The space between the stars (interstellar space)
of the Milky Way Galaxy and other galaxies is filled
with gaseous ions, atoms, molecules (interstellar gas)
and tiny dust grains (interstellar dust).
The first direct evidence which pointed to
the existence of interstellar gas came from
the ground-based detection of Na and Ca+ optical absorption lines
(Hartmann 1904).
1
This did not gain wide acceptance until
Struve (1929)
showed that the strength of the Ca+ K-line was
correlated with the distance of the star.
The true interstellar nature of this gas
was further supported by the detection of
the first interstellar molecules CH, CH+ and CN
(Swings & Rosenfeld 1937,
McKellar 1940,
Douglas & Herzberg 1941).
The presence of dark, obscuring matter in
the Milky Way Galaxy was also recognized at
the beginning of the 20th century (e.g. see
Barnard 1919).
Trumpler (1930)
first convincingly showed that
this "Obscuring Matter" which dims and reddens starlight
consists of small solid dust grains.
The dust and gas are generally well mixed in
the interstellar medium (ISM), as demonstrated
observationally by the reasonably uniform correlation
in the diffuse ISM between the hydrogen column density
NH and the dust extinction color excess
or reddening E(B - V)
AB -
AV:
NH / E(B - V)
5.8 ×
1021 mag-1 cm-2
(Bohlin et al. 1978),
where AB and AV are
the extinction at the B
(
= 4400 Å)
and V ( = 5500
Å) wavelength bands. From this correlation one can estimate the
gas-to-dust ratio to be ~ 210 in the diffuse ISM.
2
Despite its tiny contribution to the total mass
of a galaxy, 3
interstellar dust has a dramatic effect on the physical
conditions and processes taking place within the universe,
in particular, the evolution of galaxies and the formation
of stars and planetary systems (see the introduction section
of Li & Greenberg 2003).
In this review I will concentrate on the radiative
properties of interstellar dust. I will distinguish
the dust in terms of 3 components:
cold dust with steady-state temperatures of 15 K
T
25 K in thermal
equilibrium with the solar neighbourhood interstellar
radiation field, very cold dust with T < 10 K
(typically T ~ 5 K), and warm dust undergoing
"temperature spikes" via single-photon heating.
I will first summarize in Section 2
the observational constraints on the physical
and chemical properties of interstellar dust.
I will then discuss in Section 3 the heating and
cooling properties of the warm and cold interstellar dust components.
In Section 4 I will demonstrate the robustness of
the silicate-graphite-PAHs interstellar grain model
by showing that this model closely reproduces
the observed infrared (IR) emission of the Milky Way,
the Small Magellanic Cloud, and the ringed Sb galaxy
NGC7331. The possible existence of a population of
very cold dust (with T < 10 K) in interstellar space
will be discussed in Section 5. Whether
ultrasmall grains
can really cool down to T < 2.7 K and appear in
absorption against the cosmic microwave background (CMB)
radiation will be discussed in Section 6.