Annu. Rev. Astron. Astrophys. 2001. 39: 249-307
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3.9. UV-Optical Background Observations

In order to relate the energy in the infrared background with that in the closely related UV-optical background (Section 4.1), we include a brief summary of UV-optical background measurements (Table 4). Measurement of the extragalactic background at these wavelengths is also extremely difficult, and numerous conflicting results have been reported. The reader is referred to reviews by Bowyer (1991), Henry (1991, 1999), Leinert et al. (1998) for more information.

Table 4. UV-optical extragalactic background light a

lambda nu Inu Comments Reference
(µm) (nW m-2 sr-1)

0.10 <1.2 (0 ± 0.6) Voyager UVS Murthy et al. 1999
0.10 <11 Voyager UVS Edelstein et al. 2000
0.1595 <14 (10 ± 2) HST/STIS Brown et al. 2000
0.165 <7.0 (5.6 ± 0.7) Shuttle UVX Martin et al. 1991
0.300 12 ± 7 HST & LCO Bernstein 1999
0.4000 37 ± 11 "Dark cloud" method Mattila 1976
0.4000 <46 (26 ± 10) "Dark cloud" method Mattila 1990
0.41 <10 OAO-2/WEP Lillie 1968
0.4400 <20 (7 ± 7) Pioneer 10 Toller 1983
0.4400 <60 (10 ± 25) Pioneer 10 Toller 1983b
0.44 <26 (1sigma "Dark cloud" method Spinrad & Stone 1978
0.5115 <26 (8 ± 9) Ground-based photometer Dube et al. 1979
0.5115 <48 (30 ± 9) Ground-based photometer Dube et al. 1979b
0.53 <33 Ground-based photometer Roach & Smith 1968
0.555 17 ± 7 HST & LCO Bernstein 1999
0.814 24 ± 7 HST & LCO Bernstein 1999

a Error bars are 1sigma. Upper limits are 2sigma unless otherwise noted. Values in parentheses are measurements and their 1sigma uncertainties. See text for abbreviations.
bValue revised according to Leinert et al. (1998).

The major obstacle to measurement of the extragalactic optical background, as in the infrared, is the bright foreground radiation. From the ground, the dominant sources are atmospheric airglow and zodiacal light. Additional foreground sources include starlight and diffuse Galactic light: optical light scattered by interstellar dust. Most of the early investigations reported upper limits. Dube et al. (1979) ingeniously assessed the zodiacal light contribution by measuring the sky brightness in and outside of Fraunhofer lines in the solar spectrum. Lillie (1972) used the Wisconsin Experiment Package (WEP) aboard the Orbiting Astronomical Observatory 2 to observe above the atmosphere. Pioneer 10 measurements reported by Toller (1983) were taken at 3 AU from the Sun, avoiding both of the strong diffuse foreground sources. However, Toller's results were limited by the starlight contribution in the large field of view of the photometer (2.3° × 2.3°).

Mattila (1976) introduced the novel technique of using a Galactic dark cloud as an opaque screen to chop on and off the extragalactic background, thus canceling most of the bright foreground contributions to the sky brightness. However, diffuse Galactic light scattered from the cloud must still be removed. Mattila initially reported detection of the optical background light with this technique. However, his results are brighter than the upper limits set by other investigators, including Spinrad & Stone (1978), who used the same technique. Mattila continued his measurements and subsequently reported an upper limit close to the value of his earlier detection, which he attributed to Mattila & Schnur (see Mattila 1990; see also Leinert et al. 1998). Leinert et al. (1998) discuss systematic corrections to both the Toller (1983), Dube et al. (1979) results (Table 4).

Bernstein (1999) has described a potentially significant new result in a preliminary report on detection of the diffuse EBL at 0.3, 0.55, and 0.8 µm. These results were obtained by combining absolute sky brightness measurements made with the HST Wide Field/Planetary Camera 2 (WFPC2) with simultaneous spectrophotometry from the duPont telescope at Las Campanas Observatory (LCO) and the HST Faint Object Spectrograph. The zodiacal light contribution was determined from the depth of the Fraunhofer lines in the sky background. After excluding light from detected stars and a correction for diffuse Galactic light, these measurements yielded the EBL due to extragalactic sources fainter than VAB = 23, the brightest sources statistically well-represented in the small HST field of view (5 arcmin2). Bernstein added the integrated light from galaxy counts brighter than this limit to obtain the total optical background shown in Table 4. This claimed detection of the EBL must be regarded as tentative since the uncertainties are substantial and the background has not been shown to be isotropic.

Bernstein also reported lower limits on the EBL determined by a simplified aperture photometry method applied to the ensemble of detected galaxies in the WFPC2 field in the range 23 < VAB < 28. Bernstein added the light from counts of galaxies with VAB < 23 to obtain the 2 sigma lower limits to the total resolved galaxy light shown in Table 3. Even though the Bernstein lower limits include only the light from detected faint galaxies, they are generally higher than the results of Madau & Pozzetti (2000) (Table 3), a result Bernstein attributes to the systematic underestimation of individual galaxy brightnesses in traditional aperture photometry.

The EBL has proven to be difficult to determine at UV wavelengths. Observations in the FUV have the advantage that the zodiacal light is not significant (Henry 1991; Figure 3). However, terrestrial airglow and Galactic sources remain a challenge. Martin et al. (1991) used a spectrometer on the Shuttle UVX mission to measure the FUV (1400-1900 Å) background by studying several directions with low H I column density. Since Galactic sources could contribute much of this background, they regarded their result as a firm upper limit to the EBL. Brown et al. (2000) measured the FUV (1450-1900 Å) background using the STIS instrument on HST. Excluding resolved sources and selecting data only from the night side of the orbit to eliminate airglow, they measured the diffuse background along a line of sight with low H I and extinction. Their result is somewhat brighter than that of Martin et al. and must also be regarded as an upper limit to the EBL (Table 4). Murthy et al. (1999) analyzed 17 years of data on the diffuse FUV (912-1100 Å) background from the Voyager Ultraviolet Spectrometer, obtaining a very low 1sigma upper limit on the isotropic extragalactic continuum. However, Edelstein et al. (2000) analyzed the same data, concluding that systematic uncertainties dictate a substantially higher upper limit to the EBL (Table 4). Clearly, determination of the extragalactic UV background remains a difficult problem.

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