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Title: The Effect of Mixing on the Observed Metallicity of the Smith Cloud
Author: David B. Henley, Jeffrey A. Gritton, Robin L. Shelton (U. of Georgia)

Measurements of high-velocity clouds' metallicities provide important clues about their origins, and hence on whether they play a role in fuelling 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 ~5 x 10^6 solar masses. We calculated the fraction of the high-velocity material that had originated in the galactic halo, Fh, for various sight lines passing through our model cloud. We find that Fh generally increases with distance from the head of the cloud, reaching ~0.5 in the tail of the cloud. Models in which the metallicities (relative to solar) of the original cloud, Zcl, and of the halo, Zh, are in the approximate ranges 0.1 \lesssim Zcl \lesssim 0.3 and 0.7 \lesssim Zh \lesssim 1.0, respectively, are in rough agreement with the observations. Models with Zh~0.1 and Zcl \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.

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Monstrous Cloud Boomerangs Back to Our Galaxy

The old adage "what goes up must come down" even applies to an immense cloud of hydrogen gas outside our Milky Way galaxy. First discovered in the 1960s, the comet-shaped cloud is 11,000 light-years long and 2,500 light-years across. If the cloud could be seen in visible light, it would span the sky with an apparent diameter 30 times greater than the size of the full moon. The cloud, which is invisible at optical wavelengths, is plummeting toward our galaxy at nearly 700,000 miles per hour. Hubble was used to measure the chemical composition of the cloud as a means of assessing where it came from.
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Title: On the Metallicity and Origin of the Smith High-Velocity Cloud
Author: Andrew J. Fox, Nicolas Lehner, Felix J. Lockman, Bart P. Wakker, Alex S. Hill, Fabian Heitsch, David V. Stark, Kathleen A. Barger, Kenneth R. Sembach, Mubdi Rahman

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-constrained 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 photoionisation modelling, 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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Title: A Search For Star Formation in the Smith Cloud
Author: David V. Stark, Ashley D. Baker, Sheila J. Kannappan

Motivated by the idea that a subset of HVCs trace dark matter substructure in the Local Group, we search for signs of star formation in the Smith Cloud, a nearby ~2x10^6 Msun HVC currently falling into the Milky Way. Using GALEX NUV and WISE/2MASS NIR photometry, we apply a series of colour and apparent magnitude cuts to isolate candidate O and B stars that are plausibly associated with the Smith Cloud. We find an excess of stars along the line of sight to the cloud, but not at a statistically significant level relative to a control region. The number of stars found in projection on the cloud after removing an estimate of the contamination by the Milky Way implies an average star formation rate surface density of 10^(-4.8 ± 0.3) Msun yr^(-1) kpc^(-2), assuming the cloud has been forming stars at a constant rate since its first passage through the Milky Way ~70 Myr ago. This value is consistent with the star formation rate expected based on the average gas density of the cloud. We also discuss how the newly discovered star forming galaxy Leo P has very similar properties to the Smith Cloud, but its young stellar population would not have been detected at a statistically significant level using our method. Thus, we cannot yet rule out the idea that the Smith Cloud is really a dwarf galaxy.

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Title: The Smith Cloud and its dark matter halo: Survival of a Galactic disc passage
Author: Matthew Nichols, Nestor Mirabal, Oscar Agertz, Felix J. Lockman, Joss Bland-Hawthorn

The current velocity of the Smith Cloud indicates that it has undergone at least one passage of the Galactic disc. Using hydrodynamic simulations we examine the present day structure of the Smith Cloud. We find that a dark matter supported cloud is able to reproduce the observed present day neutral hydrogen mass, column density distribution and morphology. In this case the dark matter halo becomes elongated, owing to the tidal interaction with the Galactic disc. Clouds in models neglecting dark matter confinement are destroyed upon disc passage, unless the initial cloud mass is well in excess of what is observed today. We then determine integrated flux upper limits to the gamma-ray emission around such a hypothesised dark matter core in the Smith Cloud. No statistically significant core or extended gamma-ray emission are detected down to a 95% confidence level upper limit of 1.4 x 10^-10 ph cm^-2 s^-1 in the 1-300 GeV energy range. For the derived distance of 12.4 kpc, the Fermi upper limits set the first tentative constraints on the dark matter cross sections annihilating into T^+T^- and bb¯ for a high-velocity cloud.

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Gail Bieger-Smith is a 69 old woman who lives a quiet life in Wassenaar, a small, wealthy town west of Leiden, the Netherlands. She never expected to be dragged again by her brief astronomy career 45 years ago, but in early January she started getting phone calls from reporters and radio astronomers. The extragalactic cloud she discovered in 1963 had been found to be on a collision course with our Milky Way Galaxy. Some 20 to 40 million years from now, a million Suns' worth of hydrogen gas will smash into the galactic plane, likely causing a huge burst of star formation in the Perseus Arm about a quarter of the way around the galaxy from us. Long forgotten, Smith's Cloud was suddenly headline news.
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Dark galaxy crashing into the Milky Way
The Milky Way's neighbourhood may be teeming with invisible galaxies, one of which appears to be crashing into our own.
In 2008, a cloud of hydrogen with a mass then estimated at about 1 million suns was found to be colliding with our galaxy. Now it appears the object is massive enough to be a galaxy itself.

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Smith Cloud
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Title: Ionised Gas in the Smith Cloud
Authors: Alex S. Hill, L. Matthew Haffner, Ronald J. Reynolds (UW-Madison)

We present Wisconsin H-Alpha Mapper observations of ionised gas in the Smith Cloud, a high velocity cloud which Lockman et al. have recently suggested is interacting with the Galactic disk. Our H-alpha map shows the brightest H-alpha emission, 0.43 ±0.04 R, coincident with the brightest H I, while slightly fainter H-alpha emission (0.25 ±0.02 R) is observed in a region with H I intensities < 0.1 times as bright as the brightest H I. We derive an ionised mass of \gtrsim 3 x 10^6 Solar masses, comparable to the H I mass, with the H^+ mass spread over a considerably larger area than the H I. An estimated Galactic extinction correction could adjust these values upwards by 40 %. H-alpha and [S II] line widths towards the region of brightest emission constrain the electron temperature of the gas to be between 8000 K and 23000 K. A detection of [N II] \lambda 6583 in the same direction with a line ratio [N II] / H-alpha = 0.32 ±0.05 constrains the metallicity of the cloud: for typical photoionisation temperatures of 8000-12000 K, the nitrogen abundance is 0.15-0.44 times solar. These results lend further support to the claim that the Smith Cloud is new material accreting onto the Galaxy.

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Title: Smith's Cloud: A High-velocity Cloud Colliding with the Milky Way
Authors: Felix J. Lockman, Robert A. Benjamin, A. J. Heroux, Glen I. Langston

New 21cm HI 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 x 1 kpc in size and its tip at (l,b)=(39 deg,-13 deg) is 7.6 kpc from the Galactic centre and 2.9 kpc below the Galactic plane. It has greater than 10^6 M solar masses in HI. Its leading section has a total space velocity near 300 km/s, is moving toward the Galactic plane with a velocity of 73±26 km/s, 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 persistent anomalies in Galactic chemical evolution.

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SmithCloud
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Credit: NRAO/AUI/NSF

Smiths cloud in Aquila
l=40, b=15

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