Abstract We present Data Release 6 of ThrUMMS, consisting of complete data cubes and various moments of line emission ( 12 CO, 13 CO, C 18 O) 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 CO . Combining I 12 CO and N 12 CO data, we derive a global mass conversion law N = N 0 I p , where N 0 ≈ 10 18 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 CO and N 12 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.
Barnes et al. (Mon,) studied this question.