Annu. Rev. Astron. Astrophys. 1991. 29: 59-88
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4.2 Observational Results

Given the difficulty of measuring weak diffuse radiation intermixed with stellar signals, it is not surprising that there have been relatively few attempts to obtain spectra of the far ultraviolet background, nor is it surprising that the results obtained have been disparate. Spectral measurements made by Henry et al. (1978) obtained with a spectrometer flown on Apollo 17 during Moon-Earth coast are shown in Figure 3. These data are from four view directions at high Galactic latitudes. Two sets of error bars, based on two different analyses of the data, are shown. A broad hump in the spectrum is apparent.

Figure
 3
Figure 3. Spectral measurements reported by Henry et al. (1978) obtained with a spectrometer flown on Apollo 17 during Moon-Earth coast. These data are from four view directions at high Galactic altitude. Two sets of error bars are shown based on two different analyses of the data.

The results of Henry et al. (1978) received initial support from the observations by Hua et al. (1978), who obtained data on the diffuse far ultraviolet background with a spectrometer flown on a Soviet spacecraft in a highly elongated high-apogee (200,000 km) orbit. These results are shown in Figure 4; the upper curve shows the sky background uncorrected for stars, and the lower curve shows the background after subtraction of a computed stellar contribution.

Figure
 4
Figure 4. Observations by Hua et al (1978) who obtained data on the diffuse far ultraviolet background with a spectrometer flown on a Soviet spacecraft in a highly elongated, high apogee (200,000 km) orbit. The upper solid curve shows the sky background uncorrected for stars, and the lower solid curve shows the background after subtraction of the computed stellar contribution, which is shown as a dashed line.

Anderson et al. (1979) reported data obtained with a complement of instruments flown on an Aries sounding rocket that observed three regions at high Galactic latitude. The data from the spectrographic instrument are shown as solid circles in Figure 5 (the remaining points refer to previous results). The spectrum obtained is essentially flat over most of the band, with an increase longward of ~ 1700 Å.

Figure
 5
Figure 5. Data obtained by Anderson et al (1979) with instruments flown on an Aries sounding rocket that observed three regions at high Galactic latitude. The data from the spectrographic instrument are shown as solid circles in this figure. (The remainign points refer to previously reported results.)

Following the suggestion of Jakobsen and Paresce (1981) that a hot Galactic corona will produce emission lines in the far ultraviolet, Feldman, Brune, and Henry (1981) reanalyzed the data from two of the three view directions observed by Anderson et al. (1979), using a different method of analysis. Their results are shown in Figure 6. The two peaks show the response of the spectrometer to terrestrial oxygen line emission at 1304 and 1356 Å. The dashed, dotted, and solid lines are estimates of various contributions to the background not made by diffuse radiation. Intensity enhancements in the data after subtraction of these components were considered by Feldman et al. as possible emission from forbidden NIV at 1490 Å, CIV at 1550 Å, and forbidden OIII at 1660 Å. However, the intensities obtained were approximately two orders of magnitude larger than that predicted by Jakobsen and Paresce (1981); Edgar and Chevalier (1986) concluded these intensities were at least a factor of 5 larger than could be present without violating other observational constraints. In the light of these analyses and in consideration of further experimental work discussed below, I conclude that these results were spurious.

Figure
 6
Figure 6. Data from two of the three view directions observed by Anderson et al (1979) and reanalyzed by Feldman et al (1981) using a different method of analysis in order to search for line emission. The two peaks show the response of the spectrometer to terrestrial oxygen line emission at 1304 and 1356 Å. The dashed, dotted and solid lines are estimates of various contributions to the background not due to diffuse radiation. Intensity enhancements in the data after subtraction of these components were considered by Feldman et al as possible emission from forbidden NIV at 1490 Å, CIV at 1550 Å, and OIII] at 1660 Å

Murthy et al. (1989) reported ~ 17 Å spectra from the Johns Hopkins UVX experiment flown on the Space Shuttle covering the wavelength range from 1200 to 1700 Å. They found a spectrally flat background over this band, with no evidence for the lines suggested by Feldman, Brune, and Henry (1981); however, because of the sensitivity of their measurement, they could not formally rule out lines of this intensity.

Martin and Bowyer (1990) reported the discovery of emission lines of CIV and OIII in several high Galactic latitude view directions with the Berkeley UVX far ultraviolet spectrometer and OIV/SiIV and NIII line detections in a summed high latitude spectrum. In Figure 7, I show data obtained in one of several high latitude view directions. The dashed line shows the best fit solution; the dotted line shows the assumed continuum. CIV emission is clearly evident in the data (formally, it is present at 8 sigma). The intensities observed were roughly those predicted by Jakobsen and Paresce (1981) and Edgar and Chevalier (1986) and are about two orders of magnitude below those reported by Feldman, Brune, and Henry (1981). These flux levels were sufficiently low that they would not have been detected by the Johns Hopkins instrument, which was taking data in the same view direction simultaneously (Murthy et al. 1989).

Figure
 7
Figure 7. A subset of the data obtained by Martin & Bowyer (1989) with the Berkeley UVX spectrometer from a high Galactic latitude view direction. The dashed line shows the best fit solution to the data; the dotted line shows the assumed continuum. These authors attribute the emission to CIV 1550 Å. Statistically significant detections of the CIV line were obtained in several high latitude view directions.

Martin and Bowyer combined their emission results with absorption data obtained with the IUE instrument and showed that the flux emanated from 2 to 3 kpc above the galactic plane. The data are not compatible with photoionized Galactic halo models, but they are consistent with the Galactic fountain model of Shapiro and Field (1976). The mass infall rate derived from these data is 6-25 Msmsun yr-1, confirming that the halo must be continuously replenished.

Martin, Hurwitz, and Bowyer (1990) found molecular hydrogen emission in four view directions with the Berkeley UVX experiment. Molecular hydrogen had been detected previously in two very high density cloud complexes with nearby OB stars (Brown et al. 1981; Witt et al. 1989) but had not been observed in the diffuse interstellar medium. All of the four detections were in view directions with only modest neutral hydrogen column densities (~ 1021 HI cm-2), but three of the four view directions included molecular clouds, as indicated by the presence of CO emission. Analysis of these data indicated the molecular hydrogen was clumped with a filling factor of < 0.2. Evidence was presented that in one case the cloud appeared to be in the process of being destroyed by the radiation field.

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