Arecibo Emission-Absorption Studies of the Interstellar Medium.
Abstract
Arecibo emission-absorption studies of HI, high velocity HI, OH, and H(,2)CO are presented. Errors in galactic HI spectra caused by stray radiation into the far sidelobes of the antenna power pattern are estimated from calibration measurements and observations toward the moon. From observations toward nineteen low galactic latitude continuum sources, the variation of HI properties with lati- tude are explored. The observations of column density and inte- grated optical depth are consistent with high latitude studies when allowance is made for the well known deficiency of HI in the solar neighborhood. The spin temperatures and column densities of low latitude absorption features are overestimated by a factor of two because of blending with low optical depth emission. There is a defi- ciency of low optical depth features because of overlap with larger optical depth features. Limits to the structure with widths less than 0.5 km s('-1) in HI spectra towards twenty high latitude sources imply limits to the HI in molecular regions of a few percent of the total hydrogen. These studies also yield limits of N(,HI(, ))< 5 x 10('18) cm('-2) for cold, isolated HI features with (DELTA)(upsilon)(, )< 0.5 km s('-1). There is little corre- lation between the properties of HI emission fluctuations on angular scales of 3.'8 and the properties of either the absorption or the emission spectra. A survey for emission and absorption from high velocity HI shows that there is little cold, high velocity HI. OH observations towards the nineteen low latitude continuum sources have resulted in the detec- tion of eleven absorption features. In three cases, the ratio of the two OH main lines is not the thermal equilibrium ratio. A comparison of the OH velocities and linewidths with HI and CO studies indicates that the OH is confined to small volumes within the CO and HI regions. H(,2)CO absorption toward 3C 154 and 3C 123 has been studied. Toward 3C 123, the H(,2)CO line has a dispersion of only 0.13 km s('-1). The abundance of H(,2)CO relative to OH and CH varies by at least a factor of five between two otherwise similar regions. A VLA map shows that the absorption across 3C 123 is constant to 10%.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1986
- Bibcode:
- 1986PhDT.........4C
- Keywords:
-
- Physics: Astronomy and Astrophysics;
- Absorption Spectra;
- Carbon Monoxide;
- Emission Spectra;
- Interstellar Matter;
- Spin Temperature;
- Abundance;
- Astrophysics;
- H I Regions;
- H Lines;
- Interstellar Gas;
- Solar Neighborhood;
- Astrophysics