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Session 89 - Starburst Galaxies.
Oral session, Wednesday, January 15
Piers 4/5,

[89.03] Dense Molecular Gas in Galaxies and the Evolution of Luminous Infrared Galaxies

Y. Gao (U. Illinois and Univ. at Stony Brook)

This thesis explores the molecular gas, especially the dense molecular gas in luminous infrared (IR) galaxies (LIRGs) and `normal' galaxies. Most LIRGs are closely interacting/merging galaxies and `normal' spiral galaxies are believed to be the building blocks of LIRGs. We here study and compare the distributions and masses of the molecular gas and the dense molecular gas, traced by CO and HCN emission respectively, in LIRGs, starburst galaxies and `normal' spirals. The molecular gas properties are then compared with the stages of galaxy-galaxy interaction/merging and the far-IR luminosity to understand the star formation process and the evolution of LIRGs.

We have surveyed more than \sim 50 IR/CO bright `normal' galaxies and LIRGs including (at least major-axis) maps of 10 nearby galaxies. We present the first observational evidence that the dense molecular gas is not confined to the inner \sim 1 kpc nuclear region, although the highest concentrations of dense molecular gas are in the center. We find that LIRGs, especially ultraluminous ones, contain tremendous amount of dense molecular gas fueling the starbursts. We show that HCN emission is better correlated with IR emission than that of CO. We also confirm that the star formation efficiency indicated by L_IR/L_CO depends on the fraction of dense molecular gas (L_HCN/L_CO, also a measure of molecular gas density) and that the L_IR/L_HCN ratio is similar in all galaxies, ultraluminous or not, hot or cold in dust temperature, illustrating the starburst nature of ultraluminous IR galaxies.

A second goal of this thesis is to study LIRGs in the intermediate merging process, to determine the relationship between the various IR/CO properties and galaxy-galaxy interactions. We find a correlation between the CO luminosity and the projected separation of merger nuclei in \sim 50 LIRG mergers, which suggests that the molecular content is decreasing as merging advances. We have also conducted high resolution interferometry CO imaging in two spectacular LIRGs, Arp 118 and Arp 119. We detected strong CO emission from rings/tails more than 10 kpc away from the merging nuclei, and most molecular gas is extranuclear. Detailed study of the distribution and kinematics of the molecular gas can provide the dynamical clues to the origin and evolution of these LIRGs.

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