Top panel: maximum outflow velocity as a function of the dynamical mass. Red and green dots distinguish ULIRGs and LIRGs systems. Orange dots are objects without L_IR determinations. Blue circles indicate objects with evidence of being affected by an AGN in the optical lines (see text). The solid line represents the mean escape velocity for r = 3 kpc, considering a simple model of a truncated isothermal sphere of r_max/r = 10. The dotted lines correspond to values for r_max/r = 1 and 100 (see text). Bottom panel: maximum outflow velocities represented in units of the escape velocity for the case r_max/r = 10.