Astrophysical Tests of Modified Gravity: the Morphology and Kinematics
  of Dwarf Galaxies
  This paper is the third in a series on tests of gravity using observations of
stars and nearby dwarf galaxies. We carry out four distinct tests using
published data on the kinematics and morphology of dwarf galaxies, motivated by
the theoretical work of Hui et al. (2009) and Jain and Vanderplas (2011). In a
wide class of gravity theories a scalar field couples to matter and provides an
attractive fifth force. Due to their different self-gravity, stars and gas may
respond differently to the scalar force leading to several observable
deviations from standard gravity. HI gas, red giant stars and main sequence
stars can be displaced relative to each other, and the stellar disk can display
warps or asymmetric rotation curves aligned with external potential gradients.
To distinguish the effects of modified gravity from standard astrophysical
phenomena, we use a control sample of galaxies that are expected to be screened
from the fifth force. In all cases we find no significant deviation from the
null hypothesis of general relativity. The limits obtained from dwarf galaxies
are not yet competitive with the limits from cepheids obtained in our first
paper, but can be improved to probe regions of parameter space that are
inaccessible using other tests. We discuss how our methodology can be applied
to new radio and optical observations of nearby galaxies.
