High-Velocity Neutral Hydrogen

Verschuur, Gerrit L.

doi:10.1146/annurev.aa.13.090175.001353

Cited by 48 publications

(32 citation statements)

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“…In early studies (Habing 1966;Hulsbosch & Wakker 1988), this complex was considered to be an HVC. Verschuur (1975) and Haud (1992) have modelled the complex as part of the outer disc of the Milky Way. Based on the deviation velocity, a recent review (Wakker 2004) considers the OAC to be an intermediate-velocity cloud.…”

Section: High Velocitiesmentioning

confidence: 99%

Gaussian decomposition of H I surveys

Haud¹

2008

A&A

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Context. Traditionally intermediate-(IVC)and high-velocity hydrogen clouds (HVC) were defined to be concentrations of H  gas, with line-of-sight velocities that are inconsistent with data on the differential rotation of the Galaxy. Aims. We attempt to demonstrate that IVCs and HVCs can be identified from density enhancements in parameter distributions of Galactic H  21 cm radio lines.Methods. To investigate the properties of the 21 cm radio lines, the profiles of "The Leiden/Argentine/Bonn (LAB) Survey of Galactic H " are decomposed into Gaussian components using a fully automatic algorithm. We focus on some regions with an increased number of Gaussians in phase space, defined by the component central velocity (V C ) and the full width at the level of half maximum (FWHM). To separate the Gaussians responsible for the phase-space density enhancements, we model the width distributions of Gaussian components at equally-populated velocity intervals, using lognormal distributions. Results. We study the Gaussians, which parameters fall into the regions of the phase-space density enhancements at (V C , FWHM) ≈ (−131 This definition is less dependant than the traditional one, on the differential rotation model of the Galaxy. The consideration of line-width information may enable IVCs and HVCs to be better distinguished from each other, and from the ordinary Galactic H .

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Section: High Velocitiesmentioning

confidence: 99%

Gaussian decomposition of H I surveys

Haud¹

2008

A&A

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“…This net negative velocity would imply an infall towards the Local Group barycenter at some 100 km s −1 . The possibility that some of the high-velocity clouds might be essentially extragalactic has been considered in various contexts by, among others, Oort (1966Oort ( , 1970Oort ( , 1981, Verschuur (1975), Eichler (1976), Einasto et al (1976), Giovanelli (1981), Bajaja et al (1987), Wakker & van Woerden (1997), BB99, and Blitz et al (1999). Blitz et al revived the suggestion that high-velocity clouds are the primordial building blocks fueling galactic growth and evolution.…”

Section: W B Burton Et Al: Arecibo Imaging Of Chvcs 617mentioning

confidence: 99%

Arecibo imaging of compact high-velocity clouds

Burton

Braun²,

Chengalur

2001

A&A

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Abstract. Ten isolated compact high-velocity clouds (CHVCs) of the type cataloged by Braun & Burton (1999) were imaged with the Arecibo telescope and were found to have a nested core/halo morphology. We argue that a combination of high-resolution filled-aperture and synthesis data is crucial to determining the intrinsic properties of the CHVCs. We identify the halos as Warm Neutral Medium surrounding one or more cores in the Cool Neutral Medium phase. These halos are clearly detected and resolved by the Arecibo filled-aperture imaging, which reaches a limiting sensitivity (1σ) of NHI ∼ 2 10 17 cm −2 over the typical 70 km s −1 linewidth at zero intensity. The F W HM linewidth of the halo gas is found to be 25 km s −1 , consistent with a WNM thermal broadening within 10 4 K gas. Substantial asymmetries are found at high NHI (>10 18.5 cm −2 ) levels in 60% of our sample. A high degree of reflection-symmetry is found at low NHI (<10 18.5 cm −2 ) in all sources studied at these levels. The column-density profiles of the envelopes are described well by the sky-plane projection of a spherical exponential in atomic volume density, which allows estimating the characteristic central halo column density, NHI(0) = 4.1 ± 3.2 10 19 cm −2 , and characteristic exponential scale-length, hB = 420 ± 90 arcsec. For plausible values of the thermal pressure at the CNM/WNM interface, these edge profiles allow distance estimates to be made for the individual CHVCs studied here which range between 150 and 850 kpc. An alternate method of distance estimation utilizing the mean exponential scale-length found in nearby low mass dwarf galaxies, hB = 10.6 ± 4.0 kpc, yields distances in the range 320 to 730 kpc. A consequence of having exponential edge profiles is that the apparent size and total flux density of these CHVCs will be strongly dependent on the resolution as well as on the sensitivity of the data used; even a relatively deep observation with a limiting sensitivity of ∼10 19 cm −2 over 70 km s −1 will detect only the central 30% of the source area and less than 50% of the total flux density. The exponential profiles also suggest that the outer envelopes of the CHVCs are not tidally truncated. Several CHVC cores exhibit a kinematic gradient, consistent with rotation. The halos appear kinematically decoupled from the cores, in the sense that the halos do not display the velocity gradients shown by the dense cores; the gradients are therefore not likely to be due to an external cause such as tidal shear. The much higher degree of symmetry observed in the halos relative to the cores also argues against an external cause of asymmetries in the cores.

