Historically, the term Diffuse Galactic Light (DGL) denotes the diffuse component of the galactic background radiation which is produced by scattering of stellar photons by dust grains in interstellar space (Elvey & Roach 1937; Roach & Gordon 1973). This scattering process is the dominant contributor to the general interstellar extinction of starlight; thus, the DGL is most intense in directions where the dust column density and the integrated stellar emissivity are both high. This is generally the case at the lowest galactic latitudes, in all spectral regions extending from the far-ultraviolet into the near-infrared. Typically, the DGL contributes between 20% to 30% of the total integrated light from the Milky Way.
However, for the purpose of this reference we are also interested in other sources of diffuse galactic background radiation, and they will be mentioned in the following where appropriate.
No comprehensive map of the DGL for the entire sky or even a
significant fraction of the sky exists at this time. Groundbased
observations in the visual face the difficult requirement that
airglow, zodiacal light, and integrated starlight all need to be known
to very high precision (
) if the DGL is
to be derived by subtraction of the above components from the total
sky brightness. In addition, the problems of atmospheric extinction
and atmospheric scattering (Staude 1975) need to be solved.
Observations of the DGL at visual wavelengths carried out with rocket- or satellite-borne photometers still have the same major sources of uncertainty, i.e. the integrated starlight and the zodiacal light, remain principal contributors to the measured intensity.
The best prospect for a comprehensive measurement of the
DGL in the visual was offered by the Pioneer 10 probe (see the
more detailed description in Sect. 10.4 (click here)), which carried
out an all-sky photometric mapping in two wavebands centered near 440
nm and 640 nm from beyond the asteroid belt (R > 3 AU), where the
contributions from zodiacal light are negligible (Hanner et al.\
1974). The instantaneous field of view of the Pioneer 10 photometer
was 
square, which due to spacecraft spin (12.5 s period)
and finite integration time (0.2 s) was drawn into elongated
effective fields-of-view of variable size depending on the look angle.
Contributions due to unresolved stars begin to enter the Pioneer 10
data at 
for an average galactic latitude; thus, stars
dominate the measured fluxes.
Toller (1981) derived DGL intensities from the Pioneer 10 blue data (440 nm) by subtracting integrated starlight intensities of Roach & Megill (1961) and Sharov & Lipaeva (1973) at the positions of 194 Selected Areas (Blaauw & Elvius 1965). The residuals, interpreted as the sum of DGL and extragalactic background light, are most representative in terms of sky coverage.
Figure 76: The average intensity of the DGL as a function of galactic
latitude based on the analysis by Toller (1981) of
Pioneer 10 photometry of 194 Selected Areas at 
nm.
Error bars denoting one standard deviation of the means are a measure
of the longitudinal variation of the DGL intensity
For reference purposes, several directions may be taken to
estimate the intensity of the DGL at 
nm:
In Fig. 76 (click here) we present the mean galactic latitude dependence of Toller's (1981) values of the sum of DGL and extragalactic background, averaged over all galactic longitudes. The error bars, representing one standard deviation of the mean, reflect in part the real variations of the DGL intensity with galactic longitude, especially at lower latitudes.
A second avenue toward a DGL estimate can be found in ratios of
DGL to total line-of-sight starlight (LOS*) intensities. In
Table 39 (click here) we list the average ratios of DGL/LOS*
for nm based on Toller's data. The use of the
values in Table 39 (click here) may be advisable, if one wants to
estimate the variation of DGL with galactic longitude, where large
differences in LOS* may occur. Due to the strongly forward
scattering nature of interstellar grains the DGL intensity generally
tracks the LOS* intensity at constant latitude.
b [ | - | N [SA] |  | |
| 0 | - | |5| | 19 |  |
| |5| | - | |10| | 11 |  |
| |10| | - | |15| | 28 |  |
| |15| | - | |20| | 15 |  |
| |20| | - | |30| | 29 |  |
| |30| | - | |40| | 22 |  |
| |40| | - | |60| | 41 |  |
| |60| | - | |90| | 29 |  |
|
| - |
A third approach toward a DGL estimate might rely on the mean
correlation between DGL intensities found in Selected Areas by Toller
(1981) and corresponding column densities of atomic hydrogen. Toller
finds:
A good source for 
values is the Bell Lab HI survey by
Stark et al. (1992). This approach is based on the fact that
the dust and HI column densities are well-correlated (Bohlin et al.\
1978) and that the DGL intensity is determined in part by the dust
column density, as long as the line-of-sight is not optically thick. This
third approach is therefore recommended mostly for higher galactic
latitudes, or 
atoms cm-2.
