For the extragalactic background radiation no generally-accepted measured values exist in the UV, optical or infrared wavebands. However, upper limits from surface photometry and lower limits from galaxy counts are available. We present a critical evaluation and tabulation of the available results.
Extragalactic background light (EBL) in UV, optical and near-IR
(
)
is thought to consist mainly of redshifted starlight from unresolved galaxies;
more hypothetical contributions would be, e.g., from stars or gas in the
intergalactic space, and from decaying elementary particles (e.g. neutrinos).
In the mid- and far-infrared the main contribution is thought to be
redshifted emission from dust particles, heated by starlight in galaxies.
Observations of the EBL are hampered by the much stronger foreground components of the night sky brightness described in the other sections. Unlike the other components the EBL is isotropic which, in combination with its weakness, complicates its separation. Recent reviews of the observational and theoretical status of the EBL have been given by Mattila (1990), Tyson (1990, 1995), Mattila et al. (1991) for the optical; by Bowyer (1991), Henry (1991), Henry & Murthy (1995) and Jakobsen (1995) for the ultraviolet; by Matsumoto (1990), Franceschini et al. (1991a), Hauser (1995a,b, 1996) and Lonsdale (1995) for the infrared; Longair (1995) has given a general review covering all wavelengths.
The observational results presented here are summarised for each wavelength range in a separate table in the corresponding subsection. They are also put together in overview in Fig. 79 (click here) at the end of this section, where in the visual and near-infrared region some model predictions are added for comparison with the data, which stretch over a wide range of brightnesses at these wavelengths. Otherwise, model prediction of EBL brightness are not the topic of this reference. For this matter see, e.g., the conference proceedings by Bowyer & Leinert (1990) and Rocca-Volmerange et al. (1991) or the work of Franceschini et al. (1991b).
Figure 79: Summary of present observational limits
and model predictions for the EBL.
The photometric upper limits by Dube et al. (DWW), Toller (T) and
Mattila &
Schnur (MS) in optical and the UV upper limit of 300 photon units
at 170 nm (see text) are shown as downward pointing arrows.
The COBE/DIRBE and COBE/FIRAS dark sky
(total) brightnesses between 1.25 
m and 500 m are shown as crosses
and open triangles, respectively.
The ranges of isotropic residuals after foreground subtraction
are shown by vertical bars for the DIRBE 1.25 m - 240 m bands
(Hauser 1996). The Mather et al. (1994) estimate
for an upper limit of possible sub-mm excess above the CMB spectrum is shown
as a dashed line between 500 m and 1000 m. The claimed tentative
detection of CIBR by Puget et al. (1996) is shown as a solid
line between 400 m and 1000 m. Solid lines at 10 m - 40
m are the possible detections from Dwek & Slavin (1994);
the upper line is for H0 = 100 km s-1 Mpc-1 and the lower one
for 50 km s-1 Mpc-1.
The results from galaxy counts are are shown with different symbols:
Cowie et al.: black squares; Tyson: solid circles; Morgan and Driver:
open circles (two values at each wavelength band, see
Table 46 (click here) and text).
In the visual range, some model calculation results are shown as well
for comparison: solid lines are after
Yoshii & Takahara (1988) for evolving galaxy models, labeled
with q0 and 
, where means the epoch (measured by
redshift) of galaxy formation; the dashed line is for a non-evolving galaxy
model with q0 = 0.02.
The two dash-dotted lines are after Väisänen (1996) for
models which include the estimated
effect of low-surface-brightness and faint blue galaxy populations:
the upper line is with Ferguson & McGaugh (1995) luminosity
function and with luminosity evolution (model labeled FMB-LE in
Väisänen 1996); the lower line is with a
luminosity function evolution in accordance with the findings of Lilly
et al. (1995), i.e. extra brightening of the blue galaxies over the
passive luminosity evolution, and an excess of a non-evolving blue
population of faint galaxies (model labeled BBG in
Väisänen 1996)
An extragalactic component is certainly present in the UV/FUV since the summed flux of galaxies is present at some level. Early in the Space Age it was realized that searches in the FUV had substantial advantages over searches in the UV, because the zodiacal light component is not present at a measurable level and contributions from stellar sources were expected to be small. In particular, it was hoped that emission from a very hot (10 6 K) or hot (105 K) intergalactic medium might be detected. These measurements were far more difficult to carry out than was originally anticipated, and a wide range of conflicting results were reported.
