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$n\alpha$ 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 $\tau$ 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.
