The HI/OH/Recombination line survey of the inner Milky Way (THOR): data release 2 and H I overview

Abstract

Context. The Galactic plane has been observed extensively by a large number of Galactic plane surveys from infrared to radio wavelengths at an angular resolution below 40 . However, a 21 cm line and continuum survey with comparable spatial resolution is lacking. Aims. The first half of THOR data (l = 14.0 • − 37.9 • , and l = 47.1 • − 51.2 • , |b| ≤ 1.25 • ) has been published in our data release 1 paper. With this data release 2 paper, we publish all the remaining spectral line data and Stokes I continuum data with high angular resolution (10 -40 ), including a new H i dataset for the whole THOR survey region (l = 14.0 − 67.4 • and |b| ≤ 1.25 • ). As we published the results of OH lines and continuum emission elsewhere, we concentrate on the H i analysis in this paper. Methods. With the Karl G. Jansky Very Large Array (VLA) in C-configuration, we observed a large portion of the first Galactic quadrant, achieving an angular resolution of ≤ 40 . At L Band, the WIDAR correlator at the VLA was set to cover the 21 cm H i line, four OH transitions, a series of Hnα radio recombination lines (RRLs; n = 151 to 186), and eight 128 MHz-wide continuum spectral windows (SPWs), simultaneously.Results. We publish all OH and RRL data from the C-configuration observations, and a new H i dataset combining VLA C+D+GBT (VLA D-configuration and GBT data are from the VLA Galactic Plane Survey) for the whole survey. The H i emission shows clear filamentary substructures at negative velocities with low velocity crowding. The emission at positive velocities is more smeared-out, likely due to higher spatial and velocity crowding of structures at the positive velocities. Compared to the spiral arm model of the Milky Way, the atomic gas follows the Sagittarius and Perseus Arm well, but with significant material in the inter-arm regions. With the C-configuration-only H i+continuum data, we produced a H i optical depth map of the THOR areal coverage from 228 absorption spectra with the nearest-neighbor method. With this τ map, we corrected the H i emission for optical depth, and the derived column density is 38% higher than the column density with optically thin assumption. The total H i mass with optical depth correction in the survey region is 4.7×10 8 M , 31% more than the mass derived assuming the emission is optically thin. If we applied this 31% correction to the whole Milky Way, the total atomic gas mass would be 9.4-10.5×10 9 M . Comparing the H i with existing CO data, we find a significant increase in the atomic-to-molecular gas ratio from the spiral arms to the inter-arm regions.Conclusions. The high-sensitivity and resolution THOR H i dataset provides an important new window on the physical and kinematic properties of gas in the inner Galaxy. Although the optical depth we derive is a lower limit, our study shows that the optical depth correction is significant for H i column density and mass estimation. Together with the OH, RRL and continuum emission from the THOR survey, these new H i data provide the basis for high-angular-res...

Atomic and molecular gas properties during cloud formation

“…The following analysis employed the H I and 1.4 GHz continuum data from the THOR survey (The H I/OH Recombination line survey of the inner Milky Way; Beuther et al 2016;Wang et al 2020b). The H I and 1.4 GHz continuum data include observations from the Karl G. Jansky Very Large Array (VLA) in both C-and D-configuration as well as single-dish observations from the Green Bank Telescope (GBT) and Effelsberg, respectively, to recover missing flux on short uv spacings.…”

“…The THOR-only data have an angular resolution of ∆Θ ∼ 20 , depending slightly on Galactic longitude. For more details about the THOR data, we refer to the two data release papers by Beuther et al (2016) and Wang et al (2020b).…”

“…However, the HICA method requires strong continuum emission sources. As most strong continuum sources are discrete point sources, this method results in an incomplete census of optical depth measurements (see Wang et al 2020b, for a compilation of all optical depth measurements in the THOR survey). Consequently, the intrinsic structure of individual H I clouds cannot be determined.…”

“…In contrast to Wang et al (2020b), where they used a 6σ threshold to select continuum sources, we measured the optical depth of atomic hydrogen toward the brightest continuum sources with brightness temperatures T cont > 200 K to not suffer from low saturation limits since we expect the optical depth to be high. Four sources have been identified with this threshold.…”

“…Therefore, the clouds do not have to be spatially associated for HISA to be observable. While absorption against strong continuum sources does yield a direct measurement of the optical depth, the discreteness of the sources only delivers an incomplete grid of optical depth measurements (e.g., Wang et al 2020b). The interpolation of optical depths across an entire H I cloud is challenging.…”