At this point, the most cited argument that some of the Far UV diffuse background is extragalactic in origin is that most measurements of this flux show a correlation with galactic neutral hydrogen column density, and the extrapolation to zero hydrogen columns yields fluxes that are in the range of 50 to 300 photon units. These results are only upper limits to an extragalactic background, however, since there is no guarantee that galactic components are not producing this flux.
While the total picture is far from clear, some aspects of a possible extragalactic flux have been established. Quasar absorption line studies definitely constrain emission from a diffuse intergalactic medium to a marginal role (Jakobsen 1991). Paresce & Jacobsen (1980) had shown before that integrated light from QSOs and AGNs will not produce a significant contribution to the diffuse FUV background. However, Armand et al. (1994) have used data on galaxy counts obtained at 2000 Å with a limiting magnitude of 18.5, to calculate the ultraviolet flux due to the integrated FUV light of all galaxies. The extrapolation is small and leads to an expected flux of 40 to 130 photon cm-2 s-1 sr-1Å-1. Hence it seems certain that there is at least some extragalactic flux present in the diffuse FUV background. It is interesting to note that the flux predicted by Armand et al. is consistent with the (uncertain and controversial) observational results for a possible extragalactic diffuse FUV background.
| Summed from all galaxies | 40 to 200 |
| QSOs/AGNs | <10 |
| Intergalactic medium | <10 |
| observed upper limit | 50 to 300 |
| Author(s) |  |  | |
 (revised) | Method |
| (Å\) | revised, 1 | limits erg s-1cm-2sr-1 | |||
| Dube et al. (1979) | 5115 |
1.0  1.2 S10 | 4.0 1.2 S10 | photometry | |
 3.4 S10 | 5.2 S10 | 4.0 10-5 | |||
| Toller (1983) | 4400 |
1.3 1.3  | 1.9 4.8 | photometry | |
|
3.9 | 6.7 | 3.2
10-5 | |||
| Mattila & Schnur | 4000 |
6.5 2.5 cgs* | 9.0 cgs* | 3.6 10-5
| photometry |
| (1990) | |||||
| Cowie et al. (1994) | 3400 (U') | 1.3 10-6 | galaxy counts | ||
| 4470 (B) | 1.8 10-6 | ( ) | |||
| 5425 (V) | 3.1 10-6 | ||||
| 8340 (I) | 4.7 10-6 | ||||
| 22000 (K) | 5.2 10-6 | ||||
| Tyson (1995) | 3600 (U) | 2.5(+.07 -.04) 10-6 | galaxy counts | ||
| 4500 (B) | 2.9(+.09 -.05) 10-6 |
(Bj29 / ) | |||
| 6500 (R) | 2.9(+.09 -.05) 10-6 | ||||
| 9000 (I) | 2.6(+.3 -.2 ) 10-6 | ||||
| 22000 (K) | 7.2(+1 -1 ) 10-6 | ||||
| Morgan & Driver | 4500 (B) | 1.9 10-6 | galaxy counts | ||
| (1995) | 5500 (V) | 1.3 10-6 | ( ) | ||
| 6500 (R) | 3.2 10-6 | ||||
| 9000 (I) | 3.5 10-6 | ||||
| 4500 (B) | 4.7 10-6 | galaxy counts | |||
| 5500 (V) | 6.4 10-6 | (
38) | |||
| 6500 (R) | 8.2 10-6 | ||||
| 9000 (I) | 10.0 10-6 |
A selection of upper limits from photometric measurements as well as
lower limits
obtained from galaxy counts are summarised in Table 46 (click here).
In the table, the author(s) and date of publication are given in Col. (1).
The
wavelength of observation and the 
value (or its upper limit)
as given in
the original publication are listed in Cols. (2) and (3). In Col. (4)
we give our critical revision (upper limit) of each value;
in deriving
these "revised values" we have tried to consider the effects of
some additional
uncertainties or corrections which in our opinion were not sufficiently
discussed in the original paper. In Col. (5) we give
for the revised EBL values.
The last Col. (6) gives the method used.
Three surface photometric measurements are included in Table 46 (click here):
(1) Dube et al. (1979) observed the total night
sky brightness from the ground in eleven high-latitude fields.
