Continuum sources from the THOR survey between 1 and 2 GHz^⋆

¹ Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: name@mpia.de
² School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
³ Department of Physics and Astronomy, West Virginia University, Morgantown, WV 26506, USA
⁴ National Radio Astronomy Observatory, PO Box O, 1003 Lopezville Road, Socorro, NM 87801, USA
⁵ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁷ Department of Astronomy, University of Massachusetts, Amherst, MA 01003-9305, USA
⁸ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁹ Research School of Astronomy and Astrophysics, The Australian National University, Canberra, ACT, Australia
¹⁰ Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada
¹¹ Centre for Astrophysics and Planetary Science, University of Kent, Canterbury CT2 7NH, UK
¹² International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987, Perth WA 6845, Australia

Received: 4 November 2015
Accepted: 30 December 2015

Abstract

We carried out a large program with the Karl G. Jansky Very Large Array (VLA): “THOR: The H i, OH, Recombination line survey of the Milky Way”. We observed a significant portion (~100 deg²) of the Galactic plane in the first quadrant of the Milky Way in the 21 cm H i line, 4 OH transitions, 19 radio recombination lines, and continuum from 1 to 2 GHz. 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 ~0.3−1 mJy beam^-1. 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 2 GHz, 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 H ii 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.