Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Head-tail molecular clouds falling onto the Milky Way disk

Mikito Kohno, Yasuo Fukui, Takahiro Hayakawa, Yasuo Doi, Rin I. Yamada, Fumika Demachi, Kazuki Tokuda, Hidetoshi Sano, Shinji Fujita, Rei Enokiya, Asao Habe, Kisetsu Tsuge, Atsushi Nishimura, Masato I. N. Kobayashi, Hiroaki Yamamoto, Kengo Tachihara

Abstract

We report discovery of two CO clouds which 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 $l=331.6^{\circ}$ 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 $4.8\times 10^3\ M_{\odot}$ and $3.5\times 10^3\ M_{\odot}$, respectively, and show kinetic temperature 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 of the 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 onto the Milky Way disk and are significantly heated up by the strong shock interaction with the disk HI gas. We suggest that the clouds represent part of the HI intermediate velocity clouds falling to the Galactic plane which were converted into molecular clouds by shock compression. This is the first case of falling CO clouds having direct observed signatures of the falling motion including clear directivity and shock heating. Possible implications of the CO clouds in the evolution of the Galactic interstellar medium are discussed.

Head-tail molecular clouds falling onto the Milky Way disk

Abstract

We report discovery of two CO clouds which are likely falling down to the Galactic plane at more than km s. The clouds show head-tail distributions elongated perpendicular to the Galactic plane at and as revealed by an analysis of the Mopra CO 1-0 survey data. We derived the distance of the clouds to be kpc based on the Gaia Data Release 3. The CO clouds have molecular masses of and , respectively, and show kinetic temperature of 30-50 K as derived from the line intensities of the CO =2-1, CO =1-0, and CO =1-0 emission. The temperature in the heads of the clouds is significantly higher than 10 K of the 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 onto the Milky Way disk and are significantly heated up by the strong shock interaction with the disk HI gas. We suggest that the clouds represent part of the HI intermediate velocity clouds falling to the Galactic plane which were converted into molecular clouds by shock compression. This is the first case of falling CO clouds having direct observed signatures of the falling motion including clear directivity and shock heating. Possible implications of the CO clouds in the evolution of the Galactic interstellar medium are discussed.
Paper Structure (15 sections, 3 equations, 9 figures, 2 tables)

This paper contains 15 sections, 3 equations, 9 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Data
  3. Mopra Southern Galactic plane CO $J=$1--0 survey
  4. SEDIGISM: APEX Galactic plane $^{13}$CO $J=$2--1 survey
  5. The Herschel far-infrared data
  6. Gaia Data Release 3
  7. Results
  8. Spatial and velocity distributions of molecular clouds
  9. The $^{13}$CO $J$ = 2--1/1--0 intensity ratio
  10. Comparison with the infrared image
  11. Discussion
  12. Distance to the head-tail molecular clouds
  13. Interaction of the head-tail molecular clouds with the gas disk component
  14. The falling scenario onto the Milky Way disk
  15. Conclusion

Figures (9)

  • Figure 1: (a) The Mopra $^{12}$CO $J$ = 1--0 integrated intensity map of the head-tailed molecular clouds. The lowest contour levels and intervals are 5.0 K km s$^{-1}$ and 3.0 K km s$^{-1}$ . (b) Same as (a), but for $^{13}$CO $J$ = 1--0. The lowest contour levels and intervals are 1.6 K km s$^{-1}$ and 1.6 K km s$^{-1}$ . The integrated velocity ranges are from $-36$ km s$^{-1}$ to $-34$ km s$^{-1}$ . The yellow dotted lines show the Galactic plane ($b=\timeform{0D}$). Alt text: Mopra 12-CO and 13-CO integrated intensity maps.
  • Figure 2: The Mopra $^{12}$CO $J$ = 1--0 velocity-field (first-moment) map. (b) The $^{12}$CO $J$ = 1--0 velocity dispersion (second-moment) map. The adopted velocity range extends from $-40$ km s$^{-1}$ to $-30$ km s$^{-1}$ . The yellow dotted lines show the Galactic plane ($b=\timeform{0D}$). The contour levels and intervals are the same as in Figure \ref{['mopra']}(a). The data points are plotted above $4.5$ K km s$^{-1}$ of the $^{12}$CO $J$ = 1--0 integrated intensity. Alt text: Mopra 12-CO first and second moment maps.
  • Figure 3: The $^{13}$CO $J$ = 2--1/1--0 ratio map above the integrated intensity of 1.8 K km s$^{-1}$ . The integrated velocity range is from $-37$ km s$^{-1}$ to $-32$ km s$^{-1}$ . The data are smoothed to the spatial resolution of 120". The lowest contour level and interval are 1.8 K km s$^{-1}$ and 0.6 K km s$^{-1}$ of $^{13}$CO $J=$2--1, respectively. A, B, C, and D indicate the position of the LVG calculation as shown in Figure \ref{['lvg']} and Table \ref{['lvgtable']}. Alt text: The 13-CO intensity ratio map.
  • Figure 4: Spectra and results of the LVG calculation at the point A, B, C, and D. The black, green, and red spectra show the $^{12}$CO $J$ = 1--0, $^{13}$CO $J$ = 2--1, and $^{13}$CO $J$ = 1--0, respectively. The vertical dotted lines indicate the integrated velocity range. Blue and red curves of constant $R^{13/12}_{\rm 1-0}$ and $R^{13}_{\rm 2-1/1-0}$ as functions of the molecular gas density [$n({\rm H_2})$] and kinetic temperature [$T_{\rm kin}$]. $Z({\rm CO})$ is the CO and H$_2$ abundance ratio given by $[^{12}{\rm CO}]/[{\rm H_2}]=10^{-4}$1982ApJ...262..590F2010ApJ...721..686P. The dotted lines show the $\pm 1\ \sigma$ error of the intensity ratio. The yellow cross marks indicate the solution of each data point. Alt text: Spectra and results of LVG calculation at the point A, B, C, and D.
  • Figure 5: (a) $^{12}$CO $J$ = 1--0 spatial distributions of head-tail molecular clouds superposed on the Herschel 160 $\mu$m continuum image 2010PASP..122..314M. The lowest contour levels and intervals are 5.0 K km s$^{-1}$ and 3.0 K km s$^{-1}$ . (b) The close-up image of panel (a). Alt text: Mopra 12-CO spatial distributions of head-tail molecular clouds superposed on the Herschel 160 mircrometer continuum image.
  • ...and 4 more figures