Estimates based on Eq. (31 (click here)) are at best good to within a
factor of two, because Eq. (31 (click here)) reflects only the
dependence of the DGL intensity on the dust column density and ignores
the dependence on the intensity of the illuminating radiation field.
The red band (
nm) data from Pioneer 10 have not
been subjected to a DGL analysis so far for lack of suitable star
count data.
The U-B and B-V colours of the DGL have been measured and have been
found to be bluer than the colour of the integrated starlight, as
expected from scattering by interstellar grains with scattering cross
sections varying as 
in the visible region
(Witt 1968; Mattila 1970).
Table 40 (click here), to give an example,
contains UBV colours of the DGL and of the
integrated starlight in Cygnus (upper panel), respectively in Crux
(lower panel).
| DGL | Integrated Starlight | Reference | ||
| l, |b| | U-B | B-V | U-B | B-V | |
 ,  | -0.05 | +0.57 | +0.07 | +0.73 | Witt |
 ,  | -0.10 | +0.44 | +0.00 | +0.68 | (1968) |
|
| -0.10 | +0.50 | -0.01 | +0.71 | Mattila |
| (1970) | |||||
|
| |||||
Recently, Gordon (1997) reported the detection of extended red emission (ERE) on a galaxy-wide scale in the diffuse interstellar medium of the Milky Way Galaxy (see also Gordon et al. 1997; Gordon & Witt 1997). The ERE consists of a broad emission band (FWHM 800 Å) with a peak wavelength found in the 6500 Å\ to 8000 Å\ range, depending on environment, with a long-wavelength tail extending well into the I-band. The ERE is believed to result from a photoluminescence process in hydrogenated carbonaceous grain mantles, and it has been previously detected photometrically and spectroscopically in numerous reflection nebulae (Witt & Schild 1988; Witt & Boroson 1990), in carbon-rich planetary nebulae (Furton & Witt 1990, 1992), in HII regions (Perrin & Sivan 1992; Sivan & Perrin 1993), and in the scattered light halo of the starburst galaxy M 82 (Perrin et al. 1995). Gordon (1997) derived the galactic ERE intensity from Pioneer 10 and 11 sky photometry obtained at heliocentric distances greater than 3.3 AU, where the contribution from zodiacal light is no longer detectable (see Sect.nbsp;10.4 (click here)). The integrated star light due to stars of m > 6.5 was determined by integrating recent starcount data from the APS Catalog (Pennington et al. 1993), the HST Guide Star Catalog, and photometric catalogs on brighter stars and was subtracted from the Pioneer 10 and 11 in both the blue and red bands. The diffuse residuals consist of DGL in the blue band, and of a sum of DGL and ERE in the red band. As a result, the B - R colour of the diffuse galactic background radiation is substantially redder than that of the DGL alone. The excess ERE in the R-band can be estimated to be about equal in intensity to the R-band DGL. This ERE intensity is consistent with the measured B-R and B-I colour excesses of individual galactic cirrus filaments (Guhathakurta & Tyson 1989), found to be 0.5 - 1.0 mag and 1.5 - 2.0 mag redder, respectively, than expected for scattered disk starlight.
Quantitatively, Gordon (1997) finds the galactic ERE
and the atomic hydrogen column density at intermediate and high
latitudes (
) to be well-correlated, yielding an
average ERE intensity of (
H-atom-1. This correlation may therefore be used to
estimate the expected ERE intensity in the R-band in different
portions of the sky.
Partial linear polarization of the DGL at a level of 
is
expected, and some tentative detections of this polarization have been
reported by Schmidt & Leinert (1966), Weinberg
(1969), Sparrow & Ney (1972), and Bandermann &
Wolstencroft (1976). Both the scattering by grains partially aligned
with their short axes parallel to the galactic plane and the scattering of
the non-isotropic galactic radiation field by dust in the galactic plane
should produce partially polarized scattered light with the electric vector
perpendicular to the galactic plane when observed near 
. A
review of existing polarization measurements is given by Leinert
(1990).
The diffuse galactic background radiation in the near-infrared (near-IR) is composed of several components, each produced by different constituents of the diffuse interstellar medium by different physical processes. The most important ones are the DGL, caused by scattering of star light on larger interstellar grains; the near-IR continuum emission, caused by a non-equilibrium emission process probably associated with small carbonaceous grains; and the set of so-called unidentified infrared bands which have been attributed to emission from interstellar aromatic hydrocarbon molecules, such as polycyclic aromatic hydrocarbons (PAH). We will refer to them as aromatic hydrocarbon bands.
