A Dusty Haze around Blue Compact Dwarf Galaxies
Max-Planck scientists discover large amounts of ultra-cold cosmic dust in the Virgo Cluster / Up to 50 percent of stellar light in galaxies is transformed into infrared light.
Just
as dust in the earth’s atmosphere causes the setting sun to dim and
glow red, so dust in galaxies shows its presence by preferentially blocking
out the blue components of light emitted from stars. But just how much
dust is in galaxies, and how much of the starlight is blocked out? 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. On
top of this, analysis of the infrared emission indicates that some blue
compact dwarf galaxies are surrounded by huge conglomerations of cosmic
dust, quite possibly tracing intergalactic gas streaming down into the
galaxies and giving rise to new generations of stars.
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.
The team
of Max-Planck scientists tackled this issue by measuring the brightness
of galaxies in the invisible infrared spectrum of light expected from
the dust particles blocking the visible starlight. 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.
Unexpectedly
large amounts of infrared emission were discovered from very cold dust
particles in almost all the galaxies measured. 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.
Figure:
The
Virgo cluster spiral galaxy NGC4178. The colors show the brightness
of the optical light from stars in this galaxy, ranging from blue (faint)
through green and yellow and red to white (bright). The contours are
lines of equal infrared brightness from dust, as observed using ISO
within the rectangular box.
Photo:
Max-Planck-Institute for Nuclear Physics
But the
biggest surprise was caused by the so-called Blue Compact Dwarf galaxies
in the Virgo cluster. 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. The scientists propose that this dust is so cold because it
surrounds the galaxy, far away from the stars. 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."
Original
publications:
Tuffs,
R.J., Popescu, C.C., Pierini, D., Völk, H.J., Hippelein, H., Leech,
K., Metcalfe, L, Heinrichsen, I. & Xu, C.:
"Far-Infrared Photometry of a Statistical Sample of Late-Type Virgo
Cluster Galaxies"
The Astrophysical Journal Supplement Series, vol. 139, p37-79,
March 15th 2002.
Popescu,
C.C., Tuffs, R.J., Völk, H.J., Pierini, D. & Madore, B.F.:
"Cold Dust in Late-Type Virgo Cluster Galaxies"
The Astrophysical Journal, vol 567, p221-236, March 1st 2002
