  We study the non-equilibrium dissipative dynamics of the center of mass of a
particle coupled to a field via its internal degrees of freedom. We model the
internal and external degrees of freedom of the particle as quantum harmonic
oscillators in 1+1 D, with the internal oscillator coupled to a scalar quantum
field at the center of mass position. Such a coupling results in a nonlinear
interaction between the three pertinent degrees of freedom -- the center of
mass, internal degree of freedom, and the field. It is typically assumed that
the internal dynamics is decoupled from that of the center of mass owing to
their disparate characteristic time scales. Here we use an influence functional
approach that allows one to account for the self-consistent backaction of the
different degrees of freedom on each other, including the coupled
non-equilibrium dynamics of the internal degree of freedom and the field, and
their influence on the dissipation and noise of the center of mass. Considering
a weak nonlinear interaction term, we employ a perturbative generating
functional approach to derive a second order effective action and a
corresponding quantum Langevin equation describing the non-equilibrium dynamics
of the center of mass. We analyze the resulting dissipation and noise arising
from the field and the internal degree of freedom as a composite environment.
Furthermore, we establish a generalized fluctuation-dissipation relation for
the late-time dissipation and noise kernels. Our results are pertinent to open
quantum systems that possess intermediary degrees of freedom between system and
environment, such as in the case of optomechanical interactions.
