On the Metallicity and Origin of the Smith High-Velocity Cloud

Abstract

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 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.

Accepted for publication in ApJL

References in corpus (13)

• A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
• Cold gas accretion in galaxies
• The COS/UVES Absorption Survey of the Magellanic Stream. III: Ionization, Total Mass, and Inflow Rate onto the Milky Way
• Smith's Cloud: A High-velocity Cloud Colliding with the Milky Way
• Ultrafaint Dwarf Galaxies - the lowest mass relics from before reionization
• The origin of the high-velocity cloud complex C
• Galactic fountains and their connection with high and intermediate velocity clouds
• The Ophiuchus Superbubble: A Gigantic Eruption from the Inner Disk of the Milky Way
• Ionized Gas in the Smith Cloud
• The Smith Cloud and its dark matter halo: Survival of a Galactic disc passage
• Mixing between High Velocity Clouds and the Galactic Halo
• A Search For Star Formation in the Smith Cloud
• Accretion Onto the Milky Way: The Smith Cloud

Cited by in corpus (47)

• Gas Accretion via Condensation and Fountains
• Nuclear Outflow of the Milky Way: Studying the Kinematics and Spatial Extent of the Northern Fermi Bubble
• A new all-sky map of Galactic high-velocity clouds from the 21-cm HI4PI survey
• Age-resolved chemistry of red giants in the solar neighbourhood
• Bimodality of [α/Fe]-[Fe/H] distributions is a natural outcome of dissipative collapse and disc growth in Milky Way-type galaxies
• The Mass Inflow and Outflow Rates of the Milky Way
• Magnetic fields in the Galactic halo restrict fountain-driven recycling and accretion
• Intermediate- and high-velocity clouds in the Milky Way I: covering factors and vertical heights
• The COS CGM Compendium (CCC). I: Survey Design and Initial Results
• Intermediate- and high-velocity clouds in the Milky Way II: evidence for a Galactic fountain with collimated outflows and diffuse inflows
• Cloudy with A Chance of Rain: Accretion Braking of Cold Clouds
• The Circumgalactic Medium of Milky Way-like Galaxies in the TNG50 Simulation -- II: Cold, Dense Gas Clouds and High-Velocity Cloud Analogs
• Gas Accretion onto the Milky Way
• Chemical Abundances in the Leading Arm of the Magellanic Stream
• Discovery of a disrupting open cluster far into the Milky Way halo: a recent star formation event in the leading arm of the Magellanic stream?
• Constraining the Magnetic Field of the Smith High Velocity Cloud using Faraday rotation
• Magnetized High Velocity Clouds in the Galactic Halo: A New Distance Constraint
• Mass, Morphing, Metallicities: The Evolution of Infalling High Velocity Clouds
• A High Precision Survey of the D/H Ratio in the Nearby Interstellar Medium
• The Smith Cloud: surviving a high speed transit of the Galactic disc
• {\it HST}/COS Observations of the Warm Ionized Gaseous Halo of NGC 891
• The Effect of Mixing on the Observed Metallicity of the Smith Cloud
• $Chempy$: A flexible chemical evolution model for abundance fitting - Do the Sun's abundances alone constrain chemical evolution models?
• Attack of the Flying Snakes : Formation of Isolated HI Clouds By Fragmentation of Long Streams
• The role of the halo magnetic field on accretion through High-Velocity Clouds
• Spatially Resolved Gas Flows Around the Milky Way
• Extraplanar H II Regions in Spiral Galaxies. I. Low-Metallicity Gas Accreting through the Disk-Halo Interface of NGC 4013
• Diverse metallicities of Fermi bubble clouds indicate dual origins in the disk and halo
• Caught in the Act: A Metal-Rich High-Velocity Cloud in the Inner Galaxy
• VERA astrometry toward the Perseus arm gap
• Simulating the impact of the Smith Cloud
• Exploring Hydrodynamic Instabilities along the Infalling High-Velocity Cloud Complex A
• The First Distance Constraint on the Renegade High Velocity Cloud Complex WD
• On the Origin of High-velocity Clouds in the Galaxy
• Synchrotron Emission from Dark Matter in Galactic Subhalos. A Look into the Smith Cloud
• DIISC-I: The Discovery of Kinematically Anomalous HI Clouds in M 100
• DIISC-II: Unveiling the Connections between Star Formation and ISM in the Extended Ultraviolet Disk of NGC 3344
• Three-Dimensional Orientation of Compact High Velocity Clouds
• Properties of 3D HI Filaments in the Smith High Velocity Cloud
• An Intermediate Velocity HI Cloud Falling to the Galactic Disk; Possible Evidence for Low Metallicity HI Gas Originated Outside the Galactic Disk
• A Component of the Smith High Velocity Cloud Now Crossing the Galactic Plane
• SDSS-IV/MaNGA: Can impulsive gaseous inflows explain steep oxygen abundance profiles \& anomalously-low-metallicity regions?
• A path towards constraining the evolution of the interstellar medium and outflows in the Milky Way using APOGEE
• Neutral Gas Accretion onto Nearby Galaxies
• Cosmic Rays and Non-thermal Emission Induced by Accretion of Cool Gas onto the Galactic Disk
• Simulating High-Velocity Clouds in the Observational Plane: An Initial Study with the Smith Cloud
• Angular Momentum Accretion onto Disc Galaxies