  Galaxy-scale outflows, which are thought to provide the link connecting the
central black hole to its host galaxy, are now starting to be observed.
However, the physical origin of the mechanism driving the observed outflows,
whether due to energy-driving or radiation-driving, is still debated; and in
some cases, it is not clear whether the central source is an active galactic
nucleus (AGN) or a nuclear starburst. Here we study the role of radiation
pressure on dust in driving galactic-scale AGN outflows, and analyse the
dynamics of the outflowing shell as a function of the underlying physical
parameters. We show that high-velocity outflows ($\gtrsim$1000 km/s) with large
momentum flux ($\gtrsim 10 L/c$) can be obtained, by taking into account the
effects of radiation trapping. In particular, the high observed values of the
momentum boosts can be reproduced, provided that the shell is initially
optically thick to the reprocessed infrared radiation. Alternatively, the
inferred measurements of the momentum flux may be significantly biased by AGN
variability. In this context, the observations of powerful outflows on
kiloparsec scales, with no or weak signs of ongoing nuclear activity at the
present time, could be re-interpreted as relics of past AGN episodes.
