The Distance and Stability of the HI Filament that Comprises HVC A0

Details The Distance and Stability of the HI Filament that Comprises HVC A0 The Distance and Stability of the HI Filament that Comprises HVC A0

May 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21832103v&link_type=abstract

American Astronomical Society, AAS Meeting #218, #321.03; Bulletin of the American Astronomical Society, Vol. 43, 2011

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Scientific paper

The neutral hydrogen structure of high-velocity "cloud” A0 has been mapped using the Green Bank Telescope of the NRAO. The HI gas is in the form of a narrow, twisted filament with a typical line width of order 23 km/s within which regions of enhanced emission with line widths of order 4 to 6 km/s and as high as 15 km/s are located. Gaussian decomposition of the profiles is used to reveal the filament most clearly. If it is in equilibrium with surrounding galactic halo pressure its distance must be about 5,400 pc. However, the observed HI line widths are not a measure of the kinetic temperature of the gas. Instead their magnitude appears to be consistent with the presence of Alfvén waves for a magnetic field of order 2.8 µG. The existence of the bright, narrow line width features is more problematic. Geometry alone may account for their brightness and line widths. On the other hand, if the small-scale structures have angular scales of order 1.'5, lost inside the resolution of our data, these features, too, could be in equilibrium with halo pressure. In either case, the bright features exist relatively independent of the filament structure itself. This work suggests that the distance to HI filaments can be determined provided reliable estimates of surrounding gas pressure are available and if we are reasonably certain that a segment of filament is being viewed side-on.

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