Luminous infrared galaxies (LIRGs), denned by the criterion LIR ≳ 2 × 1011L⊙ (for H0=75 kms−1 Mpc−1), are the most powerful IR sources in the Universe, with most of their emission (~ 90%) in the far-IR. Most LIRGs are interacting/merging galaxies with large amounts of molecular gas as revealed by CO surveys (Sanders et al. 1991; Solomon et al. 1996). However, whether starbursts or dust-enshrouded AGNs/QSOs dominate the IR luminosity is not resolved.

CO may not trace the active star-forming regions where gas density is more than one order of magnitude higher than the average. Dense molecular gas is better traced by high dipole-moment molecules like HCN and CS (e.g., Nguyen-Q-Rieu et al. 1992; Gao & Solomon 1996). Therefore, it is essential to survey HCN emission in a large sample of LIRGs to better reveal the nature of LIRGs. We here study IR and molecular gas properties vs. galaxy-galaxy interactions in LIRGs over various merging phases to trace their evolution and explore some links among interactions, starbursts, and AGN phenomena.