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ABSTRACT We study the physical drivers of slow molecular cloud mergers within a simulation of a Milky Way-like galaxy in the moving-mesh code arepo, and determine the influence of these mergers on the mass distribution and star formation efficiency of the galactic cloud population. We find that 83 per cent of these mergers occur at a relative velocity below 5 km s−1, and are associated with large-scale atomic gas flows, driven primarily by expanding bubbles of hot, ionized gas caused by supernova explosions and galactic rotation. The major effect of these mergers is to aggregate molecular mass into higher-mass clouds: mergers account for over 50 per cent of the molecular mass contained in clouds of mass M > 2 × 106 M⊙. These high-mass clouds have higher densities, internal velocity dispersions and instantaneous star formation efficiencies than their unmerged, lower mass precursors. As such, the mean instantaneous star formation efficiency in our simulated galaxy, with its merger rate of just 1 per cent of clouds per Myr, is 25 per cent higher than in a similar population of clouds containing no mergers.
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This content will become publicly available on July 7, 2026
A New High-latitude H I Cloud Complex Entrained in the Northern Fermi Bubble
We report the discovery of 11 high-velocity H I clouds at Galactic latitudes of 25°–30°, likely embedded in the Milky Way’s nuclear wind. The clouds are detected with deep Green Bank Telescope 21 cm observations of a 3.2° × 6.2° field around QSO 1H1613-097, located behind the northern Fermi Bubble. Our measurements reach 3sigma limits on NHI as low as 3.1 × 10^17/cm^2, more than twice as sensitive as previous HI studies of the bubbles. The clouds span −180 ≤v_LSR≤ −90 km/s and are the highest-latitude 21 cm high-velocity cloud detected inside the bubbles. Eight clouds are spatially resolved, showing coherent structures with sizes of 4–28 pc, peak column densities of log HI = 17.9–18.7, and HI masses up to 1470M⊙. Several exhibit internal velocity gradients. Their presence at such high latitudes is surprising, given the short expected survival times for clouds expelled from the Galactic center. These objects may be fragments of a larger cloud disrupted by interaction with the surrounding hot gas.
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- Award ID(s):
- 2206853
- PAR ID:
- 10633230
- Publisher / Repository:
- IOP Publishing
- Date Published:
- Journal Name:
- The Astrophysical Journal Letters
- Volume:
- 987
- Issue:
- 2
- ISSN:
- 2041-8205
- Page Range / eLocation ID:
- L32
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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