Mon. Not. R. Astron. Soc., 473, 5514-5531 (2018)
The Smith Cloud: surviving a high-speed transit of the Galactic disc.
TEPPER-GARCIA T. and BLAND-HAWTHORN J.
Abstract (from CDS):
The origin and survival of the Smith high-velocity H I cloud has so far defied explanation. This object has several remarkable properties: (i) its prograde orbit is ≃100 km s–1 faster than the underlying Galactic rotation; (ii) its total gas mass (>= 4 x 106 M☉) exceeds the mass of all other high-velocity clouds (HVCs) outside of the Magellanic Stream; (iii) its head-tail morphology extends to the Galactic H I 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 (<= 10pc) 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 on to 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 subhaloes with MDM <= 109 M☉ have accreted ∼109 M☉ of gas into the Galaxy over cosmic time - a small fraction of the total baryon budget.
methods: numerical - ISM: clouds - ISM: general - ISM: individual objects: Smith Cloud - Galaxy: halo - intergalactic medium