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The extragalactic background light (EBL) is an indicator of the total luminosity of the universe. It provides unique information on the evolution of cosmic structures at all epochs, as the cumulative emission from galactic systems and active galactic nuclei (AGNs) is expected to be recorded in this background.

Figure 2

Figure 2. The contribution of known galaxies to the extragalactic background light per magnitude bin as a function of U (filled circles), B (open circles), V (filled triangles), I (open squares) and K (filled squares) magnitudes. For clarity, the B, V, I and K values have been multiplied by a factor of 2, 8, 20, and 40, respectively.

The contribution of known galaxies to the optical EBL can be calculated directly by integrating the emitted flux times the differential galaxy number counts down to the detection threshold. The leveling off of the counts is clearly seen in Figure 2, where the function inu = 10-0.4(m+48.6) x N(m) is plotted against apparent magnitude in all bands [44]. While counts having a logarithmic slope of alpha geq 0.40 continue to add to the EBL at the faintest magnitudes, it appears that the HDF survey has achieved the sensitivity to capture the bulk of the extragalactic light from discrete sources (an extrapolation of the observed counts to brighter and/or fainter magnitudes would typically increase the sky brightness by less than 20%). To AB = 29 mag, the sky brightness from resolved galaxies in the I-band is approx 2 x 10-20 ergs cm-2 s-1 Hz-1 sr-1, increasing roughly as lambda2 from 2000 to 8000 Å. The flattening of the number counts has the interesting consequence that the galaxies that produce ~ 60% of the blue EBL have B < 24.5. They are then bright enough to be identified in spectroscopic surveys, and are indeed known to have median redshift < z > = 0.6 [32]. The quite general conclusion is that there is no evidence in the number-magnitude relation down to very faint flux levels for a large amount of star formation at high redshift. Note that these considerations do not constrain the rate of starbirth at early epochs, only the total (integrated over cosmic time) amount of stars - hence background light - being produced, and neglect the effect of dust reddening.

Figure 3 shows the total optical EBL from known galaxies together with the recent COBE results. The value derived by integrating the galaxy counts [44] down to very faint magnitude levels [because of the flattening at faint magnitudes of the number-magnitude relation most of the contribution to the optical EBL comes from relatively bright galaxies] implies a lower limit to the EBL intensity in the 0.3-2.2 µm interval of Iopt approx 12 n W m-2 sr-1. (3) When combined with the FIRAS and DIRBE measurements (IFIR approx 16 n W m-2 sr-1 in the 125-5000 µm range), this gives an observed EBL intensity in excess of 28 n W m-2 sr-1. The correction factor needed to account for the residual emission in the 2.2 to 125 µm region is probably ltapprox 2 [11]. (We shall see below how a population of dusty AGNs could make a significant contribution to the FIR background.) In the rest of this talk I will adopt a conservative reference value for the total EBL intensity associated with star formation activity over the entire history of the universe of IEBL = 40 I40 n W m-2 sr-1.

Figure 3

Figure 3. Spectrum of the extragalactic background light as derived from a compilation of ground-based and space-based galaxy counts in the U, B, V, I, and K-bands (filled dots), together with the FIRAS 125-5000 µm (solid and dashed lines) and DIRBE 140 and 240 µm (filled squares) detections [15], [25]. The empty squares show the DIRBE points after correction for WIM dust emission [29].

3 The direct detection of the optical EBL at 3000, 5500, and 8000 Å derived from HST data [3] implies values that are about a factor of two higher than the integrated light from galaxy counts. Back.

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