Wide-field VLBI observations of the COSMOS field

The aim of our project is to observe ∼3000 radio sources in the COSMOS extragalactic field with the Very Long Baseline Array (VLBA) at 1.4GHz. The goals are to study with statistically relevant numbers the faint radio source population, and so to complement the sensitive multiwavelength COSMOS data with milli-arcsec resolution radio images. The scientific motivation is to investigate the interplay between Active Galactic Nuclei and their host galaxies. The radio ejecta from AGN can severely impact on the way that stars are formed in galaxies - they can either heat the gas and so prevent it from collapsing into stars, or they can compress clouds of gas, thereby triggering star formation. It is therefore necessary to determine which galaxies do have radio-active AGN. Whilst there are several diagnostics, a relatively easy and direct way is a detection in a Very Long Baseline Interferometry (VLBI) observation. The reason is that the emission needed to make a detection must come from a very small volume in the target object, because the resolution is so high in VLBI observations. Here we present the survey design, observations and calibration, along with a few first results.