To answer these questions Dr Richard J. Tuffs, Dr. Cristina C. Popescu, Dr. D. Pierini and Prof. Heinrich J. Völk of the Astrophysics department of the Max-Planck-Institute for Nuclear Physics in Heidelberg, in cooperation with scientists from foreign institutes, have measured the infrared brightness of spiral and dwarf galaxies in the Virgo cluster.

They discovered surprisingly large quantities of cold dust, typically ten times that previously known (The Astrophysical Journal, March 2002).

Furthermore, estimates of the total amount of light emitted by the stars in the galaxies must now be revised sharply upwards, since the same observations show that up to 50 percent of the visible and ultraviolet light is being transformed into infrared radiation undetectable with human eyes.

The possible effects of dust on the optical appearance of galaxies have preoccupied astronomers ever since Edwin Hubble realised 70 years ago that our Milky Way is just one galaxy amongst many in the universe.

Direct observations of dust are needed to understand how much starlight is being released into the universe, what its colour is, and, therefore, what sort of stars were being formed through history in galaxies like our own Milky Way.

They pointed ESA's Infrared Space Observatory (ISO) at 63 spiral and dwarf galaxies in the nearby Virgo cluster, carefully chosen to be representative of the overall population of galaxies in the local universe. 

Making use of ISO's unprecedented sensitivity at very long infrared wavelengths, this work represents the first statistical "census" of the total infrared energy output from "normal" galaxies.

An example is shown in the picture, where the infrared emission from the spiral galaxy NGC4178 is depicted as contours overlaid on an image of the optical light. Remarkably, the temperature of the cold dust measured in the Virgo galaxies was found to range down to just ten degrees above absolute zero (-263 degrees Centigrade).

As a consequence, astronomers are having to revise upwards by a factor of typically 10 the weight of dust in these galaxies compared to previous measurements. 

The data also show that up to half the total energy output of stars in normal galaxies has been converted from visible optical into infrared photons, much more than previously suspected, necessitating an upwards revision in the total amount of emitted starlight.

These are so called because when viewed with an optical telescope they are smaller than a tenth of the size of our own galaxy, and dominated by newly born massive blue stars, each shining up to 10,000 times more brightly that the sun.

It had previously been supposed that dust exposed to this intense light should be very warm.
The ISO observations, however, revealed huge amounts of very cold dust, in fact the coldest found from all the observed galaxies in the Virgo cluster.

They believe that "the infrared eyes of ISO may be tracing a dusty mixture of intergalactic gas, which the galaxies are still accumulating, leading to their further evolution."