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“…The possibility that some of the high-velocity clouds might be essentially extragalactic has been considered in various contexts by, among others, Oort (1966Oort ( , 1970Oort ( , 1981, Verschuur (1975), Eichler (1976), Einasto et al (1976), Giovanelli (1981), Bajaja et al (1987), Wakker & van Woerden (1997), , and Blitz et al (1999). It is interesting to note that the principal earlier arguments given against a Local Group deployment, most effectively in the papers cited above by Oort and Giovanelli, were based on the angular sizes of the few large complexes and on the predominance of negative velocities in the single hemisphere of the sky for which substantial observational data were then available.…”

Section: Introductionmentioning

confidence: 99%

An all–sky study of compact, isolated high–velocity clouds

Heij

Braun²,

Burton

2002

A&A

105

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Abstract.We combine the catalog of compact high-velocity H  clouds extracted by de Heij et al. (2002) from the Leiden/Dwingeloo Survey in the northern hemisphere with the catalog extracted by from the Parkes HIPASS data in the southern hemisphere, and analyze the all-sky properties of the ensemble. Compact high-velocity clouds are a subclass of the general high-velocity cloud phenomenon which are isolated in position and velocity from the extended high-velocity Complexes and Streams down to column densities below 1.5 × 10 18 cm −2 . Objects satisfying these criteria for isolation are found to have a median angular size of less than one degree. We discuss selection effects relevant to the two surveys; in particular the crucial role played by obscuration due to Galactic H . Five principal observables are defined for the CHVC population: (1) the spatial deployment of the objects on the sky, (2) the kinematic distribution, (3) the number distribution of observed H  column densities, (4) the number distribution of angular sizes, and (5) the number distribution of H  linewidth. Two classes of models are considered to reproduce the observed properties. The agreement of models with the data is judged by extracting these same observables from simulations, in a manner consistent with the sensitivities of the observations and explicitly taking account of Galactic obscuration. We show that models in which the CHVCs are the H  counterparts of dark-matter halos evolving in the Local Group potential provide a good match to the observables. The best-fitting populations have a maximum HI mass of 10 7 M , a power-law slope of the HI mass distribution in the range −1.7 to −1.8, and a Gaussian dispersion for their spatial distributions of between 150 and 200 kpc centered on both the Milky Way and M 31. Given its greater mean distance, only a small fraction of the M 31 sub-population is predicted to have been detected in present surveys. An empirical model for an extended Galactic halo distribution for the CHVCs is also considered. While reproducing some aspects of the population, this class of models does not account for some key systematic features of the population.

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High-Velocity Neutral Hydrogen

Cited by 48 publications

References 0 publications

Gaussian decomposition of H I surveys

Gaussian decomposition of H I surveys

Arecibo imaging of compact high-velocity clouds

An all–sky study of compact, isolated high–velocity clouds

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