Molecular Clouds in the Outer Milky Way Disk: Sample, Integrated Properties, and Radial Trends with Galactocentric Radius

Abstract

We present a catalog of 32,162 ^12 CO molecular clouds that covers the northern outer Galactic plane ( l = [15°, 165°] and l = [195°, 230°], and b = [−5.°25, +5.°25]). The catalog was produced using a DBSCAN algorithm applied to the Milky Way Imaging Scroll Painting project data. We systematically analyze both the integrated properties and scaling relationships of these molecular clouds across different Galactocentric radii (8 < R _GC < 26 kpc). An interesting finding is that each cloud property’s mean, median, and maximum values generally decrease systematically with increasing R _GC , approximated quantitatively by exponential functions. The mass and size spectra for the entire sample show power-law indices of γ = −2.00 ± 0.13 and an upper mass limit of M _0 = (4.5 ± 2.8) × 10 ^6 M _⊙ , and γ = −3.68 ± 0.48 with an upper size limit of R _0 = 144 ± 80 pc, respectively. These spectra exhibit steeper slopes and reduced upper limits as R _GC increases. The derived scaling relations are $M=12.00{R}_{\mathrm{eff}}^{2.41\,\pm \,0.003}$ , ${\sigma }_{v}=0.25{R}_{\mathrm{eff}}^{0.66\,\pm \,0.003}$ , and α _vir = 37.2 M ^−0.40 ± 0.002 for the whole outer Galaxy clouds. These scaling relations show slightly steeper correlations observed in more distant locations and notably deviate from Larson’s relations. We find that 44.3% of the molecular mass resides in gravitationally bound structures, a proportion significantly higher than systematic studies from the CfA survey and external galaxies. Moreover, comparisons among different R _GC subsamples also reveal a decrease in the amount of bound mass with increasing R _GC . The scale length of the molecular disk is estimated to be approximately 2 kpc. These results, based on a constant CO conversion factor of ${X}_{\mathrm{CO}}=2.0\times {10}^{20}\,{\mathrm{cm}}^{-2}\,{\left({\rm{K}}\,\mathrm{km}\,{{\rm{s}}}^{-1}\right)}^{-1}$ , may be slightly altered by variations in X _CO . In summary, our findings provide robust evidence for the influence of Galactic evolution on molecular cloud properties, at least in the outer Galaxy region studied.

Keywords

• Molecular clouds
• Milky Way Galaxy
• Milky Way disk
• CO line emission
• Interstellar line emission