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

With the $Karl~G.~Jansky$ Very Large Array (VLA) in C-configuration, we observed a large portion of the first Galactic quadrant ($l=14.0-67.4^\circ $ and $\lvert b \rvert \leq 1.25^\circ $) achieving an angular resolution of $\leq 40^{\prime\prime}$. At $L$ Band, the WIDAR correlator at the VLA was set to cover the 21~cm HI line, four OH transitions, a series of H$nα$ radio recombination lines (RRLs; $n=151$ to 186), and eight 128~MHz wide continuum spectral windows (SPWs) simultaneously. The HI 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. Comparing 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 HI+continuum data, we produced a HI optical depth map of the THOR areal coverage from 228 absorption spectra with the nearest-neighbor method. With this $τ$ map, we corrected the HI emission for optical depth and the derived column density is 38% higher than the column density with optically thin assumption. The total HI mass with optical depth correction in the survey region is 4.7$\times10^8~M_\odot$, 31% more than the mass derived assuming the emission is optically thin. If we apply this 31% correction to the whole Milky Way, the total atomic gas mass would be 9.4-10.5$\times 10^9~M_\odot$. Comparing the HI 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.