1.8 THE INTERSTELLAR MEDIUM

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1.8 THE INTERSTELLAR MEDIUM
Apart from the 1011 stars that are distributed
throughout the Galaxy, there are many other
components (gas, dust, fields etc.) which are
distributed throughout the space between the
stars. The interstellar medium is thus not a
vacuum and the constituents have an important
impact on the stellar evolutionary cycle, the
understanding of stars and the Galaxy, as well as
our ability to make observations.
Neutral Hydrogen - HI Regions.
Neutral Hydrogen, discovered by the 21 cm radio
line which derived from the hyperfine structure
of the H atom (i.e.. proton/electron magnetic
moments flip between parallel and antiparallel
states). It is found to exist in clouds
throughout the plane of the Galaxy. Typical HI
clouds have the following characteristics.
NH 108 m-3 Radii 300 pc Temperatures
100 K Mass 103 M vrandom 10 km/s Low
ionised state
The study of the 21 cm hydrogen line spectral
profiles taken at different directions within the
galactic plane that have confirmed the spiral
structure of the Galaxy and its rotation.
A
B
C
Intensity
D
0
Blue
Red
Spectral shift
The rotation curve of the galaxy is derived from
the radial Doppler velocities of the 21 cm line
emission. Curves are taken at successive viewing
angles in the galactic plane to compile a spiral
structure map of the neutral hydrogen. Note that
transverse motions cannot be used for Doppler
measurements, leaving a gap in the apparent
structure, and that although a HI clouds are
inferred from the Doppler velocity profile, no
direct estimate of their distance from the earth
is possible.
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Diffuse Nebulae - HII Regions
These ionised diffuse nebulae are produced by
neighbouring hot stars. The ionisation is usually
caused by photoionization by uv photons in
regions of recent star formation. Recombination
causes flourescence in dynamic balance. They
appear as beautiful emission nebulae in the
colours of the characteristic lines of the
elemental constituents Ha, Hb, Hg, O(II),
N(II) etc. Their sizes are 200 pc, they are
roughly spherical (Stromgren sphere) with their
boundaries sharply delineated.
M20 The Trifid Nebula
Some famous HII regions are
Interstellar Molecules
The advent of astronomical observations at
microwavelengths led to the discovery of
molecules in the interstellar medium through the
detection of their characteristic emission and
absorption lines. e.g.
CS carbon monosufide HCN hydrogen
cyanide HNO nitroxyl H2CO formaldehide CN cyanoge
n CO carbon monoxide H3COOH methonoic
acid CH3OH methanol CH3C2H methylacetylene H3COOCH
3 methyl methanoate C2H5OH ethanol (CH3)2O dimeth
yl ether
are a few examples from many. It is remarkable
that such complex organic molecules can exist in
the hostile (uv, energetic particles)
interstellar environment.
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Interstellar Grains - Dust
The Horsehead Nebula in Orion
Dust Clouds in Sagittarius
galaxy very difficult in the optical band.
Similar obscuration is detected in other
galaxies, notably the nearby Sombrero spiral
galaxy (M104) in Virgo. It is due to the presence
of dust clouds.
The extinction has a l-1 dependence rather than
the l-4 expected from Rayleigh scattering. This
implies the scattering centres must have
dimensions similar to the wavelength of the light
i.e.. 1 - 10 mm. Furthermore the observed
polarisation of the transmitted starlight implies
that the particles are asymmetric and
preferentially aligned. Possible candidates are
silicates or graphite with a magnetic moment
aligned in a magnetic field. The origin of these
dust grains is unclear. A likely possibility is
that they are formed in the outer atmospheres of
cool giant stars which expand and contract, thus
providing good conditions to both cause the
particle condensations and eject them into the
interstellar medium.
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Hubble Gaseous Pillars in M16
The Hubble Space Telescope image of the star
forming region in the Eagle nebula provides a
spectacular representation of diffuse material in
the galactic plane. The massive fingers of gas
and dust, typically a parsec in length protrude
from an enormous cloud of molecular hydrogen.
The image was taken by Jeff Haster and Paul
Scowen of Arizona State University on April 1st
1995, using the Wide Field and Planetary Camera
2. The gas is dense enough in these columns to
collapse under its own weight, forming
evaporating gaseous globules (EGGs). This is an
early stage of star formation, the young stars
continue to grow as long as they can accumulate
material from the surroundings. Ultraviolet
radiation from hot newly formed stars nearby
continuously boils away gas from the surfaces of
the columns, causing the ghostly fringes visible
around the edges of the columns.
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The Galactic Magnetic Field There is convincing
evidence for a large scale ordered magnetic field
which permeates the Galaxy. Key evidence is as
follows Polarisation of Starlight. The optical
polarisation of the starlight is strongly
correlated with the extinction by the
interstellar dust This is attributed to
preferential scattering of photons with their
electric vectors parallel to elongated grains
aligned over large regions of space by a magnetic
field. Faraday Rotation of Polarised Radio
Emission. Faraday rotation is observed for
linearly polarised radio emission from e.g.
distant galaxies which has to travel through the
interstellar medium. The rotation f is caused by
the propagation of the waves through a magnetised
plasma and can be evaluated as
SI units. Where f is in radians, Ne electrons
m-3, Bll (B ll to line of sight) in Tesla, the
wavelength l is in metres and the path length
in pc.
Measurement of f vs l2 estimates of the product
NeBll along the line of sight
Dispersion of radio pulsars gives
Typical values are Bgalactic 2 10-10 Tesla
Cosmic Rays. Intense fluxes of energetic
particles are incident upon the top of the
earths atmosphere. Some key features are
The composition is dominated by protons, but all
nuclei of all elements up to and including the
Uranium group are present, as are electrons and
antiparticles.
Solar Modulation
The spectrum appears to be a composite of power
laws, and extends up to particles having 1021 eV
per nucleon
g 2.6
g 3.1
N(E)dE
g 2.5
Particles m-2 s-1 MeV-1, g 2.5
1010
1020
NOTE
KE per Nucleon (eV)
The low energy cosmic ray fluxes are modulated
throughout the solar cycle as the interplanetary
field changes and also by the earths magnetic
field. (Rigidity effect). Their origin is still
not known (Pulsars?). However the most energetic
are likely to come from outside the Galaxy, and
travel in straighter lines. Possible imaging?
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2. PHOTON GENERATION MECHANISMS
2.1 THERMAL RADIATION
J s-1 m-2 Hz-1 sr-1
At hn The Rayleigh Jeans Radiation Law provides a
useful approximation at radio wavelengths. Hence
the expression temperature of a source. The
temperature a BB source would have to have to
emit radiation of the observed intensity at a
given wavelength.
2.2 BREMSSTRAHLUNG AND THERMAL BREMSSTRAHLUNG
Free-Free Emission
If charged particles are accelerated in a
Coulomb field, then we may expect radiation to be
generated. Electrons are generally the only
particles worth considering because of their low
mass. The emitted photons can have energies all
the way up to the full energy of the electron.
If the electrons have a spectrum
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Then the photon flux at the Earth will be
Thermal Bremsstrahlung
If the temperature of a gas is hot enough it will
become fully ionised. This will mean that the
bremsstrahlung process will be very efficient
since the energetic electrons will have many
readily available Coulomb targets nearby. This is
the Thermal Bremsstrahlung process, and because
the energies of the electrons are typically just
above the ionisation potentials then the emitted
photons are most plentiful at X-ray energies.
(keV).
The electrons have a Maxwell-Boltzman
distribution, so that the probability that a
particle has a velocity in the range d3v is
If the particles are isotropic
If T approximation.
So that for emitted photons
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The resulting photon spectrum is
Ph m-2 s-1 keV-1
NOTE

