The Smith Cloud: A High-Velocity Cloud Colliding with the Milky Way

Abstract

New 21cm H I observations made with the Green Bank Telescope show that the high-velocity cloud known as Smith's Cloud has a striking cometary appearance and many indications of interaction with the Galactic ISM. The velocities of interaction give a kinematic distance of 12.4 ± 1.3 kpc, consistent with the distance derived from other methods. The Cloud is > 3 × 1 kpc in size and its tip at (ℓ, b) ≈ 39 • − 13 • is 7.6 kpc from the Galactic center and 2.9 kpc below the Galactic plane. It has > 10 6 M ⊙ in H I. Its leading section has a total space velocity near 300 km s −1 , is moving toward the Galactic plane with a velocity of 73 ± 26 km s −1 , and is shedding material to the Galaxy. In the absence of drag the Cloud will cross the plane in about 27 Myr. Smith's Cloud may be an example of the accretion of gas by the Milky Way needed to explain certain persistant anomalies in Galactic chemical evolution.

Neutral Gas Accretion onto Nearby Galaxies

“…Figure 5 shows an H I channel map from a recent 21cm survey made with the Green Bank Telescope (GBT). The cloud has a good distance estimate (Wakker et al, 2008), and thus a well defined mass and size; it is moving toward the Galactic plane, which it should intersect in ∼ 30 Myr if it is survives as a coherent entity (Lockman et al, 2008). Its mass in H I is ∼ 2 × 10 6 M and it has an ionized component with a similar mass detected in faint H α emission (Hill et al, 2009).…”

“…With an M HI of 2 × 10 6 M , the Smith Cloud has about the median H I mass of the M33 HVCs and lies at the upper range of those around M31. Given the observational uncertainties, the Smith Cloud would be an inconspicuous addition to either galaxy (Lockman et al, 2008;Westmeier et al, 2008;Keenan et al, 2016). However, as the brighter parts of the Smith Cloud lie only ∼ 3 kpc away from the Galactic plane, if it were at the distance of M31 or M33 it would lie projected on the disk of those galaxies, and from even the most favorable vantage would be separated by only 12 from their disk.…”

Magnetized High Velocity Clouds in the Galactic Halo: A New Distance Constraint

“…The initial density in the cloud core is set to n c = 0.1 cm −3 (n c = 0.5 cm −3 ) for the low (high) density case, yielding an initial cloud mass M c ≈ 7 × 10 5 M (M c ≈ 3 × 10 6 M ), the latter being comparable to the mass of e.g. the Smith Cloud (Lockman et al 2008). In either case, the cloud radius is set to r c = 0.5 kpc, and the parameter s -which determines the steepness of the profile -to s = 9 (see Figure 1).…”

Accretion Onto the Milky Way: The Smith Cloud

“…Observations with the 100 meter Green Bank Telescope (GBT) at 9 ′ resolution reveal a spectacular cometary cloud, showing extensive evidence of interaction between the cloud and the gaseous halo of the Milky Way (Lockman et al(2008)). Figure 1 shows an HI image from new unpublished GBT observations (Lockman et al 2016).…”