Cloud-to-cloud velocity dispersions across a Local arm segment
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2025
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| author | Yuan, Lixia Yang, Ji |
| author_facet | Yuan, Lixia Yang, Ji |
| contents | Using a large sample of 9617 molecular clouds (MCs) from the Milky Way Imaging Scroll Painting survey, we mainly measure one-dimensional cloud-to-cloud velocity dispersions across a 450 deg$^{2}$ segment of the Local arm in the Galactic second quadrant. We define the cloud-to-cloud velocity dispersion using two metrics: the standard deviation ($σ_{\rm bin}$) and flux-weighted root-mean-squared value ($σ_{\rm bin, w}$) of the centroid velocities of $^{12}$CO-detected MCs within spatial bins. The typical values of $σ_{\rm bin}$ and $σ_{\rm bin, w}$ are 7.5$\pm$0.5 km s$^{-1}$ and 6.2$\pm$0.5 km s$^{-1}$, respectively. After categorizing clouds by sizes into three types: Type $S$ (0.15 $-$ 1.2 pc), Type $M$ (1.2 $-$ 4.8 pc), and Type L ($\gtrsim$ 4.8 pc), we find that the spatial distribution of Type $S$ and $M$ MCs projected onto the Galactic longitude-latitude (l-b) plane is generally uniform. Additionally, the cloud-to-cloud velocity dispersion among Type $S$ clouds ($\sim$ 7.6 and 7.4 km s$^{-1}$ for $σ_{\rm bin}$ and $σ_{\rm bin, w}$, respectively) is systematically greater than that among Type $M$ clouds ($\sim$ 6.7 and 6.0 km s$^{-1}$ for $σ_{\rm bin}$ and $σ_{\rm bin, w}$, respectively), with differences of 0.9 $-$ 1.4 km s$^{-1}$. From these measurements, we estimate merger timescales between MCs to be approximately 0.3 to 0.9 Myr, which is shorter than their internal crossing timescales ($\sim$ 1 Myr for Type $S$, $\sim$ 2 Myr for Type $M$, and $\gtrsim$ 5 Myr for Type $L$). This disparity, particularly pronounced for larger Type $L$ clouds, suggests that MCs are dynamically transient structures, with their gas content changing due to frequent interactions with neighboring clouds. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2506_13424 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Cloud-to-cloud velocity dispersions across a Local arm segment Yuan, Lixia Yang, Ji Astrophysics of Galaxies Using a large sample of 9617 molecular clouds (MCs) from the Milky Way Imaging Scroll Painting survey, we mainly measure one-dimensional cloud-to-cloud velocity dispersions across a 450 deg$^{2}$ segment of the Local arm in the Galactic second quadrant. We define the cloud-to-cloud velocity dispersion using two metrics: the standard deviation ($σ_{\rm bin}$) and flux-weighted root-mean-squared value ($σ_{\rm bin, w}$) of the centroid velocities of $^{12}$CO-detected MCs within spatial bins. The typical values of $σ_{\rm bin}$ and $σ_{\rm bin, w}$ are 7.5$\pm$0.5 km s$^{-1}$ and 6.2$\pm$0.5 km s$^{-1}$, respectively. After categorizing clouds by sizes into three types: Type $S$ (0.15 $-$ 1.2 pc), Type $M$ (1.2 $-$ 4.8 pc), and Type L ($\gtrsim$ 4.8 pc), we find that the spatial distribution of Type $S$ and $M$ MCs projected onto the Galactic longitude-latitude (l-b) plane is generally uniform. Additionally, the cloud-to-cloud velocity dispersion among Type $S$ clouds ($\sim$ 7.6 and 7.4 km s$^{-1}$ for $σ_{\rm bin}$ and $σ_{\rm bin, w}$, respectively) is systematically greater than that among Type $M$ clouds ($\sim$ 6.7 and 6.0 km s$^{-1}$ for $σ_{\rm bin}$ and $σ_{\rm bin, w}$, respectively), with differences of 0.9 $-$ 1.4 km s$^{-1}$. From these measurements, we estimate merger timescales between MCs to be approximately 0.3 to 0.9 Myr, which is shorter than their internal crossing timescales ($\sim$ 1 Myr for Type $S$, $\sim$ 2 Myr for Type $M$, and $\gtrsim$ 5 Myr for Type $L$). This disparity, particularly pronounced for larger Type $L$ clouds, suggests that MCs are dynamically transient structures, with their gas content changing due to frequent interactions with neighboring clouds. |
| title | Cloud-to-cloud velocity dispersions across a Local arm segment |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2506.13424 |