Fluctuations in the Extragalactic Background Light: Analysis of the Hubble Deep Field

Statistical analysis of the unresolved light in the Hubble Deep Field (HDF) strongly constrains possible sources of the optical Extragalactic Background Light (EBL). This constraint is crucial for determining the spectrum of the EBL because reported upper limits on the optical EBL are several times larger than the surface brightness from detected galaxies, suggesting the possibility of additional galaxy populations. To test for the statistical signature of previously undetected sources, we estimate the auto, cross, and color correlations of the “sky” in the HDF that remains after masking objects brighter than I814 = 30mag. Auto and cross correlations of surface brightness in the V606 and I814 bandpasses are well-fitted by ω(θ) ∼ 10 (θ/1) up to 10. Probable contributions of several instrumental systematics ensure that these correlations are firm upper limits on the true EBL fluctuations. This measurement yields the most stringent limits to date on small-scale structure in the night sky; analysis of shallower imaging would be dominated by galaxies now detected by the HDF. Unless there is a truly uniform optical background, the mean EBL is likely to be within a small fraction of the surface brightness from detected galaxies. No currently plausible sources of additional EBL satisfy the constraints that they (1) would not have already been detected, (2) contribute EBL comparable to that from detected galaxies, and (3) do not produce EBL fluctuations in excess of the upper limits set by correlations in the HDF. These constraints admit only 1 Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, under NASA contract NAS 5-26555