I Emission and Absorption in the Southern Galactic Plane
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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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 ($\sim 1-2$ km ${\rm s}^{-1}$) 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.

Figure 6: This grey-scale plot is an H I channel map at v=-41.7 km ${\rm s}^{-1}$ with a 55 K contour of the same channel overlaid. The grey scale is linear and runs from 40 to 105 K, as shown in the wedge on the right. The contours outline several prominent sources of absorption, including a supernova remnant. The extended areas marked as HISA are cold clouds seen in absorption.
\begin{figure} \psfig{file=hisa.ps,height=10cm,angle=-90} \end{figure}

There are numerous HISA features in the SGPS Test Region. These cold clouds can be seen in the channel map shown in Figure 6. An absorption profile towards the cloud at

$l=330\mbox{$.\!\!^\circ$}5$,

$b=0\hbox {$^\circ $}$ shows a characteristically narrow profile with a spin temperature of $\sim 30$ K (Figure 7). Most of these HISA features are in the velocity range -50 km ${\rm s}^{-1}$

$\leq v \leq -40$ km ${\rm s}^{-1}$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).

Figure 7: The top panel is the emission profile near

$l=330\hbox {$^\circ $}$

$b=0\hbox {$^\circ $}$. The bottom panel shows the absorption profile for the cold cloud seen at

$l=330\mbox{$.\!\!^\circ$}5$,

$b=0\hbox {$^\circ $}$. The dotted lines trace the $1\sigma $ error envelope. The narrow width of the absorption feature is one of the defining characteristics of HISA.

\begin{figure} \par\psfig{file=hisa_abs.ps,height=8cm} \par \end{figure}

The smallest and most dramatic features tend to stand out in initial searches for HISA. In order to study the HISA in a statistically significant way, it will be necessary to develop non-subjective searching techniques. Such techniques have been developed for the CGPS data by Gibson et al. (2000). Similar techniques are being developed for the SGPS data. The many cold clouds detected in the CGPS and SGPS suggest excess emission fluctuations on the small side of the spatial power spectrum. It is possible that there are many warmer, less dense clouds that we are not detecting.
Next Section: Questions and Future Work
Title/Abstract Page: H I Emission and Absorption
Previous Section: H I Shells
Contents Page: Volume 18, Number 1

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