Multifractal structure of Ly α clouds : An example with the spectrum of QSO 0055 – 26

Lyα forests are usually associated with intergalactic gas clouds intercepting the quasar sight–line. Using as an example the Lyα forest of QSO 0055–26 (zem = 3.66), we show that the probability of observing a line at the velocity difference ∆v has an intermittent structure in the redshift space. On small scales (70 km s ≤ ∆v ≤ 1580 km s) the signature of intermittency appears as a self–similar structure of spikes systematically visible at the same redshift on all scales. This behaviour can be interpreted as due to the presence of clustering of the Lyα lines, which appear as singularities of the density of the probability measure. From a direct measurement of the generalized dimensions, we show that the intermittency can be described by a multifractal structure which is due to a Fourier phase correlation of the signal. The multifractal structure disappears at scales larger than ∆v ≃ 1580 km s.