The high-velocity clouds above the disk of the outer Milky Way: misty precipitating gas in a region roiled by stellar streams

Abstract

The high-velocity clouds (HVCs) in the outer Milky Way at $20^{\circ }<l<190^{\circ }$ have similar spatial locations, metallicities, and kinematics. Moreover, their locations and kinematics are coincident with several extraplanar stellar streams. The HVC origins may be connected to the stellar streams, either stripped directly from them or precipitated by the aggregate dynamical roiling of the region by the stream progenitors. This paper suggests that these HVCs are "misty" precipitation in the stream wakes based on the following observations. New high-resolution (2.6 km/s) ultraviolet spectroscopy of the QSO H1821+643 resolves what appears to be a single HVC absorption cloud (at 7 km/s resolution) into five components with $T\lesssim 3\times 10^4$ K. Photoionization models can explain the low-ionization components but require some depletion of refractory elements by dust, and model degeneracies allow a large range of metallicity. High-ionization absorption lines (SiIV, CIV, and OVI) are kinematically aligned with the lower-ionization lines and cannot be easily explained with photoionization or equilibrium collisional ionization; these lines are best matched by non-equilibrium rapidly cooling models, i.e., condensing/precipitating gas, with high metallicity and a significant amount of HI. Both the low- and high-ionization phases have low ratios of cooling time to freefall time and cooling time to sound-crossing time, which enables fragmentation and precipitation. The H1821+643 results are corroborated by spectroscopy of six other nearby targets that likewise show kinematically correlated low- and high-ionization absorption lines with evidence of dust depletion and rapid cooling.

Published in MNRAS. Replaced version matches minor changes in section numbering required by MN editors

References in corpus (45)

• Array Programming with NumPy
• The Gaia mission
• Cold streams in early massive hot haloes as the main mode of galaxy formation
• The Circumgalactic Medium
• The effect of photo-ionization on the cooling rates of enriched, astrophysical plasmas
• A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
• Time-Dependent Ionization in Radiatively Cooling Gas
• The Discovery of 1000 km/s Outflows in Massive Post-starburst Galaxies at z=0.6
• The Search for the Missing Baryons at Low Redshift
• The Cosmic Baryon and Metal Cycles
• The COS-Halos Survey: Metallicities in the Low-Redshift Circumgalactic Medium
• A High-Resolution Survey of Low-Redshift QSO Absorption Lines: Statistics and Physical Conditions of O VI Absorbers
• Characterizing the Circumgalactic Medium of Nearby Galaxies with HST/COS and HST/STIS Absorption-Line Spectroscopy
• The COS-Dwarfs Survey: The Carbon Reservoir Around sub-L* Galaxies
• Regulation of star formation in giant galaxies by precipitation, feedback, and conduction
• Magnetized Gas Clouds can Survive Acceleration by a Hot Wind
• The COS-Halos Survey: Origins of the Highly Ionized Circumgalactic Medium of Star-Forming Galaxies
• Resolving small-scale cold circumgalactic gas in TNG50
• On the possible environmental effect in distributing heavy elements beyond individual gaseous halos
• Distances to Galactic high-velocity clouds. Complex C
• Substructure in Tidal Streams; Tributaries in the Anticenter Ring
• An Accurate Distance to High-Velocity Cloud Complex C
• The Far Ultraviolet Spectroscopic Explorer Survey of OVI Absorption in the Disk of the Milky Way
• Efficiency of gas cooling and accretion at the disc-corona interface
• The Properties of Low Redshift Intergalactic O VI Absorbers Determined from High S/N Observations of 14 QSOs with the Cosmic Origins Spectrograph
• A Magellanic Origin for the Warp of the Galaxy
• CalFUSE v3: A Data-Reduction Pipeline for the Far Ultraviolet Spectroscopic Explorer
• Smith's Cloud: A High-velocity Cloud Colliding with the Milky Way
• COS-Burst: Observations of the Impact of Starburst-Driven Winds on the Properties of the Circum-Galactic Medium
• The origin of the high-velocity cloud complex C
• Metallicity and Ionization in High Velocity Cloud Complex C
• The evolution of interstellar clouds in a streaming hot plasma including heat conduction
• It's Cloud's Illusions I Recall: Mixing Drives the Acceleration of Clouds from Ram Pressure Stripped Galaxies
• The Survival of Multiphase Dusty Clouds in Hot Winds
• The COS Absorption Survey of Baryon Harbors: Unveiling the Physical Conditions of Circumgalactic Gas through Multiphase Bayesian Ionization Modeling
• The Physics of Galactic Winds Driven by Cosmic Rays II: Isothermal Streaming Solutions
• An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
• Interaction of Cosmic Rays with Cold Clouds in Galactic Halos
• A Graphical Interpretation of Circumgalactic Precipitation
• Mass, Morphing, Metallicities: The Evolution of Infalling High Velocity Clouds
• Kinematics beats dust: unveiling nested substructure in the perturbed outer disc of the Milky Way
• Molecular Gas within the Milky Way's Nuclear Wind
• The HI Column Density Distribution of the Galactic Disk and Halo
• Exploring Hydrodynamic Instabilities along the Infalling High-Velocity Cloud Complex A
• Resolving the formation of cold HI filaments in the high velocity cloud complex C

Cited by in corpus (8)

• Cloudy with A Chance of Rain: Accretion Braking of Cold Clouds
• Radiative turbulent mixing layers at high Mach numbers
• Directly constraining the spatial coherence of the $z\sim 1$ circumgalactic medium
• Detection of Dust in High-Velocity Cloud Complex C -- Enriched Gas Accreting onto the Milky Way
• On the Origin of High-velocity Clouds in the Galaxy
• On the patchy appearance of the circum-Galactic medium and the influence of foreground absorption
• The Warm-Hot Disk-Halo Interface Below the Perseus Spiral Arm
• The Observed O VI is Just the Tip of the Iceberg: Estimating the Hidden Material in Circumgalactic and Intergalactic Clouds