Neutral atomic hydrogen gas (``H I'') emits and absorbs energy at a characteristic wavelength of 21cm, which can be observed with radio telescopes. Usually, H I is seen as bright emission against a darker background, but under the right viewing conditions, it can also be seen as dark absorption against a brighter background. In the case where the background is itself H I emission, the hydrogen is said to be self-absorbed, and the radiative transfer process is termed H I self-absorption (HISA). A more detailed explanation of this process is available.
We are studying the large number of HISA clouds revealed by the ongoing Canadian Galactic Plane Survey (CGPS), which is mapping out a large section of the Milky Way at arcminute resolution in several radio and infrared wavelengths. We wish to understand the physical properties and structure of these enigmatic objects, many of which are revealed for the first time by the unprecedented spatial dynamic range of our survey. The relationship between HISA and other phases of the interstellar matter, such as molecular gas and dust, is also of considerable interest to us, given the frequently assumed but hardly universal correlations between HISA, molecular tracers like carbon monoxide (CO), and far-infrared dust emission features. The broad spatial coverage, high angular resolution, and multiwavelength composition of the CGPS make it a very powerful tool for this endeavor.
Shown below is a single CGPS H I 21cm spectral line channel at a radial velocity of -41 km/s in the Local Standard of Rest, at l=140°, b=+1°. The intensity ranges from 50 (black) to 130 K (white) in brightness temperature units. The red contours indicate 12CO J=1-0 brightness at the same radial velocity, ranging from 1-10 K. Note how the CO coincides with some instances of HISA, but more often the two seem independent of each other. Where they do appear associated, there is no simple proportionality between CO brightness and HISA darkness. The CO morphology also leans more toward knotlike concentrations, while the HISA is more extended and filamentary.
The yellow box in the figure above marks one particularly good alignment of CO and HISA. ``ON'' spectra extracted from this region are shown below (solid), along with an ``OFF'' spectrum (dashed) of the H I from the bright area to the north of the HISA feature. The HISA and CO velocity structures agree quite well. This velocity of -41 km/s falling within the range predicted by models of shocked gas in the Perseus Arm spiral density wave in this direction leads us to wonder whether we might be seeing atomic gas compressed by the spiral shock and on its way to forming molecular clouds, and eventually, new stars.
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