Confusion of extragalactic sources in the far-infrared: A baseline assessment of the performance of PRIMAger in intensity and polarization

Aims. Because of their limited angular resolution, far-infrared telescopes are usually affected by the confusion phenomenon. Since several galaxies can be located in the same instrumental beam, only the brightest objects emerge from the fluctuations caused by fainter sources. The PRobe far-Infrared Mission for Astrophysics imager (PRIMAger) will observe the mid- and far-infrared (25-235 mu m) sky both in intensity and polarization. We aim to provide predictions of the confusion level and its consequences for future surveys. Methods. We produced simulated PRIMAger maps affected only by the confusion noise using the simulated infrared extragalactic sky (SIDES) semi-empirical simulation. We then estimated the confusion limit in these maps and extracted the sources using a basic blind extractor. By comparing the input galaxy catalog and the extracted source catalog, we derived various performance metrics as completeness, purity, and the accuracy of various measurements (e.g., the flux density in intensity and polarization or the polarization angle). Results. In intensity maps, we predict that the confusion limit increases rapidly with increasing wavelength (from 21 mu Jy at 25 mu m to 46 mJy at 235 mu m). The confusion limit in polarization maps is more than two orders of magnitude lower (from 0.03 mJy at 96 mu m to 0.25 mJy at 235 mu m). Both in intensity and polarization maps, the measured (polarized) flux density is dominated by the brightest galaxy in the beam, but other objects also contribute in intensity maps at longer wavelengths (similar to 30% at 235 mu m). We also show that galaxy clustering has a mild impact on confusion in intensity maps (up to 25%), while it is negligible in polarization maps. In intensity maps, a basic blind extraction will be sufficient to detect galaxies at the knee of the luminosity function up to z similar to 3 and 10(11) M-circle dot main-sequence galaxies up to z similar to 5. In polarization for the most conservative sensitivity forecast (payload requirements), similar to 200 galaxies can be detected up to z = 1.5 in two 1500 h surveys covering 1 deg(2) and 10 deg(2). For a conservative sensitivity estimate, we expect similar to 8000 detections up to z = 2.5, opening a totally new window on the high-z dust polarization. Finally, we show that intensity surveys at short wavelengths and polarization surveys at long wavelengths tend to reach confusion at similar depth. There is thus a strong synergy between them.