| dc.description.abstract | In this thesis I present studies of the kinematics and the physical properties of molecular gas galactic nuclei. Two representative bar galaxies; (1) NGC 1097, (2) our own Galactic center, are studied.
The nucleus of NGC 1097 consists of a molecular concentration, ~350 pc in scale, and a starburst ring, ~kpc in scale. To better depict the kinematics and the star formation of the nucleus, the SubmilliMeter Array (SMA) and the Hubble Space Telescope (HST) are used to obtain high angular resolution CO(J = 2-1) line and the Pa_alpha line images. The unprecedented high resolution reveals that the turbulent/diffuse molecular gas (V ~90 km/s) is associated with the dust lane. The less turbulent/dense molecular gas (V ~45 km/s) could be determined to be associated with the starburst ring. The variations of the physical properties of the molecular gas are associated with the large scale dynamics. However, the star formation rate is not significantly affected by such dynamics. For similar type of galaxies, this work initiated the quantitative measurements of the evolution of star formation in the kilo-parsec starburst ring.
The high-J dense gas in the nuclear region of NGC 1097 are investigated with the HCN(J = 3-2) and HCO+(J = 3-2) lines observed by SMA. The purpose is to resolve and study the enhancement of the HCN abundance in the vicinity of the Seyfert 1 nucleus. In the nuclear concentration of NGC 1097, the HCN(J = 3-2) line is found to contribute 30% to the total HCN(J = 3-2) line flux. A self-consistent check of the fractional abundance enhanced by X-ray ionization chemistry of the nucleus is possible with our observation, and the results are consistent with the X-ray chemical model. In addition, the HCN(J = 3-2) and HCO+(J = 3-2) emission lines are optically thin and they show tight intensity correlation with the Spitzer 24 micron-meter emission in the starburst ring. The CO(J = 3-2) line is optically thick and shows poor correlation with the 24 micron-meter emission. This suggests that the dense molecular gas and the dust are of the same origins: the star-forming region hundred-pc scale.
Next studies on the Galactic Cener (GC) are presented.
I obtained the first CS(J = 4-3) and CS(J = 5-4) maps of GC with the Caltech Submillimeter Observatory (CSO).
The main purpose is to study the polar arc, which is a molecular ridge near the SgrA region, with apparent non-coplanar motions and acceleration perpendicular to the Galactic disk. With the new high-J CS maps, a new component in the ridge smoothly connecting the Polar Arc and the Galactic disk is found. This new component is the brightest in the CS(J = 4-3) line. The physical conditions of this new component can be determined using the rotational diagrams and the statistical equilibrium calculation. I found the physical conditions (density, temperature) to produce highest opacity of this new component in the CS(J = 4-3) line. This suggests that this new component is intrinsically the brightest in the CS(J = 4–3) line.
I also lead a project to map the entire central molecular zone (CMZ) of the GC with the CS(J = 2-1) line using the Nobeyama 45m telescope. I present the early results of the central 30 pc of the CMZ in the last part of this thesis. With the low velocity molecular gas surrounding the 20 cm radio halo, I identify a possible expanding shell with a size of ~30 pc near the SgrA complex. In addition, a possible outflow expanding/accelerating perpendicular to the Galactic plane cloud be found. The time scales of these features are 100000 years, which can not be driven by the Galactic center supernova SgrA East (~10000 years). This large scale outflow could be produced by >8 supernova explosions 100000 years ago. | en_US |