A deep 12 micron survey with ISO

Abstract

We present the first results of a study of faint 12 mu m sources detected with the Infrared Space Observatory (ISO) in four deep, high galactic latitude fields. The sample includes 50 such sources in an area of 0.1 square degrees down to a 5sigma flux limit of ~ 500 mu Jy. From optical identifications based on the US Naval Observatory (USNO) catalogue and analysis of the optical/IR colours and Digital Sky Survey (DSS) images, we conclude that 37 of these objects are galaxies and 13 are stars. We derive galaxy number counts and compare them with existing IRAS counts at 12 mu m, and with models of the 12 mu m source population. In particular, we find evidence for significant evolution in the galaxy population, with the no-evolution case excluded at the 3.5sigma level. The stellar population is well matched by existing models. Two of the objects detected at 12 mu m are associated with known galaxies. One of these is an IRAS galaxy at z=0.11 with a luminosity of 10(11) L_sun. 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 Kingdon) and with the participation of ISAS and NASA