Incidence of 21 centimeter absorption in QSO redshift systems selected for Mg II absorption: evidence for a two-phase nature of the absorbing gas
A search for 21 cm absorption in the continua of 15 radio-bright QSOs reveals that 21 cm absorption occurs in two out of 18 redshifted clouds selected for Mg II absorption. There is no evidence that optical properties such as Mg II equivalent width, Mg II doublet ratio, or Mg I equivalent width are correlated with 21 cm optical depth. Although the unsaturated appearance of the Mg II doublet detected at z = 0.395 in 1229--021 indicates that this absorber should be transparent to 21 cm radiation, it is one of two systems found to exhibit 21 cm absorption. The lack of correlation between optical and radio absorption properties is related to the multicomponent nature of the absorbing gas. While most of the narrow velocity components detected in this gas may not contain enough H I to produce 21 cm absorption, they still dominate saturated equivalent widths of UV resonance lines owing to the large internal velocity interval which they span. Occasionally, most of the H I is in one or more narrow components which produce detectable 21 cm absorption, but which make negligible contribution to optical equivalent widths. Lower limits are placed on the spin temperature of three Mg II systems, which rule out galactic disks as the absorption sites. A two-phase model is suggested to unify the entire sample of Mg II systems. It is likely that the thick phase is comprised of clouds in galactic disks which produce 21 cm absorption, while thin-phase clouds are transparent to 21 cm radiation and reside in galactic halos. The Mg II systems differ depending on how much gas is encountered along the line of sight in each phase. Comparison of the small cross section of the disk with the larger cross section of the halo explains why the incidence of 21 cm absorption in the Mg II systems is so rare.
- Research Organization:
- Department of Physics and Astronomy, University of Pittsburgh
- OSTI ID:
- 6049736
- Journal Information:
- Astrophys. J.; (United States), Vol. 268:1
- Country of Publication:
- United States
- Language:
- English
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GENERAL PHYSICS
MAGNESIUM IONS
ABSORPTION
QUASARS
ABSORPTION SPECTRA
COSMIC GASES
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INTERFEROMETRY
RADIO GALAXIES
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640103* - Astrophysics & Cosmology- Quasi-Stellar
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