Deep far infrared ISOPHOT survey in ``Selected Area 57". I. Observations and source counts

Abstract

We present here the results of a deep survey in a 0.4 deg² blank field in Selected Area 57 conducted with the ISOPHOT instrument aboard ESAs Infrared Space Observatory (ISO at both 60 mu m and 90 mu m. The resulting sky maps have a spatial resolution of 15 x 23 arcsec² per pixel which is much higher than the 90 x 90 arcsec² pixels of the IRAS All Sky Survey. We describe the main instrumental effects encountered in our data, outline our data reduction and analysis scheme and present astrometry and photometry of the detected point sources. With a formal signal to noise ratio of 6.75 we have source detection limits of 90 mJy at 60 μm and 50 mJy at 90 μm. To these limits we find cumulated number densities of 5+/- 3.5 deg^-2 at 60 μm and 14.8+/- 5.0 deg^-2 at 90 μm. These number densities of sources are found to be lower than previously reported results from ISO but the data do not allow us to discriminate between no-evolution scenarios and various evolutionary models. Based on observations with ISO, an ESA project with instruments funded by ESA member states (especially the PI countries: France, Germany, the Netherlands, and the United Kingdom) and with the participation of ISAS and NASA.