Survey1
N. M. McClure-Griffiths ,
John M. Dickey ,
B. M. Gaensler ,
A. J. Green ,
R. F. Haynes ,
M. H. Wieringa
, PASA, 18 (1), in press.
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Title/Abstract Page: H I Emission and Absorption
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H I Self-Absorption (HISA)
On the other end of the H I spatial power spectrum we are using H I self-absorption (HISA) to study the cold neutral medium (CNM). HISA occurs when cold foreground gas absorbs H I emission from warmer background gas at the same velocity. The correspondence of one velocity to two distances interior to the solar circle provides a prime opportunity for studies of HISA. HISA is characterized by extremely narrow velocity width ( km ) absorption features seen against the warmer background hydrogen emission. The small, cold clouds are also seen in the H I channel maps as in Figure 6. New high-resolution H I studies, such as the Canadian Galactic Plane Survey (Taylor et al. 1999), are revealing large numbers of cold clouds through HISA (Gibson et al. 2000). Because HISA does not require background continuum sources, it is an excellent way to study the cold neutral medium (CNM) throughout the Galaxy.,
shows a characteristically narrow profile with a spin temperature of K (Figure 7). Most of these HISA features are in the velocity range -50 km
km which, at these longitudes, corresponds to the edge of the Scutum-Crux spiral arm. We suggest that these cold clouds may mark the compressed gas at the edge of the spiral shock, where molecular clouds are likely to form. While some HISA does clearly trace molecular emission, other HISA are isolated, suggesting cold gas that has not yet been compressed enough to form molecules (Gibson et al. 2000).
Next Section: Questions and Future Work
Title/Abstract Page: H I Emission and Absorption
Previous Section: H I Shells
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