2025ApJS..280...31B

We present Data Release 6 of ThrUMMS, consisting of complete data cubes and various moments of line emission (¹²CO, ¹³CO, C¹⁸O) from molecular clouds, across 60° × 2° of the Fourth Quadrant (4Q) of the Milky Way at a resolution of 72″ in (l, b) and 0.09 km s^–1 in V_LSR. From LTE radiative transfer analysis of the data cubes, we compute cubes and moments of the lines' opacity, excitation temperature, and column density ${N}_{{}^{12}{{\rm{CO}}}}$. Combining ${I}_{{}^{12}{{\rm{CO}}}}$ and ${N}_{{}^{12}{{\rm{CO}}}}$ data, we derive a global mass conversion law N = N₀I^p^, where N₀ ≈ 10¹⁸ mol m^–2 and p = 2 at this resolution. We argue that the standard linear N = XI is only approximately valid: p ∼ 1.5-1.0 at coarser resolutions or in atypical locations, such as Galactic center clouds. The velocity dispersion distributions are very different between ${I}_{{}^{12}{{\rm{CO}}}}$ and ${N}_{{}^{12}{{\rm{CO}}}}$, the former preferentially tracing more diffuse molecular gas. We reevaluated Galactic rotation parameters for the 4Q, defining a new “BGT” model, and deprojected the (l, V) data onto (l, d) and (x, y) grids using standard kinematic procedures. To automate distance disambiguation inside the solar circle, we developed a simple ζ⁺ discriminator function and applied it to our deprojections. We discovered two previously unrecognised features of the molecular cloud population: widespread ripples in the midplane of wavelength 4 kpc and amplitude 50 pc, potentially generated by the last perigalactic passage of the Sgr dwarf; and three distant, massive molecular structures, the Far Ara Clouds, two of which exhibit an exceptional velocity gradient, possibly lying near the bar's far end or in a gas-rich dwarf galaxy ∼20-300 kpc beyond the disk.