• The spectral index is related to the slope of
  the electron spectrum by a ( g -1)/2
• Given the value of B the value of Ke can be
  obtained
• The radiation is polarised by B
• Since In µ m-3 , protons are not important.

B
Cyclotron Line Emission
hn
If the magnetic field is sufficiently intense
then it is possible that the radii of the
electron orbits are so small that their energy
levels become quantized in an analogous way to
atomic electrons in the Coulomb field of a
nucleus.
Quantized electron orbit
The Landau levels of an electron in a homogeneous
field are given by
where j 0, 1, 2 and s 1 are angular
momentum and spin quantum numbers respectively B
is the magnetic field intensity Bcr (m3c3)/eh
44.14 108 Tesla m is the electron mass and
pz is the electron momentum along the field lines.
For the case of the intense dipole fields (108
Tesla) which are associated with neutron stars
these cyclotron line emissions are in the hard
(10 keV to few hundred keV) region of the
spectrum. The measurement of the line energies
yields a measurement of the magnetic field
strength. Often it is difficult to determine
whether the line is an emission or absorption
line.
Photon Energy (keV)
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2.4 INVERSE COMPTON RADIATION
Whereas Bremsstrahlung radiation is due to the
scattering of electrons off the Coulomb field of
particles, and Magnetobremsstrahlung is the
scattering in a magnetic field, then the
remaining major source of scattering type
radiation is if electrons collide with ambient
gas photons. This is called the Inverse Compton
effect. It is simply a reversal of the well-known
laboratory Compton scatter, but in this case the
incident electron is the energetic partner and
imparts energy to a photon.
For the case of ultra-relativistic electrons,
(E/mc2) g 1, which impinge on isotropic
photons with mean energy Eph hno, the mean
energy of the scattered photon is
The rate of energy loss by the electrons is
This expression is similar to the
Magnetobremsstrahlung case with the photon energy
density nph hn0 replacing the magnetic field
energy density Umag B2/2m0
The equations are basically the same
nComp h n0 g2 nmb (eB/2pme) g2
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If the electron spectrum is
Electrons m-2 s-1 keV
Then the emitted spectrum will be
Ph m-2 s-1 keV-1
Where KC is the numerical values of the physical
constants
SYNCHROTRON SELF-COMPTON RADIATION
I(n )
If the emission region is highly localised the
Magnetobremsstrahlung photons will remain in the
locality for a period t R/c where R is the
radius of the region. The mb photons may be
further scattered by same set of electrons (not
the same individual electrons) as those which
produced the original mb radiation. They will be
further frequency shifted by the Inverse Compton
effect The mb spectrum is as above
Magnetobremsstrahlung
Synchrotron Self-Compton
n
Ph m-2 s-1 keV-1
The SSC spectrum will be
Where Nph µ F(R, Imb)
This is a very useful mechanism. Generally for
both the mb and IC processes what we obtain from
the amplitude of the measured spectrum is a
product of constants e.g. mb product is F(B
Ke). Thus we cannot determine either uniquely.
With the SSC mechanisms, if we measure both
spectra and know R, then we can obtain a value
for B. An upper limit on the SSC flux gives a
lower limit estimate of B, an unusual situation.