Head–tail molecular clouds falling on to the Milky Way disk

Abstract We report the discovery of two CO clouds that are likely falling down to the Galactic plane at more than 35 km s$^{-1}$. The clouds show head–tail distributions elongated perpendicular to the Galactic plane at $l={331{^{\circ}_{.}}6}$ and $b={0^{\circ}_{.}}$ as revealed by an analysis of the Mopra CO $J=$1–0 survey data. We derived the distance of the clouds to be $2.46 \pm 0.18$ kpc based on the Gaia Data Release 3. The CO clouds have molecular masses of $4.8\times 10^3$ and $3.5\times 10^3\ M_{\odot }$, respectively, and show kinetic temperatures of 30–50 K as derived from the line intensities of the $^{13}$CO $J$ = 2–1, $^{12}$CO $J$ = 1–0, and $^{13}$CO $J$ = 1–0 emission. The temperature in the heads of the clouds is significantly higher than 10 K for typical molecular clouds, although no radiative heat source is found inside or close to the clouds. Based on the results, we interpret that the present clouds are falling on to the Milky Way disk and are significantly heated up by the strong shock interaction with the disk H i gas. We suggest that the clouds represent part of the H i intermediate-velocity clouds falling to the Galactic plane that were converted into molecular clouds by shock compression. This is the first unambiguous case of falling CO clouds having directly observed signatures of falling motion including clear directivity and shock heating. Possible implications of the CO clouds in the evolution of the Galactic interstellar medium are discussed.