WISDOM Project – XXIV. Giant molecular clouds of the spiral galaxy NGC 5064: high fraction of retrograde rotation

ABSTRACT We present high-resolution ($0. 14\,\mathrm{ arcsec}\times 0. 12\,\mathrm{ arcsec}$ or $31\times 25$ pc$^2$) Atacama Large Millimeter/sub-millimeter Array $^{12}$CO(J = 2–1) observations of the spiral galaxy NGC 5064. Our study identifies 478 molecular clouds, of which 387 are resolved both spatially and spectrally. These clouds exhibit similarities to those of the Milky Way in terms of their sizes, molecular gas masses, velocity dispersions, velocity gradients, and Larson relations. However, the NGC 5064 clouds stand out with slightly higher gas mass surface densities, lower virial parameters ($\langle \alpha _{\rm vir}\rangle =0.48\pm 0.03$ assuming a standard conversion factor $X_{\rm CO}=2\times 10^{20}$ cm$^{-2}$ (K km s$^{-1})^{-1}$; $\alpha _{\rm vir} = 1$ for a lower conversion factor of $X_{\rm CO}=0.96\pm 0.02\times 10^{20}$ cm$^{-2}$ (K km s$^{-1})^{-1}$), and an unusually high fraction of retrograde rotation ($f_{\rm retro}\approx 67~{{\ \rm per\ cent}}$). Retrograde clouds are 18 per cent larger, 58 per cent more massive, 15 per cent more turbulent and have 17 per cent larger gas mass surface densities than prograde clouds. The velocity gradients in the clouds seem to arise from turbulence rather than cloud’s intrinsic rotation or large-scale galaxy rotation. Cloud–cloud collisions provide the most plausible explanation for the elevated retrograde fraction, though further investigation is needed to confirm this scenario. Projection effects due to the galaxy’s high inclination ($i = 70{_{.}^{\circ}} 8$) may further enhance the apparent retrograde fraction. Confirmation using less inclined systems is essential to determine whether the observed dominance of retrograde rotation reflects a genuine physical phenomenon or is significantly shaped by projection effects.