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Register a new userPhysical properties of two compact high-velocity clouds possibly associated with the Leading Arm of the Magellanic System
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We observed two compact high-velocity clouds HVC 291+26+195 and HVC 297+09+253 to analyse their structure, dynamics, and physical parameters. In both cases there is evidence for an association with the Leading Arm of the Magellanic Clouds. The goal of our study is to learn more about the origin of the two CHVCs and to use them as probes for the structure and evolution of the Leading Arm. We have used the Parkes 64 m radio telescope and the Australia Telescope Compact Array (ATCA) to study the two CHVCs in the 21 cm line emission of neutral hydrogen. We present a method to estimate the distance of the two CHVCs. The investigation of the line profiles of HVC 297+09+253 reveals the presence of two line components in the spectra which can be identified with a cold and a warm gas phase. In addition, we find a distinct head-tail structure in combination with a radial velocity gradient along the tail, suggesting a ram-pressure interaction of this cloud with an ambient medium. HVC 291+26+195 has only a cold gas phase and no head-tail structure. The ATCA data show several cold, compact clumps in both clouds which, in the case of HVC 297+09+253, are embedded in the warm, diffuse envelope. All these clumps have very narrow HI lines with typical line widths between 2 and 4 km/s FWHM, yielding an upper limit for the kinetic temperature of the gas of T_max = 300 K. We obtain distance estimates for both CHVCs of the order of 10 to 60 kpc, providing additional evidence for an association of the clouds with the Leading Arm.
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We present a catalog of high-velocity clouds in the region of the Magellanic Leading Arm. The catalog is based on neutral hydrogen (HI) observations from the Parkes Galactic All-Sky Survey (GASS). Excellent spectral resolution allows clouds with narrow-line components to be resolved. The total number of detected clouds is 419. We describe the method of cataloging and present the basic parameters of the clouds. We discuss the general distribution of the high-velocity clouds and classify the clouds based on their morphological type. The presence of a significant number of head-tail clouds and their distribution in the region is discussed in the context of Magellanic System simulations. We suggest that ram-pressure stripping is a more important factor than tidal forces for the morphology and formation of the Magellanic Leading Arm and that different environmental conditions might explain the morphological difference between the Magellanic Leading Arm and Magellanic Stream. We also discuss a newly identified population of clouds that forms the LA IV and a new diffuse bridge-like feature connecting the LA II and III complexes.
We present a high-resolution study of five high-velocity clouds in the Magellanic Leading Arm region. This is a follow-up study of our widefield Parkes survey of the region in order to probe the multiphase structures of the clouds and to give an insight to their origin, evolution and distance. High-resolution data were obtained from the Australia Telescope Compact Array. By combining with single-dish data from the Galactic All-Sky Survey (GASS), we are able to probe compact and diffuse emission simultaneously. We identify resolved and unresolved clumps. Physical parameters were derived for both diffuse structure and compact clumps. The latter are cold with typical velocity linewidths of 5 km/s. We find a gradient in thermal halo pressure, hydrogen density and HI column density of HVC as a function of Galactic latitude. This is possibly the first observational evidence of varying distance in the Leading Arm region, with the leading part of the Leading Arm (LA II and III) probably being closer to the Galactic disc than the trailing end (LA I).
The Leading Arm of the Magellanic System is a tidally formed HI feature extending $sim 60arcdeg$ from the Magellanic Clouds ahead of their direction of motion. Using atomic hydrogen (HI) data from the Galactic All Sky-Survey (GASS), supplemented with data from the Australia Telescope Compact Array, we have found evidence for an interaction between a cloud in the Leading Arm and the Galactic disk where the Leading Arm crosses the Galactic plane. The interaction occurs at velocities permitted by Galactic rotation, which allows us to derive a kinematic distance to the cloud of 21 kpc, suggesting that the Leading Arm crosses the Galactic Plane at a Galactic radius of $Rapprox 17$ kpc.
The Leading Arm (LA) of the Magellanic Stream is a vast debris field of H I clouds connecting the Milky Way and the Magellanic Clouds. It represents an example of active gas accretion onto the Galaxy. Previously only one chemical abundance measurement had been made in the LA. Here we present chemical abundance measurements using Hubble Space Telescope/Cosmic Origins Spectrograph Green Bank Telescope spectra of four sightlines passing through the LA, and three nearby sightlines that may trace outer fragments of the LA. We find low oxygen abundances, ranging from 4.0(+4.0,-2.0) percent solar to 12.6(+6.2,-4.1) percent solar, in the confirmed LA directions, with the lowest values found in the region known as LA III, farthest from the LMC. These abundances are substantially lower than the single previous measurement, S/H=35+/-7 percent solar (Lu et al. 1998), but are in agreement with those reported in the SMC filament of the trailing Stream, supporting a common origin in the SMC (not the LMC) for the majority of the LA and the trailing Stream. This provides important constraints for models of the formation of the Magellanic System. Finally, the HVCs in two of the three nearby sightlines show H I columns, kinematics, and oxygen abundances consistent with LA membership. This suggests that the LA is larger than traditionally thought, extending at least 20 degrees further to the Galactic northwest.
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.
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