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تسجيل مستخدم جديدThe Effect of Mixing on the Observed Metallicity of the Smith Cloud
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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 Clouds observed metallicity may not be a true reflection of its original metallicity and that the clouds ultimate origin remains uncertain.
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The Smith Cloud is a gaseous high-velocity cloud (HVC) in an advanced state of accretion, only 2.9 kpc below the Galactic plane and due to impact the disk in 27 Myr. It is unique among HVCs in having a known distance (12.4+/-1.3 kpc) and a well-const
rained 3D velocity (296 km/s), but its origin has long remained a mystery. Here we present the first absorption-line measurements of its metallicity, using HST/COS UV spectra of three AGN lying behind the Cloud together with Green Bank Telescope 21 cm spectra of the same directions. Using Voigt-profile fitting of the S II 1250, 1253, 1259 triplet together with ionization corrections derived from photoionization modeling, we derive the sulfur abundance in each direction; a weighted average of the three measurements gives [S/H]=-0.28+/-0.14, or 0.53+0.21-0.15 solar metallicity. The finding that the Smith Cloud is metal-enriched lends support to scenarios where it represents recycled Galactic material rather than the remnant of a dwarf galaxy or accreting intergalactic gas. The metallicity and trajectory of the Cloud are both indicative of an origin in the outer disk. However, its large mass and prograde kinematics remain to be fully explained. If the cloud has accreted cooling gas from the corona during its fountain trajectory, as predicted in recent theoretical work, its current mass would be higher than its launch mass, alleviating the mass concern.
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abilize it. Our main focus is the clouds influence on the local and global star formation rate (SFR) of the Galaxy and whether or not it leads to an observable event in the far future. Our model assumes two extremes for the mass of the Smith Cloud, an upper mass limit of 10$^7$ M$_{odot}$ and a lower mass limit of 10$^6$ M$_{odot}$, compared to the observational value of a few 10$^6$ M$_{odot}$. In addition, we also make the conservative assumption that the entirety of the cloud mass of the extended Smith Cloud is concentrated within the tip of the cloud. We find that the impact of the low-mass cloud produces no noticeable change in neither the global SFR nor the local SFR at the cloud impact site within the galactic disk. For the high-mass cloud we find a short-term (roughly 5 Myr) increase of the global SFR of up to 1 M$_{odot}$ yr$^{-1}$, which nearly doubles the normal Milky Way SFR. This highly localized starburst should be observable.
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