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

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 fraction 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. 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 10916 sources. After removing spurious source detections caused by the sidelobes of the synthesised beam, we classified 10387 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 spectral window (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 ultrasteep spectral sources (α < −1.3) that could be high-redshift galaxies. Around 9000 of the sources we extracted are not classified specifically, but based on their spatial and spectral distribution, a large fraction of them is 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 cm continuum emission and the associated spectral indices allows us to characaterize a large fraction 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.

The history of dynamics and stellar feedback revealed by the H I filamentary structure in the disk of the Milky Way

“…The side lobes can introduce spurious filamentary structures in the HI emission around continuum sources, which are seen in absorption. To mitigate this effect, we mask regions of the map by using a threshold on the continuum emission noise maps introduced in Wang et al (2018), as detailed in Appendix A. For the sake of illustration, we include the orientation of the noise features characterized with the Hessian method in the examples presented in Figs.…”

ATLASGAL – relationship between dense star-forming clumps and interstellar masers

“…The H /OH/Recombination (THOR; Beuther et al 2016) line survey of the inner Milky Way has surveyed 14.5 • ≤ ≤ 66.8 • and |b| < 1.25 • of the Galactic plane using the Karl Jansky Very Large Array (VLA) in the C-array configuration. The THOR survey has observed a number of different spectral lines, including the H 21 cm, nineteen of the Hnα radio recombination lines and the four ground-state transitions of the hydroxyl maser, as well as continuum emission between 1 and 2 GHz (Bihr et al 2016;Wang et al 2018). The survey has angular resolutions of ∼ 12.5 to 19 arcsec, with a spectral resolution of 1.5 km s −1 , and typical rms noise levels of ∼10 mJy.…”

Atomic and molecular gas properties during cloud formation

“…The measured optical depths of these sources vary between 0.5 and 2.5 (lower limit). We selected the continuum source G19.075-0.287 (Wang et al 2018) as a representative source for the optical depth as it is not in the 5σ saturation at most velocities between 43 and 56 km s −1 , which gives a mean optical depth of τ ∼ 0.9 (Fig. 12).…”

“…12. Optical depth measurement toward the H II region G19.075-0.287 (Wang et al 2018). The plot shows the optical depth as a function of LSR velocity and was computed using Eq.…”