Interemdiate-velocity Clouds

Albert, C. E.; Danly, L.

doi:10.1007/1-4020-2579-3_4

Cited by 6 publications

(1 citation statement)

References 75 publications

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“…Panopoulou & Lenz 2020). IVCs have broadly been classified (Albert & Danly 2004) as having |V LSR | between 20 and 90 km s −1 . In recent surveys, the IVC ranges were chosen as 30-90 km s −1 (Richter et al 2003) and 40-90 km s −1 (Lehner et al 2022).…”

Section: Distinguishing Low-velocity and High-velocity Gasmentioning

confidence: 99%

Variations of Interstellar Gas-to-dust Ratios at High Galactic Latitudes

Shull,

Panopoulou

2024

ApJ

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Interstellar dust at high Galactic latitudes can influence astronomical foreground subtraction, produce diffuse scattered light, and soften the UV spectra of quasars. In a sample of 94 sight lines toward quasars at high latitude and low extinction, we evaluate the interstellar “gas-to-dust ratio” N H/E(B − V), using hydrogen column densities (H i and H2) and far-IR (FIR) estimates of dust reddening. In the Galactic plane, this ratio is 6.0 ± 0.2 (in units of 1021 cm−2 mag−1). On average, recent Planck estimates of E(B − V) in low-reddening sight lines are 12% higher than those from Schlafly & Finkbeiner, and N H I exhibits significant variations when measured at different radio telescopes. In a sample of 51 quasars with measurements of both H i and H2 and 0.01 ≤ E(B − V) ≲ 0.1, we find mean ratios 10.3 ± 0.4 (gas at all velocities) and 9.2 ± 0.3 (low-velocity only) using Planck E(B − V) data. High-latitude H2 fractions are generally small (2%–3% on average), although nine of 39 sight lines at ∣b∣ ≥ 40° have f H2 of 1%–17%. Because FIR-inferred E(B − V) is sensitive to modeled dust temperature T d and emissivity index β, gas-to-dust ratios have large, asymmetric errors at low E(B − V). The ratios are elevated in sight lines with high-velocity clouds, which contribute N H but little reddening. In Complex C, the ratio decreases by 40% when high-velocity gas is excluded. Decreases in dust content are expected in low-metallicity gas above the Galactic plane, resulting from grain destruction in shocks, settling to the disk, and thermal sputtering in hot halo gas.

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Section: Distinguishing Low-velocity and High-velocity Gasmentioning

confidence: 99%

Variations of Interstellar Gas-to-dust Ratios at High Galactic Latitudes

Shull,

Panopoulou

2024

ApJ

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THE GALFA-H i COMPACT CLOUD CATALOG

Saul

Peek

Grcevich

et al. 2012

ApJ

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We present a catalog of 1964 isolated, compact neutral hydrogen clouds from the Galactic Arecibo L-Band Feed Array Survey Data Release One. The clouds were identified by a custom machine-vision algorithm utilizing the difference of Gaussian kernels to search for clouds smaller than 20 . The clouds have velocities typically between |V LSR | = 20 and 400 km s −1 , line widths of 2.5-35 km s −1 , and column densities ranging from 1 to 35 × 10 18 cm −2 . The distances to the clouds in this catalog may cover several orders of magnitude, so the masses may range from less than a solar mass for clouds within the Galactic disk, to greater than 10 4 M for high-velocity clouds (HVCs) at the tip of the Magellanic Stream. To search for trends, we separate the catalog into five populations based on position, velocity, and line width: HVCs; galaxy candidates; cold low-velocity clouds (LVCs); warm, low positive-velocity clouds in the third Galactic quadrant; and the remaining warm LVCs. The observed HVCs are found to be associated with previously identified HVC complexes. We do not observe a large population of isolated clouds at high velocities as some models predict. We see evidence for distinct histories at low velocities in detecting populations of clouds corotating with the Galactic disk and a set of clouds that is not corotating.

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Multiphase neutral interstellar medium: analysing simulation with H i 21cm observational data analysis techniques

Bhattacharjee,

Roy,

Sharma

et al. 2023

Monthly Notices of the Royal Astronomical Society

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Several different methods are regularly used to infer the properties of the neutral interstellar medium (ISM) using atomic hydrogen (H i) 21cm absorption and emission spectra. In this work, we study various techniques used for inferring ISM gas phase properties, namely the correlation between brightness temperature and optical depth (TB(v), τ(v)) at each channel velocity (v), and decomposition into Gaussian components, by creating mock spectra from a 3D magnetohydrodynamic simulation of a two-phase, turbulent ISM. We propose a physically motivated model to explain the TB(v) − τ(v) distribution and relate the model parameters to properties like warm gas spin temperature and cold cloud length-scales. Two methods based on Gaussian decomposition – using only absorption spectra and both absorption and emission spectra – are used to infer the column density distribution as a function of temperature. In observations, such analysis reveals the puzzle of large amounts (significantly higher than in simulations) of gas with temperature in the thermally unstable range of ∼200–2000 K and a lack of the expected bimodal (two-phase) temperature distribution. We show that, in simulation, both methods are able to recover the actual gas distribution in the simulation till temperatures ≲2500 K (and the two-phase distribution in general) reasonably well. We find our results to be robust to a range of effects such as noise, varying emission beam size, and simulation resolution. This shows that the observational inferences are unlikely to be artefacts, thus highlighting a tension between observations and simulations. We discuss possible reasons for this tension and ways to resolve it.

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Interemdiate-velocity Clouds

Cited by 6 publications

References 75 publications

Variations of Interstellar Gas-to-dust Ratios at High Galactic Latitudes

Variations of Interstellar Gas-to-dust Ratios at High Galactic Latitudes

THE GALFA-H i COMPACT CLOUD CATALOG

Multiphase neutral interstellar medium: analysing simulation with H i 21cm observational data analysis techniques

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