Constraining the LogN-LogS Relationof Far-Infrared UnresolvedSources Using a P(D) Analysis; Application to FIRBACK Maps Obtained by ISO

Abstract

The FIRBACK survey is a deep imaging survey carried out with ISOPHOTon board ISO, at 170 μm. The survey covers ≈4 sq deg inthree high galactic latitude fields (FIRBACK South Marano,FIRBACKNorth 1 and FIRBACK/ELAIS North 2). In these fields, fluctuationsdue to unresolved extra-galactic sources were isolated for the firsttime [1]. We are using a P(D) (probability distribution of fluctuations) analysis to gaininformation about the N(≥S) curve of the unresolved sources whichmake up the background. Many plannedsurveys (e.g. SIRTF, FIRST, PLANCK...) will pose the same analysischallenges and we hope that the outcome of this study will be usefulin that context. As a preparation to the analysis of the FIRBACK maps we analyzesimulated sky maps. We create histograms of number of pixels per flux bin. The histograms arebinned such that there are enough pixels per bin to allow us to applyGaussian estimates for the errors. We then calculate the expected number ofpixels within the relevant flux ranges andevaluate the χ² statistic. We will use a maximum likelihoodalgorithm to find the parameters of N(≥S).