremely clean. Except in the vast concentrations of stars, gas and dust that make up the galaxies themselves, the Universe was supposed to be filled only by very thin traces of invisible gas. ISO's detection of dust means that the Universe is less transparent than astronomers have assumed. Their cosmic window-pane is slightly dirty and large-scale inferences based on the brightnesses of distant galaxies and quasars may be affected.
ISO reveals the energy source of luminous galaxies
Observations with ISO have revealed that the most famous of the enigmatic ultraluminous infrared galaxies (Arp220) is powered by an outburst of star formation, not by a central black hole. The same is true for all but one similar galaxies observed so far. Ultraluminous infrared galaxies (ULIRGs), the most luminous objects in the local universe, were detected in the 1980s by their prodigious emission of infrared radiation, but the origin of their enormous luminosities has yet remained unclear. The hopes of many astronomers were hence put on ISO, which allowed to study in unprecedented detail the infrared radiation, having wavelengths longer than visible light.
Two galaxies colliding
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 ability to harvest and analyse infrared rays, ISO distinguishes between the different kinds of commotion provoked by the encounter.
A population explosion of stars, long ago
"One of the major ISO results comes from the deep surveys," said Catherine Cesarsky of CEA-Saclay, France. "They show that most of the star formation in the universe has been so far hidden by the dust".
Looking far into space also means looking far back in time. ISO's deep surveys detected the faintest and farthest objects ever seen at infrared wavelengths. The results reveal an early universe much more violent than expected, with three to four times more stars being formed than the rate inferred from observations by visible light. ISO reached the epoch when the Universe was about one third of its present age. It discovered new populations of infrared galaxies producing stars 100 times faster than our own Galaxy, the Milky Way.
"The intense stages of star formation are often associated with merging and interacting galaxies," said Jean-Loup Puget of the Institut d'Astrophysique Spatiale, near Paris. "This process, considered crucial in the evolution of galaxies, can now be traced to the past through infrared observations."
Thus a key aspect of ISO's legacy is the news that infrared space telescopes will help to solve the problem of how galaxies were born and grew up. Here is a major goal for FIRST, the Far Infrared and Submillimetre Telescope due to be launched by ESA in 2007.
ISO opens its gold mine of data. Many more astronomical surprises are still hidden within nearly 30 000 observations obtained by ISO. They are now being made accessible to all scientists via the archive at the ISO Data Centre (IDC) in Villafranca, Spain. Astronomers can request the data easily through the IDC website, and ask for expert support in Villafranca or any of the National Data Centres in Groningen (Netherlands); Garching and Heidelberg (Germany); Orsay-Saclay (France); Rutherford (UK); and Pasadena, California.
Frenzied star-making in infrared galaxies
The Antennae, 60 million light-years away, are galaxies in collision. The encounter has triggered star formation within dense dust clouds, transparent to ISO's infrared eye. Laurent Vigroux and Felix Mirabel of CEA-Saclay, France, discovered that the most active star-forming regions in the Antennae are visible only in the infrared. They went undetected before ISO.
What struck astronomers, when they first saw the whole infrared sky with the IRAS satellite in 1983, was that the most luminous objects were certain galaxies that looked faint by visible light. They were very dusty, so that starlight was dimmed while intense infrared emission came from cool dust. Needing a name, astronomers called them 'ULIRGs', meaning ultra-luminous infrared galaxies. But the experts could not agree about their nature - until ISO settled the issue.
The strange beasts are scenes of collision and merger between two galaxies. Some astronomers imagined that the collision created the ULIRG by activating a giant black hole. Others thought that the shock of the collision provoked a frenzy of star-making, called a starburst.
Are the ultra-luminous objects hideouts for monsters or nests for new-born stars? After analysing as many as 60 ULIRGs with ISO, a team led by Reinhard Genzel and Dieter Lutz of the Max-Planck-Institut für Extraterrestrische Physik at Garching, Germany, favoured the latter option. Penetrating the dust enshrouding the galaxies they found, not evidence for black-hole activity, but characteristic chemical signatures of starbursts.
"This is the first time we can prove that most if not all of the luminosity in the ULIRGs comes from star formation," Reinhard Genzel said. "To understand how, and for how long, such vigorous star formation can occur in these galaxies is now one of the most interesting questions in astrophysics".
Amazing rings in the nearby Andromeda Galaxy, M31, are among the surprises from ISO. Instead of the typical starry spiral seen by visible light, ISO shows several rings of very cold dust, at about minus 260 °C, invisible to optical telescopes.
"Seen in the far infrared, M31 is a multiple ringed galaxy rather than the classical spiral type found in the optical," noted Martin Haas of the Max-Planck-Institut für Astronomie, Heidelberg, Germany.