No separate detection of the DGL at near-IR wavelengths has
been accomplished so far, although the galactic component of the
near-IR background at 1.25 
m and 2.2 m observed by the DIRBE
experiment (Silverberg et al. 1993; Hauser 1996)
undoubtedly contains a scattered light contribution. Recent evidence (Witt
et al. 1994; Lehtinen & Mattila 1996) provides a
strong indication that the dust albedo remains as high as it is in the
visible out through the K-band (2.2 m). The K-optical depth is
about 10% of that at V; hence, only at quite low galactic latitudes (
) can one find the required dust column densities which will give
rise to substantial (scattered) DGL. At the galactic equator, however, the
ratio of DGL/LOS* should be similar to the values listed in
Table 39 (click here). At higher galactic latitudes the ratio
DGL/LOS* will be substantially lower than the values listed in
Table 39 (click here).
The near-IR continuum emission was first recognized in
reflection nebulae whose surface brightnesses in the 1 m - 10 m
wavelength range exceeded that expected from scattering by factors of
several (Sellgren et al. 1983; Sellgren 1984).
Absence of polarization provided additional confirmation of the
non-scattering origin of this radiation. The non-equilibrium nature of the
radiation process was recognized from the fact that the colour temperature
of the emerging radiation was independent of distance from the exciting
star and thus independent of the density of the exciting radiation.
This leaves as the cause of this radiation non-equilibrium processes
which depend upon excitation by single photons, e.g. photoluminescence
of grain mantles or, alternatively, non-equilibrium heating of tiny
grains resulting in large temperature fluctuations. The galactic
distribution of this radiation component has yet to be studied; it
depends on a very accurate assessment of the near-IR integrated
starlight (see Sect.nbsp;10.5 (click here)) and the near-IR zodiacal light (see
Sect.nbsp;8.5 (click here)), which need to be subtracted from photometries of the
near-IR sky background.
The aromatic hydrocarbon bands centered at wavelengths 3.3 m,
6.2 m, 7.7 m, 8.6 m, and 11.3 m,
with widths in the range of 0.03
to 0.5 m, were first observed in bright nebulous regions by
Gillett et al. (1973). Thanks to the successful AROME
balloon-borne experiment (Giard et al. 1988; Ristorcelli
et al. 1994) and the more recent
missions of the Infrared Telescope in Space (IRTS, Onaka et al.
1996) and the Infrared Space Observatory (ISO, Mattila et al.
1996; Lemke et al. 1997), they
have now been observed in the diffuse interstellar medium at low
galactic latitudes. The relative bandstrengths and widths are very
similar to those observed in reflection nebulae, planetary nebulae,
and HII regions, pointing toward a common emission mechanism. Onaka
et al. (1996) show that the band intensities at 3.3 m and 7.7
m and the far-IR background intensities at 100 m along
identical lines of sight are correlated very tightly, suggesting that
the respective emitters, presumably PAH molecules in the case of the
aromatic hydrocarbon bands and classical sub-micron grains for the
100-m thermal continuum, are well-mixed spatially and are
excited by the same interstellar radiation field. The correlation of
the band intensities with the atomic hydrogen column density is also
excellent, reflected in the dust emission spectrum per hydrogen atom
given in Table 43 (click here).
Wavelength | Dust emission spectrum |
4  | |
|
(m) | (erg s-1 H-atom-1) |
| 0.65 | 1.17 10-24 |
| 3.5 | 1.27 10-25 |
| 4.9 | 1.36 10-25 |
| 7.7 | 1.13 10-25 |
| 12 | 1.02 10-24 |
| 25 | 4.11 10-25 |
| 60 | 7.04 10-25 |
| 100 | 2.64 10-24 |
| 140 | 3.64 10-24 |
| 240 | 1.41 10-24 |
| 346 | 5.77 10-25 |
| 490 | 1.77 10-25 |
| 535 | 1.20 10-25 |
| 736 | 3.33 10-26 |
| 1100 | 5.49 10-27 |
|
|
The infrared emission from the diffuse galactic ISM is
dominated by thermal and other emissions by dust, with some additional
contributions from interstellar cooling lines, mainly from CII and
NII. At wavelengths < 100 m the galactic diffuse emission is
weaker than the infrared emission from the zodiacal dust cloud (see
Fig. 1); at wavelengths > 400 m the cosmic background radiation
dominates over the galactic thermal radiation. Only in the 100 - 400
m band is the galactic emission the primary background component.
