High-Velocity Clouds

Wakker, Bart P.; Woerden, H. van

doi:10.1007/978-94-007-5612-0_12

Cited by 8 publications

(14 citation statements)

References 182 publications

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“…On the other hand, HVCs generally have sub-solar metallicities ranging from a low of ∼10% of the solar value to 20%-60% of the solar value (see Table 2, Wakker & van Woerden 2013). IV21 appears to be in the middle of this metallicity range.…”

Section: The Origins Of a Sub-solar Metallicity For Iv21mentioning

confidence: 99%

A Low-Metallicity Molecular Cloud in the Lower Galactic Halo

Hernandez¹,

Wakker²,

Benjamin

et al. 2013

ApJ

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We find evidence for the impact of infalling, low-metallicity gas on the Galactic disk. This is based on FUV absorption line spectra, 21 cm emission line spectra, and far-infrared (FIR) mapping to estimate the abundance and physical properties of IV21 (IVC135+54-45), a galactic intermediate-velocity molecular cloud that lies ∼300 pc above the disk. The metallicity of IV21 was estimated using observations toward the subdwarf B star PG1144+615, located at a projected distance of 16 pc from the cloud's densest core, by measuring ion and H i column densities for comparison with known solar abundances. Despite the cloud's bright FIR emission and large column densities of molecular gas as traced by CO, we find that it has a sub-solar metallicity of log(Z/Z ) = −0.43 ± 0.12 dex. IV21 is thus the first known sub-solar metallicity cloud in the solar neighborhood. In contrast, most intermediate-velocity clouds (IVC) have near-solar metallicities and are believed to originate in the Galactic Fountain. The cloud's low metallicity is also atypical for Galactic molecular clouds, especially in light of the bright FIR emission which suggest a substantial dust content. The measured I 100 μm /N (H i) ratio is a factor of three below the average found in high latitude H i clouds within the solar neighborhood. We argue that IV21 represents the impact of an infalling, low-metallicity high-velocity cloud that is mixing with disk gas in the lower Galactic halo.

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Section: The Origins Of a Sub-solar Metallicity For Iv21mentioning

confidence: 99%

A Low-Metallicity Molecular Cloud in the Lower Galactic Halo

Hernandez¹,

Wakker²,

Benjamin

et al. 2013

ApJ

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“…Low-metallicity matter of intergalactic origin that falls onto the Galaxy. This latter low-metallicity component is thought to inject fresh material into the Galaxy for star formation and its existence is important to current models of Galactic evolution (e.g., Wakker & Woerden 2013). In any case, HVCs are essential for understanding the flows of energy and mass towards and within the Galaxy.…”

Section: Introductionmentioning

confidence: 99%

“…Several aspects of cloud-disk collisions as well as interactions of HVCs with their environment have been studied (e.g., see Chapter 12 in Wakker & Woerden 2013). The possibility of particle acceleration in these interactions, however, remains to be explored.…”

Section: Introductionmentioning

confidence: 99%

High-energy radiation from collisions of high-velocity clouds and the Galactic disc

Valle

Müller

Romero

2018

Monthly Notices of the Royal Astronomical Society

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High-velocity clouds (HVCs) are interstellar clouds of atomic hydrogen that do not partake of the Galactic rotation and have velocities of a several hundred kilometers per second. A considerable number of these clouds are falling down towards the Galactic disk. HVCs form large and massive complexes, so their collisions with the disk must release a great amount of energy into the interstellar medium. The cloud-disk interaction produces two shocks, one propagates through the cloud and the other through the disk; the properties of these shocks depend mainly on the cloud velocity and the disk-cloud density ratio. In this work we study the conditions necessary for these shocks to accelerate particles by diffusive shock acceleration and the produced non-thermal radiation. We analyze particle acceleration in both the cloud and disk shocks. Solving a time-dependent 2-D transport equation for both relativistic electrons and protons we obtain particle distributions and non-thermal spectral energy distributions. In a shocked cloud significant synchrotron radio emission is produced along with soft gamma rays. In the case of acceleration in the shocked disk, the nonthermal radiation is stronger; the gamma rays, of leptonic origin, might be detectable with current instruments. A large number of protons are injected into the Galactic interstellar medium, and locally exceed the cosmic-ray background. We conclude that under adequate conditions the contribution from HVC-disk collisions to the galactic population of relativistic particles and the associated extended non-thermal radiation might be important.

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“…N (H I) > 2 × 10 18 cm −2 the sky-covering fraction of HVC H I gas is ∼ 30 percent (see, e.g., Murphy et al 1995;Wakker 2004), while for N (H I) > 7 × 10 17 cm −2 the covering fraction increases to almost 50 percent. Note that neutral gas structures with even lower H I column densities do exist.…”

Section: Distribution Of Neutral Gas In the Halomentioning

confidence: 99%

“…A more detailed review of the distribution of HVCs and IVCs in the Milky Way halo is provided by Wakker (2004).…”

Section: Distribution Of Neutral Gas In the Halomentioning

confidence: 99%

High‐Velocity Clouds and the Local Intergalactic Medium

Richter¹

2006

Reviews in Modern Astronomy

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In this article I review recent observations of the gaseous halos of galaxies and the intergalactic medium at low redshift. In the first part I discuss distribution, metal content, and physical properties of the Galactic intermediate- and high-velocity clouds and the hot halo of the Milky Way. Recent absorption and emission measurements show that the Galaxy's tidal interaction with the Magellanic Clouds, the infall of low-metallicity gas, as well as the circulation of gas as part of the galactic fountain contribute to the observed distribution of gas in the halo of the Milky Way. In the second part I give a short overview on the circumgalactic gaseous environment of other nearby spiral galaxies. Multi-wavelength observations demonstrate that neutral and ionized gaseous halos of galaxies are common, and that they extend deep into intergalactic space. These studies suggest that the gaseous material around spiral galaxies is tightly connected to the on-going hierarchical formation and evolution of these galaxies. In the last part of this article I summarize recent quasar absorption-line measurements of the local intergalactic medium. In accordance with cosmological simulations, absorption-line studies in the far-ultraviolet indicate that both the photoionized Ly alpha forest and the shock-heated warm-hot intergalactic medium harbor a substantial fraction of the baryons in the local Universe.Comment: 32 pages, 10 figures; to appear in Reviews in Modern Astronom

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High-Velocity Clouds

Cited by 8 publications

References 182 publications

A Low-Metallicity Molecular Cloud in the Lower Galactic Halo

A Low-Metallicity Molecular Cloud in the Lower Galactic Halo

High-energy radiation from collisions of high-velocity clouds and the Galactic disc

High‐Velocity Clouds and the Local Intergalactic Medium

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