Evidence for universal structure in galactic halos

The late infall of dark matter onto a galaxy produces structure (such as caustics) in the distribution of dark matter in the halo. We argue that such structure is likely to occur generically on length scales proportional to l ∼ t0vrot, where t0 is the age of the universe and vrot is the rotation velocity of the galaxy. A set of 32 extended galactic rotation curves is analyzed. For each curve, the radial coordinate is rescaled according to r → r̃ ≡ r(v0/vrot), where we choose v0 = 220 km/s. A linear fit to each rescaled rotation curve is subtracted, and the residuals are binned and averaged. The sample shows significant features near r̃ = 40 kpc and r̃ = 20 kpc. This is consistent with the predictions of the self-similar caustic ring model of galactic halos.