The dynamic atomic polarizability describes the response of the atom to incoming electromagnetic radiation. The functional form of the imaginary part of the polarizability for small driving frequencies ω has been a matter of long-standing discussion, with both a linear dependence and an ω 3 dependence being presented as candidate formulas. The imaginary part of the polarizability enters the expressions of a number of fundamental physical processes which involve the thermal dissipation of energy, such as blackbody friction, and non-contact friction. Here, we solve the longstanding problem by calculating the imaginary part of the polarizability in both the length (" d · E") as well as the velocity-gauge (" p · A") form of the dipole interaction, verify the gauge invariance, and find general expressions applicable to atomic theory; the ω 3 form is obtained in both gauges. The seagull term in the velocity gauge is found to be crucial in establishing gauge invariance.