a single galaxy. Instead, it provides an average halo shape from a statistically large sample of sources, by modelling the alignment of background galaxies to a large series of foreground galaxies. This allows the sources to probe the outer edges of haloes. A recent analysis by @vanUitert2012 finds, on a sample of $2.2\times10^7$ galaxies, that the halo ellipticity distribution favors oblate, with $q=0.62^{+0.25}_{-0.26}$.

Polar ring galaxies are also of interest in the study of the dark matter halo shape, as the orbits of the stars in the polar rings are very sensitive to the gravitational potential. The method was pioneered by @Schweizer1983A who noted that the polar ring in galaxy A0136-0801 indicated a massive halo that was more spherical than flat. @Whitmore1987A studied this galaxy in more detail and found $q=0.98\pm0.20$ for this galaxy. These authors also studied galaxies NGC4650A and ESO415-G26, for which they report $q=0.86\pm0.21$ and $q=1.05\pm0.17$. Galaxy NGC4650A has also been studied by @Sackett1994A, who reported a flattened halo with $q$ between 0.3 and 0.4.

Another way to place constraints on the halo shape is by carefully modelling local edge-on galaxies. The thickness of the [ <span style="font-variant:small-caps;">HI</span> ]{}layer in spiral galaxies corresponds directly on the local 