The Effect of Mixing on the Observed Metallicity of the Smith Cloud

Abstract

Measurements of high-velocity clouds' metallicities provide important clues about their origins, and hence on whether they play a role in fueling ongoing star formation in the Galaxy. However, accurate interpretation of these measurements requires compensating for the galactic material that has been mixed into the clouds. In order to determine how much the metallicity changes as a result of this mixing, we have carried out three-dimensional wind-tunnel-like hydrodynamical simulations of an example cloud. Our model cloud is patterned after the Smith Cloud, a particularly well-studied cloud of mass $\sim 5 \times 10^6~M_\odot$. We calculated the fraction of the high-velocity material that had originated in the galactic halo, $F_\mathrm{h}$, for various sight lines passing through our model cloud. We find that $F_\mathrm{h}$ generally increases with distance from the head of the cloud, reaching $\sim$0.5 in the tail of the cloud. Models in which the metallicities (relative to solar) of the original cloud, $Z_\mathrm{cl}$, and of the halo, $Z_\mathrm{h}$, are in the approximate ranges $0.1 \lesssim Z_\mathrm{cl} \lesssim 0.3$ and $0.7 \lesssim Z_\mathrm{h} \lesssim 1.0$, respectively, are in rough agreement with the observations. Models with $Z_\mathrm{h} \sim 0.1$ and $Z_\mathrm{cl} \gtrsim 0.5$ are also in rough agreement with the observations, but such a low halo metallicity is inconsistent with recent independent measurements. We conclude that the Smith Cloud's observed metallicity may not be a true reflection of its original metallicity and that the cloud's ultimate origin remains uncertain.