Conclusions

We have searched for HI absorption in the halo of NGC 3079 against the background quasar, Q 0957+561, in the first observational test of dark matter as cold fractal clouds. No absorption is detected to a limit of

. Assuming that HI exists in some proportion with H₂ in fractal clouds at 3 K, we can rule out limited regions of parameter space, i.e. the fractal dimension, D, and the scale over which fractal structure exists, R_c/R_*, as indicated in Fig. 2. Given the fine velocity resolution of the observations, it is reasonable to assume that multiple clouds in a beam/resolution element are unlikely, which places additional constraints on D and R_c/R_*. However, we cannot rule out the possibility that all the dark matter could be hidden in cold fractal clouds, since regions of parameter space which allow this possibility fall below our observational threshold. By contrast, if HI is in the form of cold, diffuse gas with unity filling factors, then it cannot constitute more than 10^-5, by mass, of the dark matter.