GLOSTAR — Radio Source Catalog I: 28◦ < ℓ < 36◦ and |b| < 1◦⋆

Context. Radio continuum surveys of the Galactic plane are an excellent way to identify different source populations such as planetary nebulae, H ii regions, and radio stars and characterize their statistical properties. The GLOSTAR survey will study the star formation in the Galactic plane between −2◦ < ℓ < 85◦ and |b| < 1◦ with unprecedented sensitivity in both, flux density (∼40 µJy beam−1) and range of angular scales (∼1.′′5 to the largest radio structures in the Galaxy). Aims. In this paper we present the first results obtained from a radio continuum map of a 16 square degree sized region of the Galactic plane centered on ℓ = 32◦ and b = 0◦ (28◦ < ℓ < 36◦ and |b| < 1◦). This map has a resolution of 18′′ and a sensitivity of ∼60-150 µJy beam−1. Methods. We present data acquired in 40 hours of observations with the VLA in D-configuration. Two 1 GHz wide sub-bands were observed simultaneously and they were centred at 4.7 and 6.9 GHz. These data were calibrated and imaged using the Obit software package. The source extraction has been performed using the BLOBCAT software package and verified through a combination of visual inspection and cross-matching with other radio and mid-infrared surveys. Results. The final catalog consists of 1575 discrete radio sources and 27 large scale structures (including W43 and W44). By crossmatching with other catalogs and calculating the spectral indices (S (ν) ∝ να), we have classified 231 continuum sources as H ii regions, 37 as ionization fronts, and 46 as planetary nebulae. The longitude and latitude distribution and negative spectral indices are all consistent with the vast majority of the unclassified sources being extragalactic background sources. Conclusions. We present a catalog of 1575 radio continuum sources and discuss their physical properties, emission nature and relation with previously reported. These first GLOSTAR results have increased the number of reliable H ii regions in this part of the Galaxy by a factor of four.