IRSKY Extragalactic Estimation

Estimating the Extragalactic Confusion Noise

The ISSA plates are of insufficient resolution to directly compute the confusion due to extragalactic sources in typical size beams. To estimate the confusion noise contributed by the background of faint galaxies we use a set of modeled galaxy source counts to estimate the statistical distribution of sources within an aperture. Then we use the variance of the total flux expected from such a sample as an estimate of the confusion noise. The method is detailed in Helou and Beichman, 1990.

In this method, source counts are taken from the analytic fits to infrared galaxy counts (Soifer et al., 1987) and are extrapolated to two sets of functions of source counts vs. limiting flux. Each set contains functions computed for wavelengths which cover the ISO wavelengths range). The two sets of functions differ in that each set is computed under a different cosmological model so as to bracket the results of the calculation. The program finds the smallest flux at which the flux variance of the faint source background will contribute an uncertainty of less than 20%, and equates this variance to the confusion noise. The two bracketing cosmological models are: Model 1: q0=0.5, H0=100, and source density~(1+z)^5; and Model 2: Euclidean.

Desert, F-X., Boulanger, F., and Puget, 
J., 1990, Interstellar Dust Models for 
Extinction and Emission, A & A 237, 215.

Gautier, T. N. et al. 1992, A 
Calculation of Confusion Noise Due
to Infrared Cirrus, AJ, 103 1313.
[provides details of the cirrus 
confusion calculation]

Helou and Beichman 1990, Proc. 29, 
Liege Intn.  Astr. Coll., ESA SP 314.
[provides an overview of the cirrus 
confusion calculation]

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