The Smith Cloud: surviving a high speed transit of the Galactic disc

Abstract

The origin and survival of the Smith high-velocity HI cloud has so far defied explanation. This object has several remarkable properties: (i) its prograde orbit is ~100 km/s faster than the underlying Galactic rotation; (ii) its total gas mass ($\gtrsim 4\times 10^6{\mathrm{M}}_{\odot }$) exceeds the mass of all other high-velocity clouds (HVC) outside of the Magellanic Stream; (iii) its head-tail morphology extends to the Galactic HI disc, indicating some sort of interaction. The Smith Cloud's kinetic energy rules out models based on ejection from the disc. We construct a dynamically self-consistent, multi-phase model of the Galaxy with a view to exploring whether the Smith Cloud can be understood in terms of an infalling, compact HVC that has transited the Galactic disc. We show that while a dark-matter (DM) free HVC of sufficient mass and density can reach the disc, it does not survive the transit. The most important ingredient to survival during a transit is a confining DM subhalo around the cloud; radiative gas cooling and high spatial resolution ($\lesssim$ 10 pc) are also essential. In our model, the cloud develops a head-tail morphology within ~10 Myr before and after its first disc crossing; after the event, the tail is left behind and accretes onto the disc within ~400 Myr. In our interpretation, the Smith Cloud corresponds to a gas 'streamer' that detaches, falls back and fades after the DM subhalo, distorted by the disc passage, has moved on. We conclude that subhalos with ${\mathrm{M}}_{\mathrm{DM}}\lesssim 10^9{\mathrm{M}}_{\odot }$ have accreted $\sim 10^9{\mathrm{M}}_{\odot }$ of gas into the Galaxy over cosmic time - a small fraction of the total baryon budget.

MNRAS accepted

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Cited by in corpus (10)

• The Magellanic System: the puzzle of the leading gas stream
• Constraining the Magnetic Field of the Smith High Velocity Cloud using Faraday rotation
• Exploring the Dust Content of Galactic Halos with Herschel III. NGC 891
• Simulating the impact of the Smith Cloud
• Young stars raining through the Galactic Halo: the nature and orbit of Price-Whelan 1
• A Component of the Smith High Velocity Cloud Now Crossing the Galactic Plane
• The Long Tails of the Pegasus-Pisces Arch Intermediate Velocity Cloud
• Low-density star cluster formation: discovery of a young faint fuzzy on the outskirts of the low-mass spiral galaxy NGC 247
• Millimeter-wave CO and SiO Observations toward the Broad-velocity-width Molecular Feature CO 16.134-0.553: a Smith cloud scenario?
• Simulating High-Velocity Clouds in the Observational Plane: An Initial Study with the Smith Cloud