Q: What is this dust you are talking about?
A: When you go to a dark place and look up at the sky, you will often
see our galaxy, the Milky Way, stretching across the sky from horizon
to horizon. Against this background of billions of stars, dark areas
can be seen - streaks with no stars. Actually, there are
stars there, but their light is blocked by interstellar dust.
The space between the stars is not empty; it is filled with a very
diffuse mixture of dust and gas. The gas is mostly hydrogen and
helium, and can be neutral or ionized. In dense clouds of gas,
molecules form, including H2, CO, HCN, NH3,
CH3 and larger hydrocarbons. In addition to this gas there
are dust grains, mostly made of graphite, silicates, and PAHs
(Polycyclic Aromatic Hydrocarbons). The dust absorbs and scatters
optical light, and emits FIR (Far Infra-Red) radiation with a
wavelength of about 100-250 microns. Because of this, the
interstellar dust interferes with nearly every kind of measurement
astronomers make.
Q: How does interstellar dust interfere with astronomical
measurements?
A: The dust affects optical light in two ways: extinction and
reddening. Extinction is the loss of light due to scattering and
absorption as it travels through clouds of dust. Because the dust
scatters blue light more than red, the color of the light also changes
- an effect known as reddening. This effect is closely related to the
scattering in our atmosphere that causes the sky to be blue and
sunsets to be red. So, when astronomers measure distant stars,
galaxies, supernovae, etc., they must correct the color and amount of
light they measure for the amount of dust it has passed
through.
Q: Does the emission in the Far Infra-Red interfere with astronomical
measurements also?
A: Yes. One of the most important astronomical achievements of the
1990s is the discovery of anisotropy in the Cosmic Microwave
Background Radiation (CMBR). The CMBR was emitted when the
Universe was very young, before any galaxies or stars existed. This
microwave radiation has traveled vast distances over billions of
years, and measurements of it give us important information about the
formation of the universe. However, data from much of the sky is
difficult to interpret, because the dust in our Galaxy contributes to
the radiation.
Recent work by several groups has shown surprisingly large
correlations between dust and CMBR. Thermal emission from the dust
itself is not enough to explain the correlation between microwave
signals and our dust map. In fact, some scientists believe that
rapidly spinning dust grains may emit microwaves if they have an
electric charge, and that this is sufficient to explain the
dust-microwave correlation. At the moment, this is still a
mystery.
Q: How do you measure the dust?
A: In addition to
microwave measurements, the COBE satellite also made maps of the sky
in the FIR with an instrument called DIRBE (Diffuse Infra-Red
Background Experiment). We also use a map made by the IRAS satellite
in 1983. This map has a very good spatial resolution, but its
relatively poor calibration has prevented widespread use of dust maps
based on IRAS data alone. Because the dust emits energy in the FIR
wave bands observed by DIRBE and IRAS, we are able to map the dust.
Q: What is so hard about that?
A: Well, one problem is the zodiacal light. This is light emitted by
dust grains in our solar system, called IPD (Interplanetary Dust)
which is much closer than the
interstellar dust we are interested in. Even though there is a very
small amount of IPD, it is hot compared to the dust we are
interested in, and radiates profusely at the wavelengths we observe.
Therefore, careful modeling and removal of the zodiacal light from
the IPD is required before the DIRBE maps are useful as dust maps.
The temperature of the dust also varies from one cloud to the next, so
we make a temperature correction based on the data from different
DIRBE channels. These are just a few of the many complications.
Q: How do you test your map?
A: One test we use is based on a relation between the magnesium
content of stars in a galaxy, and its color. As mentioned above, the
color of a galaxy depends on how much dust is in front of it, but the
magnesium line measurements do not. Therefore, a perfect dust correction
will tighten the scatter of the magnesium-color relationship, and the
error in the relation will not depend on the amount of dust. Our dust
correction is not perfect, but it passes this test better than any
existing dust correction.
Q: What other dust corrections are available?
A: The most popular is the Burstein-Heiles correction (Burstein &
Heiles, 1982) which is based on emission from neutral atomic hydrogen
(denoted HI). Although the BH correction has
been used almost exclusively for 15 years, it has become somewhat
dated. Recent radio surveys have produced HI
maps of greater sensitivity and better spatial resolution, and the FIR
maps we use are of greater resolution still. Another problem is that
HI is gas, not dust. The gas is highly
correlated with dust, it is better to measure the emission
from the dust itself.
Another dust map was introduced in 1991 by Rowan-Robinson based on
IRAS data, but due to problems with the zodiacal light model and lack
of a well-calibrated baseline like DIRBE, this map is not widely used.
Q: Can I look at the dust map?
A: Unless you are a professional astronomer, you will have no use for
the actual map. You can get a feel for what it looks like by
selecting the "Postscript and JPEGs" option on the main menu.