Context. The past decade has witnessed a large number of Galactic plane surveys at angular resolutions below 20 . However, no comparable high-resolution survey exists at long radio wavelengths around 21 cm in line and continuum emission. Aims. We remedy this situation by studying the northern Galactic plane at ∼20 resolution in emission of atomic, molecular, and ionized gas. Methods. Employing the Karl G. Jansky Very Large Array (VLA) in the C-array configuration and a large program, we observe the HI 21 cm line, four OH lines, nineteen Hnα radio recombination lines as well as the continuum emission from 1 to 2 GHz in full polarization over a large part of the first Galactic quadrant. Results. Covering Galactic longitudes from 14.5 to 67.4 deg and latitudes between ±1.25 deg, we image all of these lines and the continuum at ∼20 resolution. These data allow us to study the various components of the interstellar medium (ISM): from the atomic phase, traced by the HI line, to the molecular phase, observed by the OH transitions, to the ionized medium, revealed by the cm continuum and the Hnα radio recombination lines. Furthermore, the polarized continuum emission enables magnetic field studies. In this overview paper, we discuss the survey outline and present the first data release as well as early results from the different datasets. We now release the first half of the survey; the second half will follow later after the ongoing data processing has been completed. The data in fits format (continuum images and line data cubes) can be accessed through the project web-page. Conclusions. The HI/OH/Recombination line survey of the Milky Way (THOR) opens a new window to the different parts of the ISM. It enables detailed studies of molecular cloud formation, conversion of atomic to molecular gas, and feedback from Hii regions as well as the magnetic field in the Milky Way. It is highly complementary to other surveys of our Galaxy, and comparing the different datasets will allow us to address many open questions.

We carried out a large program with the Karl G. Jansky Very Large Array (VLA): "THOR: The H , OH, Recombination line survey of the Milky Way". We observed a significant portion (∼100 deg 2 ) of the Galactic plane in the first quadrant of the Milky Way in the 21 cm H  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  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.

Aims. This work provides an update to existing reconstructions of the Galactic Faraday rotation sky by processing almost all Faraday rotation data sets available at the end of the year 2020. Observations of extra-Galactic sources in recent years have further illuminated the previously underconstrained southern celestial sky, as well as parts of the inner disc of the Milky Way, along with other regions. This has culminated in an all-sky data set of 55 190 data points, thereby comprising a significant expansion on the 41 330 used in previous works. At the same time, this novelty makes an updated separation of the Galactic component a promising enterprise. The increased source density allows us to present our results in a resolution of about 1.3 × 10−2 deg2 (46.8 arcmin2), which is a twofold increase compared to previous works. Methods. As for previous Faraday rotation sky reconstructions, this work is based on information field theory, namely, a Bayesian inference scheme for field-like quantities that handles noisy and incomplete data. Results. In contrast to previous reconstructions, we find a significantly thinner and pronounced Galactic disc with small-scale structures exceeding values of several thousand rad m−2. The improvements can mainly be attributed to the new catalog of Faraday data, but are also supported by advances in correlation structure modeling within numerical information field theory. We also provide a detailed discussion on the statistical properties of the Faraday rotation sky and we investigate correlations with other data sets.

We present first results for Faraday rotation of compact polarized sources (1 to 2 GHz continuum) in The HI/OH/Recombination line (THOR) survey of the inner Galaxy. In the Galactic longitude range 39 • < < 52 • , we find rotation measures in the range −310 rad m −2 ≤ RM ≤ +4219 rad m −2 , with the highest values concentrated within a degree of = 48 • at the Sagittarius arm tangent. Most of the high RM s arise in diffuse plasma, along lines of sight that do not intersect H II regions. For > 49 • , RM drops off rapidly, while at < 47 • , the mean RM is higher with a larger standard deviation than at > 49 • . We attribute the RM structure to the compressed diffuse Warm Ionized Medium in the spiral arm, upstream of the major star formation regions. The Sagittarius arm acts as a significant Faraday screen inside the Galaxy. This has implications for models of the Galactic magnetic field and the expected amount of Faraday rotation of Fast Radio Bursts from their host galaxies. We emphasize the importance of sensitivity to high Faraday depth in future polarization surveys.

We present polarization and Faraday rotation for the supernova remnants (SNRs) G46.8 − 0.3, G43.3 − 0.2, G41.1 − 0.3, and G39.2 − 0.3 in the L-band (1–2 GHz) radio continuum in the H i/OH/Recombination line survey. We detect polarization from G46.8 − 0.3, G43.3 − 0.2, and G39.2 − 0.3 but find upper limits at the 1% level of Stokes I for G41.1 − 0.3. For G46.8 − 0.3 and G39.2 − 0.3, the fractional polarization varies on small scales from 1% to ∼6%. G43.3 − 0.2 is less polarized with fractional polarization ≲3%. We find upper limits at the 1% level for the brighter regions in each SNR with no evidence for associated enhanced Faraday depolarization. We observe significant variation in Faraday depth and fractional polarization on angular scales down to the resolution limit of 16″. Approximately 6% of our polarization detections from G46.8 − 0.3 and G39.2 − 0.3 exhibit two-component Faraday rotation and 14% of polarization detections in G43.3 − 0.2 are multicomponent. For G39.2 − 0.3, we find a bimodal Faraday depth distribution with a narrow peak and a broad peak for all polarization detections as well as for the subset with two-component Faraday rotation. We identify the narrow peak with the front side of the SNR and the broad peak with the back side. Similarly, we interpret the observed Faraday depth distribution of G46.8 − 0.3 as a superposition of the distributions from the front side and the back side. We interpret our results as evidence for a partially filled shell with small-scale magnetic field structure and internal Faraday rotation.