The Cool Phase of Neutral Hydrogen in the Galaxy
by Simon Strasser
Under the supervision of Dr. John Dickey & Dr. Evan Skillman
We analyze 815 emission and absorption spectra toward continuum background point sources in the Canadian, Southern, and VLA Galactic Plane Surveys (CGPS, SGPS, and VGPS). The neutral hydrogen (HI) data covers the plane between Galactic longitude 18 degrees, and 174 degrees, and between longitude 253 degrees to 355 degrees,at approximately 1 arcminute resolution. We are thus able to study the global variation in fundamental ISM parameters.
We find spin temperatures (T_s) between 20~K and 300~K, with a distribution peak around 100~K. We do not observe a strong dependence on Galactocentric radius, R, in the northern Galaxy (covered by the VGPS and CGPS) but in the SGPS T_s clearly drops outside the solar circle. The opacity generally decreases with $R$, peaking inside the solar circle in all three surveys. The ratio of the opacity to the column density rises with R in the CGPS and VGPS, but stays constant in the SGPS.
We are not able to fit Gaussian components to a majority of the emission spectra because they are highly blended. By approximating the unabsorbed warm-phase emission with a second order polynomial, we are able to derive a cool-phase temperature, T_c. T_c ranges between 20 K and 200 K, and its distribution peaks at 50 K to 60 K. Values in the SGPS are somewhat higher than in the CGPS. The fraction of cool-phase gas drops slowly with R, from 0.4 to approximately 0.25.
To investigate the presence of absorption lines narrower than 1 km/s, we obtained high spectral resolution (0.16 km/s) VLA spectra toward five background sources. Toward each of the targets at least one absorption line in the CGPS is only partially resolved. We find one absorption line with a width of only 19 +/- 9 K, significantly narrower than indicated by the lower resolution data.
We trace the Outer and Distant spiral arms in HI absorption throughout the surveys. We find several distinct clouds within these arms, and are able to calculate arm pitch angles. This is the first detection of a widespread presence of HI in the far outer Galaxy (beyond R = 17 kpc).