The high-velocity clouds (HVCs) in the outer Milky Way at 20° < l < 190° 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−1) ultraviolet spectroscopy of the QSO H1821+643 resolves what appears to be a single HVC absorption cloud (at 7 km s−1 resolution) into five components with T ≲ 3 × 104 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 (Si iv, C iv, and O vi) 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 H i. Both the low- and high-ionization phases have low ratios of cooling time to freefall time andmore »
Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher.
Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?
Some links on this page may take you to non-federal websites. Their policies may differ from this site.
The high-velocity clouds above the disc of the outer Milky Way: misty precipitating gas in a region roiled by stellar streams
We combine 126 new galaxy-O
viabsorber pairs from the CGM2survey with 123 pairs drawn from the literature to examine the simultaneous dependence of the column density of O viabsorbers ( N OVI) on galaxy stellar mass, star-formation rate, and impact parameter. The combined sample consists of 249 galaxy-O viabsorber pairs covering z= 0–0.6, with host galaxy stellar masses M*= 107.8–1011.2 M⊙and galaxy-absorber impact parameters R⊥= 0–400 proper kiloparsecs. In this work, we focus on the variation of N OVIwith galaxy mass and impact parameter among the star-forming galaxies in the sample. We find that the average N OVIwithin one virial radius of a star-forming galaxy is greatest for star-forming galaxies with M*= 109.2–1010 M⊙. Star-forming galaxies with M*between 108and 1011.2 M⊙can explain most O visystems with column densities greater than 1013.5cm−2. Sixty percent of the O vimass associated with a star-forming galaxy is found within one virial radius, and 35% is found between one and two virial radii. In general, we find that some departure from hydrostatic equilibrium in the CGM is necessary to reproduce the observed O viamount, galaxy mass dependence, and extent. Our measurements serve as a test set for CGM models over a broad range of host galaxy masses.
The COS Absorption Survey of Baryon Harbors: unveiling the physical conditions of circumgalactic gas through multiphase Bayesian ionization modellingABSTRACT Quasar absorption systems encode a wealth of information about the abundances, ionization structure, and physical conditions in intergalactic and circumgalactic media. Simple (often single-phase) photoionization models are frequently used to decode such data. Using five discrete absorbers from the COS Absorption Survey of Baryon Harbors (CASBaH) that exhibit a wide range of detected ions (e.g. Mg ii, S ii – S vi, O ii – O vi, Ne viii), we show several examples where single-phase ionization models cannot reproduce the full set of measured column densities. To explore models that can self-consistently explain the measurements and kinematic alignment of disparate ions, we develop a Bayesian multiphase ionization modelling framework that characterizes discrete phases by their unique physical conditions and also investigates variations in the shape of the UV flux field, metallicity, and relative abundances. Our models require at least two (but favour three) distinct ionization phases ranging from T ≈ 104 K photoionized gas to warm-hot phases at T ≲ 105.8 K. For some ions, an apparently single absorption ‘component' includes contributions from more than one phase, and up to 30 per cent of the H i is not from the lowest ionization phase. If we assume that all of the phases are photoionized, we cannot find solutionsmore »