MULTIFRACTAL STRUCTURE OF LY-ALPHA CLOUDS - AN EXAMPLE WITH THE SPECTRUM OF QSO-0055-26

Ly alpha forests are usually associated with intergalactic gas clouds intercepting the quasar sight line. Using as an example the Ly alpha f orest of QSO 0055 - 26 (z(em) = 3.66), we show that the probability of observing a line at the velocity difference Delta v has an intermitte nt structure in redshift space. On small scales (70 less than or equal to Delta v less than or equal to 1580 km s(-1)) the signature of inte rmittency appears as a self-similar structure of spikes systematically visible at the same redshift on all scales. This behaviour can be int erpreted as being the presence of clustering of the Ly alpha lines, wh ich 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 Delta upsilon similar or eq ual to 1580 km s(-1).