Ammonia at the Galactic Center:
A Detailed Studey of the Molecular Environment
in the Central 10 Parsecs of the Galaxy

A thesis presented by

Robeson McGary Herrnstein

Harvard University
12 May 2003


Abstract

    I investigate the dense molecular environment at the Galactic center through observations of NH3(1,1), (2,2), (3,3), and (6,6) emission made with the Very Large Array. NH3 is observed throughout the central 10 pc of the Galaxy. Because these NH3 transitions are less affected by absorption than HCN(1-0), the circumnuclear disk (CND) is extremely faint compared to nearby giant molecular clouds (GMCs). Molecular gas tends to lie along the edge of Sgr A East and is highly affected by this expanding shell. Possible connections between the GMCs and the CND are discussed. A proposed connection between the ``northern ridge'' and the CND places Sgr A East very close to the nucleus.

Because line widths at the Galactic center are >10 km/s, blending of the five quadrupole hyperfine lines results in overestimation of the line width and opacity of the main hyperfine line. I model the effect of blending and successfully recover the intrinsic line width and main line opacity for the Galactic center data. Maps of rotation temperatures and column densities are presented, and masses are calculated for the main clouds. The 50 km/s cloud appears unaffected by the impact of Sgr A East, while the less massive western streamer, which also lies along the edge of the shell, shows increased temperatures, line widths, and a large velocity gradient. Published rotation temperatures in the central 10'x11' indicate that gas is well-fitted on large scales by a two-temperature distribution similar to other clouds within 2 degrees of the Galactic center.

The velocity integrated NH3(6,6) image is dominated by emission <2 pc from Sgr A* and interior to the CND. Kinematics of this cloud are modeled with rotation and expansion. High NH3 (6,6)-to-(3,3) line ratios exceed theoretical limits and appear to result from absorption of NH3(3,3) by cool material in a shielded layer of the cloud. Apparent increases in temperature and line width as the southern streamer approaches the CND can now be explained by projection of this hot molecular cloud and the southern streamer along the line-of-sight. This result, combined with the lack of a velocity gradient and extension past the nucleus, implies that the southern streamer may not directly interact with the nucleus.


Electronic Copies:

    If you're interested in reading part (or even all!!) of my thesis, you can get electronic copies here.  I have gzipped the postscript files, but they are still pretty big.  If you have any questions, or if you are having trouble reading it, please feel free to email me.

Full thesis : single spaced, 176 pages (6.9 Mb)

By Section:
Front matter : p i - x (105 kb)
Ch. 1 Introduction: p 1 - 12 (804 kb)
Ch. 2 Morphology & Kinematics of Gas: p 13 - 48 (2.5 Mb)
Ch. 3 The Effects of Large Line Widths: p 49 - 76 (770 kb)
Ch. 4 Hot Molecular Gas at the Galactic Center: p 77 - 88 (600 kb)
Ch. 5 Interpretation of Temperatures: p 89 - 128 (1.7 Mb)
Ch. 6 Conclusions (including cartoons!): p 129 - 140 (265 kb)
App. 1 Notes on Data Reduction: p 141 - 150 (727 kb)
App. 2 Derivation of Physical Parameters: p 151 - 166 (209 kb)
App. 3 Galactic Center Rotation Temperatures: p 167 - 170 (100 kb)
References : 171 - 176 (102 kb)