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D. Helfand (Columbia University), F. Harrison, P. Mao (Caltech)
As Steidel et al. (1999) have shown, the Hubble Deep Field North is dramatically unrepresentative of the high redshift Universe, at least with regard to the number of Lyman break galaxies in the redshift range 3-4.5. Likewise, discrepancies have been reported among the handful of deep (> 100 ksec) Chandra fields published to date; normalization differences of 40 the Deep Field South (Giacconi et al. 2000) and the SSA13 field (Mushotzky et al. 2000) lead to estimates of the fraction of the background that has been resolved which differ by 25%. Given these results, it seems prudent to investigate the magnitude of the effects cosmic variance can produce when examining small (<0.1 deg2) fields at faint (<10-14 erg cm-2 s-1) fluxes.
Using results from the thirteen moderate to deep fields included to date in our SEXSI program along with the handful of other published fields, we explore variations in source number counts as well as the magnitude distributions of optical counterparts. One of the more striking anomalies is found for a field selected because it hosted a bent-double radio source at z=0.65; this field contains more than three times the number of X-ray sources within 8' of the target than expected from the mean source surface density at this flux limit. Quantitative measures of cosmic variance will be presented, and their implications for the use of faint X-ray sources and the X-ray Background in constraining large-scale structure at intermediate and high redshifts will be discussed.