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The origin and survival of the Smith high-velocity HI cloud has so far defiedexplanation. This object has several remarkable properties: (i) its progradeorbit is ~100 km/s faster than the underlying Galactic rotation; (ii) its totalgas mass ($\gtrsim 4 \times 10^6 ~{\rm M}_{\odot}$) exceeds the mass of allother high-velocity clouds (HVC) outside of the Magellanic Stream; (iii) itshead-tail morphology extends to the Galactic HI disc, indicating some sort ofinteraction. The Smith Cloud's kinetic energy rules out models based onejection from the disc. We construct a dynamically self-consistent, multi-phasemodel of the Galaxy with a view to exploring whether the Smith Cloud can beunderstood in terms of an infalling, compact HVC that has transited theGalactic disc. We show that while a dark-matter (DM) free HVC of sufficientmass and density can reach the disc, it does not survive the transit. The mostimportant ingredient to survival during a transit is a confining DM subhaloaround the cloud; radiative gas cooling and high spatial resolution ($\lesssim$10 pc) are also essential. In our model, the cloud develops a head-tailmorphology within ~10 Myr before and after its first disc crossing; after theevent, the tail is left behind and accretes onto the disc within ~400 Myr. Inour interpretation, the Smith Cloud corresponds to a gas 'streamer' thatdetaches, falls back and fades after the DM subhalo, distorted by the discpassage, has moved on. We conclude that subhalos with ${\rm M}_{\rm DM}\lesssim 10^9 ~{\rm M}_{\odot}$ have accreted $\sim10^9 ~{\rm M}_{\odot}$ ofgas into the Galaxy over cosmic time - a small fraction of the total baryonbudget.

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