NONLINEAR DEPENDENCE OF L-B ON L-FIR AND M-H2 AMONG SPIRAL GALAXIES AND THE EFFECTS OF TIDAL INTERACTION

Through the study of a carefully selected sample of isolated spiral ga laxies, we have established that two important global physical quantit ies for tracing star-forming activities, L-FIR and M-H2, have a nonlin ear dependence on another commonly cited global quantity, L-B. Further more, we show that simple power-law relations can effectively describe these nonlinear relations for spiral galaxies spanning 4 orders of ma gnitude in FIR and M-H2, and nearly 3 orders of magnitude in L-B. Whil e the existence of a nonlinear dependence of M-H2 (assuming a constant CO-to-H-2 conversion) and L-FIR on optical luminosity (L-B) has been previously noted in the literature, an improper normalization consisti ng of simple scaling by L-B has been commonly used in many previous st udies to claim enhanced molecular gas content and induced activity amo ng tidally interacting and other types of galaxies. We remove these no nlinear effects using the template relations derived from an isolated galaxy sample and conclude that strongly interacting galaxies do not h ave enhanced molecular gas content, contrary to previous claims. With these nonlinear relations among L-B, L-FIR, and M-H2 properly taken in to account, we confirm again that FIR emission and star formation effi ciency (L-FIR/M-H2) are indeed enhanced by tidal interactions. Virgo g alaxies show the same levels of M-H2 and L-FIR as isolated galaxies. W e do not find any evidence for enhanced star-forming activity among ba rred galaxies.