DENSE MOLECULAR GAS IN GALAXIES AND THE EVOLUTION OF LUMINOUS INFRARED GALAXIES

Abstract

ABSTRACT This thesis explores the molecular gas (the raw material for star formation), especially the dense molecular gas content of luminous infrared galaxies (LIGs) and ''normal'' galaxies using millimeter line observations. Most LIGs are closely interacting/merging galaxies and ''normal'' spiral galaxies are believed to be the building blocks of LIGs. We here study and compare the distributions and masses of the total molecular gas and the dense molecular gas, traced by CO and HCN emission respectively, in LIGs, starburst galaxies and ''normal'' spiral galaxies. The molecular gas properties are then compared with the stages of galaxy- galaxy interaction and the far-IR luminosity to understand the star formation process and evolution of LIGs. We first study the dense molecular gas extent and distribution in ~ 10 nearby ''normal'' galaxies by mapping HCN emission (complementary with CO) at least along the major axes. We present the first detailed observational evidence that HCN emission in galaxies, \\ie, the dense molecular gas, is not confined to the inner ~ 1 kpc nuclear region, although the highest concentrations of dense molecular gas are in the center. A significant fraction of dense molecular gas is distributed in the inner disks of galaxies outside the nuclear or inner ring starburst regions, and can be detected to radii as large as a few kpc, perhaps to diameters of ~ D25/4. Then we have further surveyed HCN emission in more than ~ 40 relatively distant IR/CO bright ''normal'' galaxies and LIGs. Therefore, a statistically significant sample of HCN data in galaxies is established. We find that LIGs, especially ultraluminous ones, contain tremendous amount of dense molecular gas fueling the starbursts. Although LIGs are rich in molecular gas, some ''normal'' galaxies could have as much molecular gas as LIGs. However, the dense molecular gas content of even gas-rich ''normal'' galaxies is much less than that of LIGs of comparable molecular gas content. We show that HCN emission is better correlated with IR emission than that of CO. The total dense molecular content, the ratio of HCN/CO luminosity and the distribution of the surface brightness ratio IHCN/ICO are important high mass star formation indicators. We also confirm that the star formation efficiency indicated by LIR/LCO depends on the fraction of dense molecular gas (LHCN/LCO) and that the LIR/LHCN 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 LIGs 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 a sample of more than ~ 50 LIG mergers, which suggests that the molecular content is decreasing as merging advances. Although the correlation is weak for the truly ultraluminous (LIR>1012 ls ) IR mergers, which could simply be due to the incompleteness of the sample since ultraluminous mergers are at great distances, the correlation is better established with less luminous LIG mergers, more close to a volume-limited statistically complete sample. The correlation slope for this nearby almost complete sample is the same as that of the large, heterogeneous sample of 50 mergers. We conducted new CO observation in ~ 20 LIG mergers to provide the CO data for this statistically complete sample of nearby LIG mergers. This correlation seems to have important constraints on both the merger- induced star formation models and the evolution scenario of LIGs. We have also conducted high resolution interferometry CO imaging in two spectacular LIGs, Arp 118 and Arp 119. We detected strong CO emission from rings/tails more than 10 kpc away from the 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 LIGs. Finally, we study the environment and spatial distribution of LIGs. We found that LIGs have stronger clustering than that of \\IRAS galaxies, yet most of them are not inside the groups of galaxies. Using various previous studies along with our own present investigations, we suggest that some LIGs are remnants of the merged groups of gas-rich galaxies.