2016A&A...588A..97B

We carried out a large program with the Karl G. Jansky Very Large Array (VLA): "THOR: The HI, OH, Recombination line survey of the Milky Way". We observed a significant portion (∼100deg²) of the Galactic plane in the first quadrant of the Milky Way in the 21cm HI line, 4 OH transitions, 19 radio recombination lines, and continuum from 1 to 2GHz. In this paper we present a catalog of the continuum sources in the first half of the survey (l=14.0-37.9° and l=47.1-51.2°, |b|<=1.1°) at a spatial resolution of 10-25'', depending on the frequency and sky position with a spatially varying noise level of ∼0.3-1mJy/beam. The catalog contains ∼4400 sources. Around 1200 of these are spatially resolved, and ∼1000 are possible artifacts, given their low signal-to-noise ratios. Since the spatial distribution of the unresolved objects is evenly distributed and not confined to the Galactic plane, most of them are extragalactic. Thanks to the broad bandwidth of the observations from 1 to 2GHz, we are able to determine a reliable spectral index for ∼1800 sources. The spectral index distribution reveals a double-peaked profile with maxima at spectral indices of α≃-1 and α≃0, corresponding to steep declining and flat spectra, respectively. This allows us to distinguish between thermal and non-thermal emission, which can be used to determine the nature of each source. We examine the spectral index of ∼300 known HII regions, for which we find thermal emission with spectral indices around α≃0. In contrast, supernova remnants (SNR) show non-thermal emission with α≃-0.5 and extragalactic objects generally have a steeper spectral index of α≃-1. Using the spectral index information of the THOR survey, we investigate potential SNR candidates. We classify the radiation of four SNR candidates as non-thermal, and for the first time, we provide strong evidence for the SNR origin of these candidates.