Work

The Three-mm Ultimate Mopra Milky Way Survey. III. Data Release 6, An Atlas of Physical Conditions, Global Mass Conversion Laws, and 3D Physical Architecture of the Molecular Interstellar Medium in the Fourth Quadrant

Year: 2025

Type: article

Abstract: 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 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 ≈ 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 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.

Source: The Astrophysical Journal Supplement Series

DOI: 10.3847/1538-4365/adf32b

Authors: Peter J. Barnes, Dylan G. H. Barnes, Audra K. Hernández, Sebastian Lopez, Erik Muller

Institutions: Space Science Institute, Florida Institute of Technology, University of Wisconsin–Madison, The Ohio State University, The University of Sydney

Language: English

Topic: Astrophysics and Star Formation Studies

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: gold

Estimated APC paid: $4,499

Funder: National Science Foundation