As a mean value of the 11 fields Dube et al. gave an EBL+DGL value of
. Because it was not possible to estimate the DGL
contribution
the result was interpreted as a 2 upper limit to the EBL of
3.4 S10 or 5.1 10-9 ergs cm-2 s-1 sterad-1 Å-1. A basic problem with this method is that it
starts with the total night-sky brightness which is a factor of 
100
brighter than the EBL. Thus, very accurate measurements of the absolute
intensities of ZL and airglow are required. The most critical point in the
data analysis of Dube et al. was the way they corrected for the airglow.
They assumed that airglow is a linear function of sec z and used
linear
extrapolation to sec z = 0 to eliminate airglow.
This method is doubtful
since the sec z - dependence of the airglow is not strictly
linear but follows the so-called van Rhijn's (1921) law.
Mattila et al. (1991) have reanalysed
the airglow problem using, as far as possible, the observational values
given in Dube et al. (1979) and in Dube (1976).
They have thus found that Dube et al. probably have
overestimated the airglow intensity by 3 S10. Thus the
residual value for EBL + DGL should be increased by this amount, resulting
in 
= 4.0 1.2 S10 or an 
upper limit
of 5.2 S10.
(2) Toller (1983) utilized measurements of a photometer
aboard Pioneer 10 as it moved out of the zodiacal dust cloud (
). From these he subtracted integrated starlight and
gave a value for the average brightness of the diffuse background
light of
= 3.3 1.2
He estimated 
to be 2.0 0.4
. As a final
result Toller thus obtained an EBL intensity of 1.3 1.3
which he expressed as a 
upper limit of
3.9 .
Since Toller's EBL value has been frequently cited as the EBL reference
value, it deserves a detailed discussion
of errors.
The basic problem for his EBL determination is the large field
of view (
) of the photometer. Thus, the starlight
entered with full weight into the measured sky brightness, and in order to
derive the small residual
EBL one must know the ISL very accurately in the Pioneer 10 photometric
system. This was not fully the case. The ISL values of Roach & Megill
(RM, 1961) and Sharov & Lipaeva (SL, 1973)
are based on the Harvard-Groningen
(Pickering et al. 1918, 1923, 1924; van Rhijn
1929) and Mount Wilson starcounts (Seares et al. 1930)
the magnitudes of which were calibrated by using photographic techniques.
Sharov & Polyakova (1972) have shown that the
Harvard-Groningen photographic magnitude scales are in need of positive
corrections of as much as 0.4 mag to 0.5 mag in order to reduce stars of 7
to 16 mag from 
to the B system. In their ISL summation SL
tried to take these photometric corrections into account and thus their ISL
values should be given the preference over the RM values.
Then an average of 3.9 , instead of
3.3 , is obtained. There is a
the remaining systematic
error of the Sharov and Lipaeva ISL values due to the
scale errors which is at least 15%.
With an average ISL value
of 25 this amounts to 3.8 .
The systematic error of the Pioneer 10
photometry itself (e.g. due to calibration) has been given as 8%
(Weinberg & Schuerman 1981), which for 25
corresponds to 2.0 .
A further uncertainty of 1.6 results from
variations in the cutoff for bright stars. The total error resulting from
quadratically adding the systematic and statistical errors then
is 4.8 .
Thus we end up with a revised EBL value of
= 1.9 4.8 , which corresponds to
2.3 5.7 10-9 ergs cm-2 s-1 sterad-1 Å-1
or to a one upper limit of 8.0 10-9 ergs cm-2 s-1 sterad-1 Å-1.
(3) Mattila & Schnur (1990), on the basis of their
observations in the dark cloud area L1642, have presented a preliminary
estimate for the EBL of 6.5 2.5 10-9 ergs cm-2 s-1 sterad-1 Å-1.
In this method an opaque dark cloud is used as a zero point where
the EBL is negligible or at least much smaller than in the transparent
surroundings of the cloud. Starlight, zodiacal light and the atmospheric
components are eliminated in the differential surface brightness
measurements used in this method. The main problem
is the elimination of the scattered light (DGL) which is present
both in the opaque parts of the cloud as well as in its (semi)transparent
surroundings. In view of the preliminary character of the above-mentioned
value we prefer to interpret
it as an upper limit, 9.0 10-9 ergs cm-2 s-1 sterad-1 Å-1.