However, as the composite spectrum of all night sky components in
Fig. 1 schematically indicates, the thermal IR spectrum of galactic
dust is complex in structure, suggesting significant contributions
from grains covering a wide range of temperatures. In particular,
there is substantial excess emission in the 5 to 50 m spectral
range. This excess is generally attributed to stochastically heated
very small grains with mean temperatures in the range 
(Draine & Anderson 1985; Weiland et al. 1986),
while the main thermal emission peak near 150 m is attributed to
classical-sized dust grains in equilibrium with the galactic interstellar
radiation field, resulting in temperatures around 20 K.
The exploration of the infrared background has been greatly advanced by the highly successful missions of the Infrared Astronomical Satellite (IRAS; Neugebauer et al. 1984), the Diffuse Infrared Background Experiment (DIRBE; Boggess et al. 1992) and the Far-Infrared Absolute Spectrophotometer (FIRAS; Fixsen et al. 1994) on board of the COBE satellite, the Infrared Telescope in Space (IRTS; Murakami et al. 1994, 1996), and the AROME balloon-borne experiment (Giard et al. 1988). Before mentioning the relation to interstellar gas, we first comment on the maps of galactic far-infrared emission available from these experiments.
From IRAS, the so-called ISSA maps are available for wavelengths of
12 m, 25 m, 60 m and 100 m. These present, after
subtraction of a zodiacal light model, 
fields with 1.5' resolution, covering the sky on a 10
grid.
For the two longer wavelengths this gives a good picture of the
variation of galactic emission (in MJy/sr). The absolute value
of these maps is not reliable,
which can be seen from the regions with negative intensities.
To access the data one can use the world wide web address
http://www.ipac.caltech.edu/ipac/iras/released-data.html.
With better modeling of the zodiacal light contribution,
Rowan-Robinson et al.
(1991) produced sky maps of galactic plus extragalactic far-infrared
emission with 0.5 pixel size. These maps give realistic
absolute values and still show the spatial variation of the galactic
diffuse emission in some detail. Tables 41 and 42 give a version of the
two tables for 60 m and 100 m in ecliptic coordinates. They are not
printed here but available in electronic form at the CDS
by anonymous ftp to 130.79.128.5.
But in particular the DIRBE Zodiacal Light-Subtracted Mission Average
("ZSMA") maps at 60 m, 100 m, 140 m and 240 m give a good
estimate of the observed intensity resulting from the sum of galactic diffuse
infrared emission and the extragalactic background at each of these
wavelengths, apart from errors in the zodiacal light model model.
Figure 77 (click here) shows the DIRBE 240 micron
ZSMA map, along with representative intensity profiles at fixed galactic
latitude. Since the extragalactic background light, which is spatially
uniform, is not necessarily negligible at these wavelengths
(see Table 47 (click here) in Sect. 12 (click here)), caution
should be exercised when making quantitative statements about the
absolute level of the diffuse Galactic infrared emission as derived
from these maps. The Galactic contribution is certainly dominant
at latitudes 
, but probably also all over
the sky. Aside from this issue of absolute
levels (how much extragalactic background light radiation exists and has to
be subtracted?), and aside from some visible zodiacal light model
artifacts in the
ecliptic plane at 60 microns, the ZSMA maps give a good picture of
the spatial variation of the diffuse Galactic emission at
all latitudes. The intensities recorded in the ZSMA maps are
reported in MJy/sr, assuming a nominal wavelength and a spectral shape
of 
= constant. The ZSMA maps are available from the NSSDC
through the COBE homepage website at
http://www.gsfc.nasa.gov/aas/cobe/cobe-home.html.
Figure 77: Map of the sky brightness at 240 m after removal of
zodiacal light, obtained from the COBE/DIRBE experiment.
The map is a Mollweide projection in Galactic
coordinates, with intensities on a logarithmic stretch from 1 to
1000 MJy/sr. Smoothed intensity profiles at
fixed Galactic latitudes of 0, 
, 
and 
are also
plotted; positive latitudes are represented by the thin black,
negative latitudes by the thick grey line
Interstellar dust appears to be well-mixed with all phases of
the interstellar gas (Boulanger & Perault 1988;
Sodroski et al. 1997); however, to obtain a
first-order representation of the emissions from galactic dust, the
well-established correlations with N(HI) provide the best guide. The
average dust emission spectrum per H-atom is given in
Table 43 (click here), as derived from the following original
sources: ERE at 0.65 m, Gordon (1997); galactic emission
in the 3.3 m aromatic feature, Giard et al. (1989),
Bernard et al. (1994); dust emission in the 6.2, 7.7, 8.6, and
11.3 m mid-infrared unidentified bands, Onaka et al.
(1996); and the broad-band thermal dust emissions, Boulanger et
al. (1996), Reach et al. (1995a), and Dwek et al.
(1997).
