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

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°, andl= 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 Idataset 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 Ianalysis in this paper. Methods.With theKarl G. JanskyVery Large Array (VLA) in C-configuration, we observed a large portion of the first Galactic quadrant, achieving an angular resolution of ≤40′′. AtLBand, the WIDAR correlator at the VLA was set to cover the 21 cm H Iline, four OH transitions, a series of Hnαradio recombination lines (RRLs;n= 151 to 186), and eight 128 MHz-wide continuum spectral windows, simultaneously. Results.We publish all OH and RRL data from the C-configuration observations, and a new H Idataset combining VLA C+D+GBT (VLA D-configuration and GBT data are from the VLA Galactic Plane Survey) for the whole survey. The H Iemission 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 produce an H Ioptical depth map of the THOR areal coverage from 228 absorption spectra with the nearest-neighbor method. With thisτmap, we corrected the H Iemission for optical depth, and the derived column density is 38% higher than the column density with optically thin assumption. The total H Imass with optical depth correction in the survey region is 4.7 × 108 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 × 109 M⊙. Comparing the H Iwith 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 Idataset 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 issignificant for H Icolumn density and mass estimation. Together with the OH, RRL and continuum emission from the THOR survey, these new H Idata provide the basis for high-angular-resolution studies of the interstellar medium in different phases.

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