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Aperture synthesis observations of OH absorption in the galactic center


Bieging, John Harold (1974) Aperture synthesis observations of OH absorption in the galactic center. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/HD0Q-HN74.


NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The galactic center radio sources Sgr A and Sgr B2 have been mapped, by an aperture synthesis technique, in the 1667 MHz absorption line of the OH molecule. The angular resolution of the maps is [...] and the velocity resolution is 1.44 km [...]. The range of velocities covered by the maps is from -200 to +100 km [...] in the local standard of rest. The data are presented in two forms: (i) spectra of brightness temperature as a function of velocity, for fixed positions in right ascension and declination; and (ii) maps of the optical depth distribution in the spatial coordinates, for fixed velocity. The deep absorption line at +40 km [...] in Sgr A is produced by two separate components. Both components appear to be very massive, dense clouds undergoing rapid rotation about axes perpendicular to the galactic plane. If the clouds are gravitationally bound, then their total masses must exceed [...]. Fifteen discrete molecular clouds are seen in absorption against Sgr B2 over a velocity range from -120 km [...] to +100 km [...]. The strong absorption feature at +60 [...] is resolved into four separate clouds, which are closely associated with the HII region GO.7-0.0. It is suggested that the ionized gas and the dense molecular clouds may be interacting dynamically to produce the observed difference in velocity between the [...] recombination line and the OH absorption lines. The implications of these observations for various kinematical models of the galactic center are discussed. Simple models with one or two expanding and rotating rings are not adequate to explain the data. The excitation temperature of the OH 1667 MHz transition is less than 10K, based on the maximum line temperature away from the continuum sources. However, one anomalous region north of G0.7-0.0 shows apparently thermal line emission, which implies an excitation temperature between 30K and 80K if the gas is optically thick. The thermal emission has a maximum angular extent of 12 arcminutes. The molecular clouds fall into two groups. Those with positive radial velocities have linewidths which are, on-the average, twice as large as those for clouds with negative radial velocities. The positive-velocity clouds tend to have larger optical depths and column densities of OH. The mean value of [...] for the positive-velocity clouds is [...] , while for negative-velocity clouds, the mean is [...]. For an assumed excitation temperature of 10K, the space densities of OH molecules exceed by at least an order of magnitude the expected densities based on a theoretical model for the reaction rates of molecule formation. The parameters of the theoretical model may be inappropriate for the galactic center region.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Astronomy
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Seielstad, George A.
Group:Owens Valley Radio Observatory (OVRO), Astronomy Department
Thesis Committee:
  • Unknown, Unknown
Defense Date:17 December 1973
Record Number:CaltechETD:etd-09292005-082550
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:3818
Deposited By: Imported from ETD-db
Deposited On:29 Sep 2005
Last Modified:21 Dec 2019 04:55

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PDF (Bieging_jh_1974.pdf) - Final Version
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