The 2.3 GHz continuum survey of the GEM project

Context. Determining the spectral and spatial characteristics of th e radio continuum of our Galaxy is an experimentally challenging endeavour for improving our understanding of the astrophysics of the interestellar medium. This knowledge has also become of paramount significance for cosmology, since Galactic emiss ion is the main source of astrophysical contamination in measurements of the Cosmic Microwave Background radiation. Aims. In this paper we present the scope of the Galactic Emission Mapping (GEM) project and its results at 2.3 GHz. Its observational program was conceived and developed to reveal the large scale properties of Galactic synchrotron radiation in total int ensity and polarisation through a self-consistent set of radio contin uum surveys between 408 MHz and 10 GHz. GEM’s unique observational strategy and experimental design aim at the production of foreground templates in order to address the mutual inconsistencies between existing surveys. Methods. The GEM experiment uses a portable and double-shielded 5.5-m radiotelescope on a rotating platform to map 60 ◦ wide declination bands, from different observational sites, by circularly scanning the sky a t 30 ◦ from the Zenith. The observations at 2.3 GHz were accomplished with a total power receiver, whose front-end HEMT was matched directly to a cylindrical horn at the prime focus of a parabolic reflector. The Moon was used to cali brate the antenna temperature scale and the preparation of the map required direct subtraction and destriping algorithms to r emove ground contamination as the most significant source of systematic error. Results. For this first GEM survey, 484 hours of observations were used from two locations in Colombia and Brazil to yield a 69% sky coverage fromδ =−53 ◦ toδ = +35 ◦ with a horizontal HPBW of 2. 30 and a vertical HPBW 1. ◦ 85. The pointing accuracy was 8. 6 and the RMS sensitivity was 9.8± 1.6 mK. The zero-level uncertainty is 103 mK with a temperature scale error of 5% after direct correlation with the Rhodes/HartRAO survey at 2326 MHz on a T -T plot.