Radio continuum emission in the northern Galactic plane: Sources and spectral indices from the THOR survey^⋆

¹ Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: wang@mpia.de
² School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
³ National Radio Astronomy Observatory, NM, PO Box O, 1003 Lopezville Road, 87801 Socorro, USA
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Department of Physics and Astronomy, West Virginia University, 26506, Morgantown, WV, USA
⁶ Adjunct Astronomer at the Green Bank Observatory, Green Bank, PO Box 2, 24944 WV, USA
⁷ Center for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, WV, 26505 Morgantown, USA
⁸ School of Physical Sciences, University of Kent, Ingram Building, Kent CT2 7NH Canterbury, UK
⁹ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
¹⁰ Universität Heidelberg, Interdisziplinäres Zentrum fur Wissenschaftliches Rechnen, INF 205, 69120 Heidelberg, Germany
¹¹ Research School of Astronomy and Astrophysics, The Australian National University, ACT, Canberra, Australia
¹² Astrophysics Research Institute, Liverpool John Moores University, Liverpool Science Park, IC2, 146 Brownlow Hill, Liverpool L3 5RF, UK

Received: 14 June 2018
Accepted: 16 August 2018

Abstract

Context Radio continuum surveys of the Galactic plane can find and characterize H II regions, supernova remnants (SNRs), planetary nebulae (PNe), and extragalactic sources. A number of surveys at high angular resolution (≤25^″) at different wavelengths exist to study the interstellar medium (ISM), but no comparable high-resolution and high-sensitivity survey exists at long radio wavelengths around 21 cm.

Aims. Our goal is to investigate the 21 cm radio continuum emission in the northern Galactic plane at < 25″ resolution.

Methods We observed a large percentage of the Galactic plane in the first quadrant of the Milky Way (l = 14.0−67.4° and |b| ≤ 1.25°) with the Karl G. Jansky Very Large Array (VLA) in the C-configuration covering six continuum spectral windows (SPW). These data provide a detailed view on the compact as well as extended radio emission of our Galaxy and thousands of extragalactic background sources.

Results We used the BLOBCAT software and extracted 10 916 sources. After removing spurious source detections caused by the side lobes of the synthesized beam, we classified 10 387 sources as reliable detections. We smoothed the images to a common resolution of 25^″ and extracted the peak flux density of each source in each SPW to determine the spectral indices α (assuming I(ν) ∝ ν^α). By cross-matching with catalogs of H II regions, SNRs, PNe, and pulsars, we found radio counterparts for 840 H II regions, 52 SNRs, 164 PNe, and 38 pulsars. We found 79 continuum sources that are associated with X-ray sources. We identified 699 ultra-steep spectral sources (α < −1.3) that could be high-redshift galaxies. About 9000 of the sources we extracted are not classified specifically, but based on their spatial and spectral distribution, a large percentage of these are likely to be extragalactic background sources. More than 7750 sources do not have counterparts in the SIMBAD database and more than 3760 sources do not have counterparts in the NED database.

Conclusions Studying the long wavelengths centimeter continuum emission and the associated spectral indices allows us to characaterize a large percentage of Galactic and extragalactic radio sources in the area of the northern inner Milky Way. This database will be extremely useful for future studies of a diverse set of astrophysical objects.