Most of the dust seen by Planck is very cold, at temperatures
as low as around −260 C, and as a result glows in
Planck's high frequencies (particularly 353 GHz and above). It
is cold, dense clumps of dust and gas that mark the earliest
stages of the birth of stars.
Although the dust is
concentrated in the disc of our Galaxy, Planck has mapped its
distribution all over the sky, showing structure at high
Galactic latitudes (looking up or down out of the plane) of
the Galaxy. Many other galaxies also contain similar dust, but
most of them have been removed from the map of dust shown
The emission from the dust seen by Planck must be
removed from the maps before cosmological analysis, but is of
great interested to astronomers studying star formation in our
own Milky Way.
The "low frequency" foregrounds include a number of types
of emission. Most of it is due to high energy electrons
moving through the Galactic magnetic field (a phenomenon
called "synchtrotron emission", and the rest is
predominantly the emission from slower electrons moving
through hot, ionised gas (a phenomenon called
"Bremsstrahlung" or "free-free" emission). These phenomena
cause strong emission at Planck's lowest frequencies
(particularly 70 GHz and below). Most of the emission if
located along the plane of our Galaxy, with the other bright
spots primarily being galaxies beyond our own.
fraction of the microwave light is not accounted for by
either of these methods, and is called "anomalous microwave
emission". Planck has previously given a great deal of
weight to the favoured theory, in which this enomalous
emission is from tiny particles of dust spinning billions of
times per second.
All of this emission is removed from
the maps before the cosmological analysis, but it is of
great interest to astronomers investigating the properties
of our own Galaxy. With more analysis, and the inclusion of
more information from other experiments, it should be
possible to separate this single "foreground" into its
Most of the cold gas in our Galaxy is made of hydrogen,
which only shines brightly at the frequencies seen by Planck
when it is very hot. However, there are other "tracer" gases
that do emit light at appropriate frequencies, such as
carbon monoxide (CO). By carefully looking for the signs of
emission by CO at 115, 230 and 345 GHz, Planck has produced
the best all-sky map of this gas throughout our
The carbon monoxide is, as expected, largely
concentrated in the Galactic Plane, though there are a
number of molecular clouds which shine brightly at these
frequencies. Planck's sensititivity has allowed it to detect
some very diffuse clouds that had not been seen
The dark regions near the ecliptic poles are
because Planck's maps are more sensitive in these regions,
reducing the noise, or "graininess", in the images. The
emission from carbon monoxide must be removed from Planck's
maps before the comsological analysis, since any
contamination could skew the results.
To do the cosmological analysis, any "contamination" from
sources of light other than the CMB (primarily our Galaxy and
others) must be removed. Although the Planck data can be used
to make maps of the various other sources of light and correct
for it, this is much more difficult to do in the centre of our
Galaxy, and near the brightest sources of light around the
rest of the sky. The "sky mask" shows the areas of sky that
are not considered when doing the cosmological analysis, which
accounts for just over half the sky.
As well as the light from the Cosmic Microwave Background, and from diffuse dust annd gas in our own Galaxy, Planck also sees seemingly small objects dotted all over the sky. Some of these really are small, such as clumps of gas and dust in the process of forming stars. But others are simply very far away, such as other galaxies.
The map of sources shown here is representative of the full range of sources, showing those seen at frequencies of 30GHz (red), 143 GHz (green) and 857 GHz (blue). The size of the dots is a measure of Planck's resolution at the different frequencies (the resolution is better at higher frequencies).
The 30GHz sources are distributed over the whole sky, indicating that they are generally distant galaxies. The galaxies seen by Planck at this frequency are emitting large amounts of radio waves, often due to the activity of the central black hole.
By contrast, the sources seen at 857 GHz are concentrated in the plane of our Galaxy, with a few in the Large and Small Magellanic Clouds and the Andromeda Galaxy (add the "Labels" to see where these are. These are clumps of dust, either in our Galaxy or in nearby ones.
To do the comological analysis, most of these source have to be taken into account, either by ignoring that relevant patch of the sky (see the "Sky Mask") or by carefully removing the light associated with each source from the data before the full analysis is performed.
As the light from the CMB travelled through the Universe
over the course of 13.8 billion years, most of it has been
completely unimpeded. But there are very subtle effects caused
by concentrations of mass in the Universe - most of which is
in the form of mysterious dark matter. The gravitational pull
of the mass distorts the image of the CMB, and these
distortions can be mapped over the sky. These distortions are
shown here, with large concentrations of mass showing up
The analysis depends on the assumptions about the patterns in
the Cosmic Microwave Background itself, namely that on average
all the hot-spots and cold-spots are circular. While that is
true when averaging over the whole sky, it might not be the
case when averaging over smaller regions, and some of the
distortions are due to inherent variations in the original CMB
light. There are also regions, particlarly in the Galactic
Plane, but also around a few other objects, where the
foregound light is so strong that it is not possible to
calculate the mass distribution.