Halo velocity profiles in screened modified gravity theories
  Screened modified gravity predicts potentially large signatures in the
peculiar velocity field that makes it an interesting probe to test gravity on
cosmological scales. We investigate the signatures induced by the Symmetron and
a Chameleon $f(R)$ model in the peculiar velocity field using $N$-body
simulations. By studying fifth force and halo velocity profiles we identify
three general categories of effects found in screened modified gravity models:
a fully screened regime where we recover $\Lambda$CDM to high precision, an
unscreened regime where the fifth force is in full operation, and, a partially
screened regime where screening occurs in the inner part of a halo, but the
fifth force is active at larger radii. These three regimes can be pointed out
very clearly by analyzing the deviation in the maximum cluster velocity.
Observationally, the partially screened regime is of particular interest since
an uniform increase of the gravitational force - as present in the unscreened
regime - is degenerate with the (dynamical) halo mass estimate, and, thus, hard
to detect.