Deep galaxy counts by Cowie et al. (1994), Tyson
(1995) and Morgan & Driver (1995) have provided
estimates for the contribution of galaxies to the EBL.
These lower limits to the EBL are given in
Table 46 (click here) for several wavelength bands between 3400 Å and 2.2 m.
(1) The EBL values of Cowie et al. are for a K-band-limited sample with
22 mag. The K-band counts have a slope 
= 0.26 at K = 22 mag which implies that the surface
brightness contribution per magnitude interval is converging. Contrary to
this the B-band counts have a divergent slope = 0.45 (Gardner et al. 1993). Thus it is
suggested that the total EBL at U to visual wavelengths may be
substantially higher than the values given in the Table.
(2) The EBL values of Tyson are for a sample to
an isophotal limiting magnitude of 29 Bj magnitude arcsec-2. The limiting
magnitudes of the counts are 27 mag at Bj, 26 mag at R,
and 24 mag at I. Galaxies fainter
than
20 mag at Bj contribute about 75% of the EBL at 4500 Å.
Most of the EBL flux originates from galaxies around B = 24 mag.
(3) The EBL values of Morgan & Driver (1995) are from counts
with limiting magnitudes of B = 26 mag, V = 24.5 mag, R = 26 mag and
I = 22.5 mag. Morgan & Driver adopted two approaches in estimating the
EBL: firstly they used direct observations of the number counts to sum up
the EBL to the limiting magnitude; secondly they used a dwarf-dominated
luminosity function to extrapolate the number counts down to a limiting
magnitude of = 38 mag. The EBL values for the second case
are seen to be a factor of 2 to 5 higher than for the first case. This
gives an impression of the possible importance of the contribution by
very faint galaxies, 
30 mag, to the EBL.
|
|  | Reference | | Reference | Ref. | |
|
m | nW m-2 sr-1 | nW m-2 sr-1 | ||||
| 1.25 | 393 | 13 | DIRBE dark sky | 50 - 104 | DIRBE residual | 1 |
| 2.2 | 150 | 5 | '' | 15 - 26 | '' | 1 |
| 3.5 | 63 | 3 | '' | 15 - 24 | '' | 1 |
| 4.9 | 192 | 7 | '' | 9 - 22 | '' | 1 |
| 12 | 2660 | 310 | '' | 102 - 164 | '' | 1 |
| 25 | 2160 | 330 | '' | 136 - 210 | '' | 1 |
| 60 | 261 | 22 | '' | 31 - 42 | '' | 1 |
| 100 | 74 | 10 | '' | 20 - 35 | '' | 1 |
| 140 | 57 | 6 | '' | 12 - 63 | '' | 1 |
| 240 | 22 | 2 | '' | 8 - 33 | '' | 1 |
| 111 | 108 | FIRAS dark sky | 1 | |||
| 143 | 63 | '' | 1 | |||
| 250 | 30 | '' | 1 | |||
| 500 | 6 | '' | 1 | |||
| 500-5000 | 680/(m) | FIRAS residual | 2 | |||
| 400-1000 | 3.4 (/400 m)-3 | FIRAS residual | 3 | |||
| 10 - 40 | 6 h (/m)0.55 |  -ray method | 4 | |||
The Diffuse Infrared Background Experiment (DIRBE) aboard
the Cosmic Background Explorer (COBE) was designed
to perform a sensitive search
for the Cosmic Infrared Background Radiation (CIBR).
Special care was paid in the design to supression
of stray radiation. During the 10-month cryogenic operation of
COBE in 1989 - 1990 DIRBE mapped the whole sky with high redundancy
in ten wavelength bands between 1.25 and 240 m. DIRBE is
completely free from any residual atmospheric effects or
contamination by rocket exhaust which have made many of the
previous balloon or rocket borne experiment results problematic.
The main obstacles for a determination of the CIBR are
the remaining strong foreground components which contribute to the infrared
sky brightness with varying importance over the whole wavelength
region. As detailed in previous section, these
are the zodiacal light, the light of
unresolved stars and the thermal emission by interstellar dust (cirrus).
Since there is no distinct spectral signature known in the CIBR, the
separation of the foreground components has to be based on modelling
of their different spatial or broad band spectral distributions.
