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Register a new userSimulations of High-Velocity Clouds. I. Hydrodynamics and High-Velocity High Ions
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We present hydrodynamic simulations of high-velocity clouds (HVCs) traveling through the hot, tenuous medium in the Galactic halo. A suite of models was created using the FLASH hydrodynamics code, sampling various cloud sizes, densities, and velocities. In all cases, the cloud-halo interaction ablates material from the clouds. The ablated material falls behind the clouds, where it mixes with the ambient medium to produce intermediate-temperature gas, some of which radiatively cools to less than 10,000 K. Using a non-equilibrium ionization (NEI) algorithm, we track the ionization levels of carbon, nitrogen, and oxygen in the gas throughout the simulation period. We present observation-related predictions, including the expected H I and high ion (C IV, N V, and O VI) column densities on sight lines through the clouds as functions of evolutionary time and off-center distance. The predicted column densities overlap those observed for Complex C. The observations are best matched by clouds that have interacted with the Galactic environment for tens to hundreds of megayears. Given the large distances across which the clouds would travel during such time, our results are consistent with Complex C having an extragalactic origin. The destruction of HVCs is also of interest; the smallest cloud (initial mass approx 120 Msun) lost most of its mass during the simulation period (60 Myr), while the largest cloud (initial mass approx 4e5 Msun) remained largely intact, although deformed, during its simulation period (240 Myr).
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In order to determine if the material ablated from high-velocity clouds (HVCs) is a significant source of low-velocity high ions (C IV, N V, and O VI) such as those found in the Galactic halo, we simulate the hydrodynamics of the gas and the time-dependent ionization evolution of its carbon, nitrogen, and oxygen ions. Our suite of simulations examines the ablation of warm material from clouds of various sizes, densities, and velocities as they pass through the hot Galactic halo. The ablated material mixes with the environmental gas, producing an intermediate-temperature mixture that is rich in high ions and that slows to the speed of the surrounding gas. We find that the slow mixed material is a significant source of the low-velocity O VI that is observed in the halo, as it can account for at least ~1/3 of the observed O VI column density. Hence, any complete model of the high ions in the halo should include the contribution to the O VI from ablated HVC material. However, such material is unlikely to be a major source of the observed C IV, presumably because the observed C IV is affected by photoionization, which our models do not include. We discuss a composite model that includes contributions from HVCs, supernova remnants, a cooling Galactic fountain, and photoionization by an external radiation field. By design, this model matches the observed O VI column density. This model can also account for most or all of the observed C IV, but only half of the observed N V.
We present a proof-of-concept study of a method to estimate the inclination angle of compact high velocity clouds (CHVCs), i.e. the angle between a CHVCs trajectory and the line-of-sight. The inclination angle is derived from the CHVCs morphology and kinematics. We calibrate the method with numerical simulations, and we apply it to a sample of CHVCs drawn from HIPASS. Implications for CHVC distances are discussed.
We examine the proposal that the HI high-velocity clouds (HVCs) surrounding the Milky Way and other disc galaxies form by condensation of the hot galactic corona via thermal instability. Under the assumption that the galactic corona is well represented by a non-rotating, stratified atmosphere, we find that for this formation mechanism to work the corona must have an almost perfectly flat entropy profile. In all other cases the growth of thermal perturbations is suppressed by a combination of buoyancy and thermal conduction. Even if the entropy profile were nearly flat, cold clouds with sizes smaller than 10 kpc could form in the corona of the Milky Way only at radii larger than 100 kpc, in contradiction with the determined distances of the largest HVC complexes. Clouds with sizes of a few kpc can form in the inner halo only in low-mass systems. We conclude that unless even slow rotation qualitatively changes the dynamics of a corona, thermal instability is unlikely to be a viable mechanism for formation of cold clouds around disc galaxies.
We consider here the class of compact, isolated, high-velocity HI clouds, CHVCs, which are sharply bounded in angular extent down to a limiting column density of 1.5x10^18 cm^-2. We describe our automated search algorithm and its application to the LDS north of dec= -28 deg. and the HIPASS data south of dec=0, resulting in an all--sky catalog numbering 246 CHVCs. We argue that these objects are more likely to represent a single phenomenon in a similar evolutionary state than would a sample which included any of the major HVC complexes. 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 HI column densities, (4) the number distribution of angular sizes, and (5) the number distribution of line widths. We show that the spatial and kinematic deployments of the ensemble of CHVCs contain various clues regarding their characteristic distance. These clues are not compatible with a location of the ensemble within the Galaxy proper. The deployments resemble in several regards those of the Local Group galaxies. We describe a model testing the hypothesis that the CHVCs are a Local Group population. The agreement of the model with the data is judged by extracting the 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 HI counterparts of dark-matter halos evolving in the Local Group potential provide a good match to the observables, if account is taken of tidal and ram--pressure disruption, the consequences of obscuration due to Galactic HI and of differing sensitivities and selection effects pertaining to the surveys.
We have imaged five compact high-velocity clouds in HI with arcmin angular- and km/s spectral-resolution using the WSRT. Supplementary total-power data, which is fully sensitive to both the cool and warm components of HI, is available for comparison for all the sources, albeit with angular resolutions that vary from 3 to 36. The fractional HI flux in compact CNM components varies from 4% to 16% in our sample. All objects have at least one local peak in the CNM column which exceeds about 10^19 cm^-2 when observed with arcmin resolution. It is plausible that a peak column density of 1-2x10^19 cm^-2 is a prerequisite for the long-term survival of these sources. One object in our sample, CHVC120-20-443 (Davies cloud), lies in close projected proximity to the disk of M31. This object is characterized by exceptionally broad linewidths in its CNM concentrations (more than 5 times greater than the median value). These CNM concentrations lie in an arc on the edge of the source facing the M31 disk, while the diffuse HI component of this source has a position offset in the direction of the disk. All of these attributes suggest that CHVC120-20-443 is in a different evolutionary state than most of the other CHVCs which have been studied. Similarly broad CNM linewidths have only been detected in one other object, CHVC111-07-466, which also lies in the Local Group barycenter direction and has the most extreme radial velocity known. A distinct possibility for Davies cloud seems to be physical interaction of some type with M31. The most likely form of this interaction might be the ram-pressure or tidal- stripping by either one of M31s visible dwarf companions, M32 or NGC205, or else by a dark companion with an associated HI condensation.
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