ESA Science & Technology - INFO 37-1997: ISO proves that intergalactic space is dusty

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. Emissions from the intergalactic dust were picked up by the photometer ISOPHOT. A team of German, British, Spanish and Danish astronomers contributed this versatile set of detectors to ISO. The leader of the ISOPHOT team is Dietrich Lemke of the Max-Planck Institut für Astronomie (MPIA) in Heidelberg, Germany. "ISOPHOT in ISO is the only instrument in existence capable of making this detection" Lemke says. "The intergalactic dust is so cold that we need a very cold telescope to detect it. The strongest emissions from the dust are at a wavelength of 0.1-0.2 millimetre, which cannot be well observed from the Earth. ISO provides telescope in space cooled by superfluid helium to within 2 degrees of absolute zero. ISOPHOT is the instrument on ISO that measures infrared intensities at the longest wavelengths, up to 0.2 millimetre."

Faint emissions from cold intergalactic dust, peaking in the middle of the Coma Cluster of galaxies

ISOPHOT's advantages made finding the intergalactic dust possible, but not easy. The observations pushed instrumental sensitivity to the limit, and emissions from cold dust clouds in the Milky Way Galaxy confused the picture. The signal of intergalactic dust emerged clearly only after extensive data analysis.

Cold dust in a hot cluster

Our home Galaxy, the Milky Way, belongs to a very small group of galaxies. Intergalactic dust may very well be present nearby, but it is likely to be sparse and scattered. A team of astronomers, from MPIA Heidelberg and Helsinki Observatory, hoped that the intergalactic dust might be easier to recognise in a large cluster of galaxies. They chose the Coma Cluster, which fills an area of the sky twice as wide as the Full Moon even though it is about 450 million light-years away.

ISO scanned the Coma Cluster twice, along different cross-sections, measuring with ISOPHOT its emissions of long-wavelength infrared rays. The hunch of the German-Finnish team turned out to be correct. Emissions indicating the presence of intergalactic dust were much stronger towards the crowded centre of the cluster than at the edges.

The results on the Coma Cluster obtained with ISO seem to contradict, at first sight, observations of the same cluster by another space observatory built in Europe. The German-US-UK Rosat satellite for X-ray astronomy has charted X-rays coming from very hot gas between the galaxies, and concentrated towards the centre of the Coma Cluster. The intergalactic gas detected by Rosat has a temperature of 80 million degrees, far hotter even than the core of the Sun.

"The dust particles are at the very cold end of the temperature scale," says Kalevi Mattila of Helsinki Observatory, "ISOPHOT allows us to measure temperatures for them, in the range minus 220 to minus 250 degrees Celsius."

How can cold dust at minus 250 degrees survive within a very hot gas? The gas is extremely tenuous, so it cannot simply warm the grains of dust like a hair-drier. Instead the hot gas subjects the dust to impacts by energetic atomic particles which knock atoms out of it, and so gradually erode the dust grains. Calculations suggest that the hot gas will destroy the cold dust in about 100 million years. Although that is very slow by human standards, it represents only one-hundredth of the age of the galaxies. Experts therefore have to consider where fresh supplies of intergalactic dust come from.

Rosat astronomers found that the Coma Cluster is not spherical, which would be the shape expected in an isolated cluster. By X-rays, the cloud of hot intergalactic gas is seen to be egg-shaped. The same shape is apparent in the intergalactic dust cloud observed by ISO at long infrared wavelengths. A smaller cluster of galaxies is colliding with the large Coma Cluster and altering its shape.

"We think that the intergalactic dust in the Coma Cluster has been ejected from galaxies during the past 100 million years," comments Manfred Stickel of MPIA Heidelberg, "The two largest galaxies in the middle of the Coma Cluster do not show up in our infrared scan. They've lost their dust, either in collisions between galaxies or more probably in the merger of the Coma Cluster with another cluster. Fierce cosmic winds generated in such an event can blow the dust right out of the galaxies and into the surrounding space. That may be typical of the way in which intergalactic dust clouds arise, throughout the Universe."

The impact on cosmology

The dirt on the cosmic window-pane has to be slight, or astronomers would have detected it long ago. Small amounts of dust can nevertheless have a big effect. Within our own Milky Way Galaxy, dust comprises only 0.1 per cent of the visible matter, yet the centre of the Galaxy appears a thousand times less bright than it would do in the absence of dust.

The newly discovered intergalactic dust is too sparse to hide any galaxies entirely, in the manner of dark dust clouds in the Milky Way which blot out some individual stars. Intergalactic space remains so transparent to visible light that astronomers can observe galaxies and quasars across billions of light-years of space. The effect of intergalactic dust on our view of these distant objects is subtle.

Moderate amounts of intervening dust, within our own Galaxy, make many stars appear redder and dimmer than they really are. As a result, astronomers may underestimate a star's luminosity, or overestimate its distance. Similarly, comparisons among galaxies and quasars that rely on their relative brightnesses may be faulty, if there is significant intervening dust. Discrepancies in the counts and colours of galaxies and quasars, near and far, led some cosmologists to imagine that intergalactic dust might be casting a shadow over the cosmic scene.

ISO's discovery confirms their suspicion. In the continuing efforts to measure the size and age of the Universe, and to study the evolution of galaxies, astronomers will have to allow for the dimming due to intergalactic dust. Galaxies much younger than the Milky Way, seen at great distances, are possibly more luminous than they look.