In the case of the zodiacal component also its temporal variation
during a year can be utilized. This modelling process is still
being worked on by the DIRBE team. The most recent, still preliminary
results have been presented by Hauser (1996). They are
reproduced in Table 47 (click here) for the ten DIRBE bands as well as for
selected wavelengths based on FIRAS data. The first column of results gives
the upper limits on the CIBR based on the darkest spots
observed on the sky. Because no foreground components were subtracted,
these values are stringent upper limits to any isotropic component
of the infrared sky brightness. In
the second column of results the range of DIRBE
sky brightness residuals at high galactic latitude after subtraction
of "best models" currently available for zodiacal light and emission,
starlight and interstellar cirrus are given. These numbers, as
emphasized by Hauser (1996), are still preliminary. The
uncompleteness of the foreground modelling is reflected in the fact that
the spectrum of the residual brightness at mid-IR wavelengths shows a
resemblance with the
zodiacal light spectrum.
For ease of comparison, part of Table 6 on total infrared
sky brightness in dark spots is repeated here in the left part of the table.
Kashlinsky et al. (1996) have tried to convert the smoothness
of the spatial distribution of DIRBE
light into upper limits to a CIBR radiation
component coming from clustered matter evolving according to standard
scenarios. They find that the upper limits to such a component
between 1.25 m - 100 m are by a factor of 4 to 100 lower
than the Hauser et al. (1996) residual isotropic brightnesses
given in Table 47 (click here). These values have to be taken with caution,
however, since their derivation is strongly model-dependent.
Using COBE/FIRAS data Mather et al. (1994) have estimated that
the CMB spectrum between 0.5 mm and 5 mm deviates from a 2.726 K blackbody
shape by less than 0.03% of the peak intensity. Taking twice this amount
as an upper limit to an additional CIBR implies
W m-2 sr-1.
Puget et al. (1996) have claimed a tentative detection of
far-IR CIBR using COBE/FIRAS data. They have modelled and removed the
foreground components above 140 m. For estimating the interstellar
cirrus emission they used its correlation with HI 21-cm data, and for
zodiacal emission its spectral and spatial distribution as
determined at shorter wavelengths, 
100 m.
The residual isotropic component claimed for the 400 m - 1000
m range can be represented by
W m-2 sr-1.
An indirect method for measurement of the mid-IR CIBR
is based on the spectra of -ray sources, since -rays
interact with intergalactic IR-photons by pair production, giving rise
to energy-dependent extinction. A recent
application gives, for 
, the result
W m-2 sr-1 (Dwek & Slavin 1994). The result depends
on the Hubble constant h = H0/100 km s-1 Mpc-1. This estimate
is by a factor of 10 lower that the DIRBE isotropic residuals at
10 and 25 m, but fits nicely to the DIRBE isotropic residuals at
shorter and longer wavelengths (see Fig. 79 (click here)). Again, there are
uncertainties in this method, since the intrinsic high energy gamma ray
spectra before attenuation by interaction with the cosmic infrared radiation
field are not really known.
Figure 79 (click here) summarises the current observational limits to the
extragalactic background light in the wavelength range from 0.1 m to
1000 m. In the visual and near-infrared range,
where discrepancies between different methods of
determination are particularly large, we also plot
a few selected model predictions for comparison, without the intent
to discuss them here. Compared to the situation ten years ago,
the gap between upper limits from
direct measurements, lower limits from galaxy counts, and model
predictions is getting smaller, being mostly less than a factor of ten by now.
A comparison with the total sky brightness values shown in Fig. 1,
which are typically brighter by two orders of magnitude, is informative.
In this comparison please note that and
are identical units of brightness.
Acknowledgements
We are grateful to M. Cohen for providing the estimates of integrated starlight shown in Figs. 61 and 62 and in Tables 24 and 29. We thank P. Feldman, R. MacQueen, H. Kimura and H. Lauche for helpful advice and discussions and M. Rowan-Robinson for supplying his infrared sky maps in digital form. We are grateful to P. Väisänen, who contributed the first and the last figure of this paper, and we very much thank D. Anders, K. Meissner-Dorn, and in particular M. Weckauf for the careful and patient work in producing or making computer-readable most of the numerous figures of this paper.