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Weak Lensing Constraints on Dark Matter Halos
Tereasa Brainerd
Institute for Astrophysical Research, Boston University, USA
It is currently thought that all large galaxies reside within massive
dark matter halos. Despite the apparent ubiquity of such halos
and their importance for developing a complete picture of the
history of galaxy formation, the characteristic parameters (e.g.,
total mass, physical extent, and shape) of these objects remain
largely unconstrained at present.
Direct constraints on the characteristic parameters of dark halos
require tracers of the potential at large physical radius (e.g., 50
to 100 kpc), and dynamical tracers at such large radii are rare
indeed. However, if it is true that most, if not all, large galaxies
reside within massive dark halos, then systematically throughout
the universe foreground galaxies should act as weak gravitational
lenses for background galaxies. In particular, the images of dis-
tant galaxies should show a weak preference for a tangential ori-
entation with respect to the locations on the sky of more nearby
galaxies. This eect has come to be known as galaxy-galaxy lens-
ing and it has recently been detected by a number of independent
observational investigations.
Here I will summarise the current status of galaxy-galaxy lensing,
including conclusions about the characteristic parameters of the
halos of eld galaxies and I will discuss the prospects for using
galaxy-galaxy lensing to measure the projected shapes of the dark
matter halos of eld galaxies.
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