Compared with emission from dust, the radiation from infrared
cooling lines of the gas is comparatively weak, reflecting the fact
that dust in interstellar space absorbs approximately one third of all
energy emitted by stellar sources. The three strongest lines are the
[CII] transition at 158 m and the [NII] lines at 122 and 205 m (Wright et al. 1991). At low galactic latitudes, the
[CII] line emission is well correlated with N(HI), yielding (2.65 
0.15) 10-26 ergs s-1 H-atom-1 (Bennett et al.
1994). With latitudes increasing beyond 
, the ratio
of I([158])/I(FIR
) decreases rapidly, leaving the
158 m line unmeasurably weak at 
.
Intense Lyman 
flux is present in this bandpass,
and this, combined with various instrumental limitations,
meant that most of the effort to measure a diffuse flux
in this band was carried out at wavelengths longer than 121.6 nm.
In retrospect, the measurement of a diffuse flux in this bandpass
is far more difficult than was originally imagined, and the
potential for obtaining erroneous results is substantial. The
literature is filled with controversial and erroneous results.
This bandpass has been studied extensively since the beginning of the Space Age, because the zodiacal light component is not present and contributions from stellar sources were expected to be sufficiently low that emission from a hot (105 K) or very hot (106 K) intergalactic medium might be detected.
A review of this literature is provided by Bowyer (1991), but for an alternative view and a detailed examination of an particular data set, see Henry (1991).
Substantial progress
has been made in identifying the components that contribute to
the diffuse flux in this band.
The vast majority of the diffuse flux is starlight scattered by
interstellar dust. In particular, Haikala et al. (1995)
found FUV emission at a high galactic latitude from
a cirrus cloud detected at 100 m with IRAS.
Emission from hot (
105 K) gas
has been detected. An analysis of this radiation establishes
that the emitting gas is well above the Galactic plane. Two-photon
emission from recombining ionized hydrogen has been recognized
as a component of this background. Molecular hydrogen fluorescence has
been found in low density clouds.
Any extragalactic flux is quite small; this component is
discussed in Sect. 12.1 (click here).
In Fig. 78 (click here)
we provide examples of the best available data on the
diffuse far ultraviolet background. The data from 912 to 1200
Angstrom are from Holberg (1986) and
are upper limits to the background from a high galactic latitude
view direction.
Two data sets are shown for the 1400 to 1840
Angstrom band. The upper line is from Hurwitz et al.
(1991) and shows
data obtained at a low galactic latitude. These data are typical
of what is observed in viewing directions with an optical depth of
1. Molecular hydrogen fluorescence is evident as an
additional component at wavelengths from 1550 to 1650 Angstrom.
The lower line is from Martin & Bowyer (1990)
and shows data obtained at
a high galactic latitude and a low total galactic neutral hydrogen column.
The CIV 1550 Angstrom line is clearly evident in emission, and
the 1663 Angstrom line of forbidden O III is also apparent though
at a lower signal-to-noise.
As already mentioned, the major components of the cosmic far ultraviolet background are summarized in Table 44 (click here) above.
| Total intensity | 300-1500 | |
| Scattering by dust | 200-1500 | |
| H II two-photon emission | 50 | |
| H2 fluorescence | 100 | |
| (in molecular clouds) | ||
| Hot gas line emission | 10 | |
| from hot Galactic gas | ||
| Extragalactic | 50 to 200 | |
| Unexplained | none to 200 |
Figure 78: Summary data on the diffuse cosmic far ultraviolet background.
The data from 902 to 1200 Å are from Holberg
(1986) and are upper limits to the flux from a high Galactic latitude
view direction.
Two data sets are shown for the 1400 to 1850 Å band.
The upper line is from Hurwitz et al. (1991)
and shows typical
data obtained in view directions with 1.
Note the H2 fluorescence emission around 160 nm.
The lower line is from Martin & Bowyer (1990)
and shows data obtained
at a high Galactic latitude; the CIV 1550 Å line
is clearly evident in emission and the 1663 Å line of OIII]
is also apparent, though at lower signal-to-noise ratio. The extragalactic
contribution to these data probably is small (see
Table 44 (click here))
The diffuse radiation in this band is the sum of zodiacal light and starlight scattered by interstellar dust. A few first studies of the zodiacal light in this band have been carried out, which suggest this component exhibits characteristics similar to that observed in the visible (see Sect. 8.6 (click here)). A few studies of scattering by dust by early type stars have been carried out. The results obtained differ, and independent of these differences, the scattering varies tremendously from place to place in the galaxy. We refer the reader to Dring et al. (1996) and references therein for a discussion of these results.
] 
,