and Cosmology

2.3 The Structure of the Galaxy
B-stars and HII regions. 7 This is the reason why spiral
arms appear blue. Obviously, star formation in our
Milky Way takes place mainly in the spiral arms. Here,
the molecular clouds – gas clouds which are sufficiently
dense and cool for molecules to form in large abundance
– contract under their own gravity and form new
stars. The spiral arms are much less prominent in red
light. Emission in the red is dominated by an older stellar
population, and these old stars have had time to move
away from the spiral arms. The Sun is located close to,
but not in, a spiral arm – the so-called Orion arm.
Observing the gas in the Galaxy is made possible
mainly by the 21-cm line emission of HI (neutral atomic
hydrogen) and by the emission of CO, the second-most
abundant molecule after H 2 (molecular hydrogen). H 2
is a symmetric molecule and thus has no electric dipole
moment, which is the reason why it does not radiate
strongly. In most cases it is assumed that the ratio of
CO to H 2 is a universal constant (called the “X-factor”).
Under this assumption, the distribution of CO can be
converted into that of the molecular gas. The Milky Way
is optically thin at 21 cm, i.e., 21-cm radiation is not
absorbed along its path from the source to the observer.
With radio-astronomical methods it is thus possible to
observe atomic gas throughout the entire Galaxy.
To examine the distribution of dust, two options are
available. First, dust is detected by the extinction it
causes. This effect can be analyzed quantitatively, for instance
by star counts or by investigating the reddening of
stars (an example of this can be seen in Fig. 2.7). Second,
dust emits thermal radiation observable in the FIR part
of the spectrum, which was mapped by several satellites
such as IRAS and COBE. By combining the sky maps of
these two satellites at different frequencies the Galactic
distribution of dust was determined. The dust temperature
varies in a relatively narrow range between ∼ 17 K
and ∼ 21 K, but even across this small range, the dust
emission varies, for fixed column density, by a factor
∼ 5 at a wavelength of 100 μm. Therefore, one needs
to combine maps at different frequencies in order to determine
column densities and temperatures. In addition,
the zodiacal light caused by the reflection of solar radiation
by dust inside our Solar system has to be subtracted
7 HII regions are nearly spherical regions of fully ionized hydrogen
(thusthenameHII region) surrounding a young hot star which photoionizes
the gas. They emit strong emission lines of which the Balmer
lines of hydrogen are strongest.
before the Galactic FIR emission can be analyzed. This
is possible with multifrequency data because of the different
spectral shapes. The resulting distribution of dust
is displayed in Fig. 2.11. It shows the concentration of
dust around the Galactic plane, as well as large-scale
anisotropies at high Galactic latitudes. The dust map
shown here is routinely used for extinction correction
when observing extragalactic sources.
Besides a strong concentration towards the Galactic
plane, gas and dust are preferentially found in spiral
arms where they serve as raw material for star formation.
Molecular hydrogen (H 2 ) and dust are generally found
at 3 kpc R 8 kpc, within |z| 90 pc of both sides of
the Galactic plane. In contrast, the distribution of atomic
hydrogen (HI) is observed out to much larger distances
from the Galactic center (R 25 kpc), with a scaleheight
of ∼ 160 pc inside the Solar orbit, R R 0 .At
larger distances from the Galactic center, R 12 kpc,
the scale-height increases substantially to ∼ 1 kpc. The
gaseous disk is warped at these large radii though the
origin of this warp is unclear. For example, it may
be caused by the gravitational field of the Magellanic
Clouds. The total mass in the two components of hydrogen
is about M(HI) ≈ 4 × 10 9 M ⊙ and M(H 2 ) ≈ 10 9 M ⊙ ,
respectively, i.e., the gas mass in our Galaxy is less than
∼ 10% of the stellar mass. The density of the gas in the
Solar neighborhood is about ρ(gas) ∼ 0.04M ⊙ /pc 3 .
2.3.4 Cosmic Rays
The Magnetic Field of the Galaxy. Like many other
cosmic objects, the Milky Way has a magnetic field. The
properties of this field can be analyzed using a variety
of methods and we list some of them in the following.
• Polarization of stellar light. The light of distant stars
is partially polarized, with the degree of polarization
being strongly related to the extinction, or reddening,
of the star. This hints at the polarization being linked
to the dust causing the extinction. The light scattered
by dust particles is partially linearly polarized,
with the direction of polarization depending on the
alignment of the dust grains. If their orientation were
random, the superposition of the scattered radiation
from different dust particles would add up to a vanishing
net polarization. However, a net polarization
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