## 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.

REFERENCES:
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]
This paper is online in PostScript form on IPAC's publicftp machine:
ftp.ipac.caltech.edu
in
/pub/irsky/doc
Look for the files:
helou_beichman.ps
helou_beichman_figure1.ps
helou_beichman_figure2.ps
helou_beichman_figure3.ps
helou_beichman_figure4.ps