The Smith Cloud: High-Velocity Accretion and Dark Matter Confinement

Abstract

The Smith Cloud is a massive system of metal-poor neutral and ionized gas (M gas 2 × 10 6 M ⊙ ) that is presently moving at high velocity (V GSR ≈ 300 km s −1 ) with respect to the Galaxy at a distance of 12 kpc from the Sun. The kinematics of the cloud's cometary tail indicates that the gas is in the process of accretion onto the Galaxy, as first discussed by Lockman et al. (2008). Here, we re-investigate the cloud's orbit by considering the possibility that the cloud is confined by a dark matter halo. This is required for the cloud to survive its passage through the Galactic corona. We consider three possible models for the dark matter halo (NFW, Einasto, Burkert) including the effects of tidal disruption and ram-pressure stripping during the cloud's infall onto and passage through the Galactic disk. For the NFW and Einasto 1 dark-matter models, we are able to determine reasonable initial conditions for the Smith Cloud, although this is only marginally possible with the Burkert model. For all three models, the progenitor had an initial (gas+dark matter) mass that was an order of magnitude higher than inferred today. In agreement with Lockman et al., the cloud appears to have punched through the disk ≈ 70 Myr ago. For our most successful models, the baryon to dark matter ratio is fairly constant during an orbital period but drops by a factor of 2 − 5 after transiting the disk. The cloud appears to have only marginally survived its transit, and is unlikely to retain its integrity during the next transit ≈ 30 Myr from now. Subject headings:

Neutral Gas Accretion onto Nearby Galaxies

“…Its total space velocity is below the escape velocity of the Milky Way, and the largest component is in the direction of Galactic rotation. It appears to be entering the Milky Way at a rather shallow angle (Lockman et al, 2008;Nichols & Bland-Hawthorn, 2009). The Smith Cloud is thus adding angular momentum to the disk.…”

Accretion Onto the Milky Way: The Smith Cloud

“…One answer, first proposed by Nichols & Bland-Hawthorn(2009) and presented in more detail by Nichols et al(2014) is that the Smith Cloud is the baryonic component of a dark matter sub-halo. A dark matter halo ∼ 3 × 10 8 M ⊙ is sufficient to stabilize the cloud core while not massive enough to induce significant star formation, which is not observed.…”

Do high-velocity clouds trace the dark matter subhalo population?

“…The latter possibility would provide the requested link to the postulated DM building blocks of hierarchical galaxy formation which means that this baryonic cloud mass should be harbored by a so-called DM mini-halo. Nichols & Bland-Hawthorn (2009) have investigated the orbit of the Smith Cloud under the consideration that the cloud is confined by a DM halo.…”