The local power spectrum and correlation hierarchy of the cosmic mass field

We analyze the power spectrum of a QSO's Lyα transmitted flux in the discrete wavelet transform (DWT) representation. Although the mean DWT power spectrum is consistent with its counterpart in Fourier representation, the spatial distribution of the local power varies greatly, i.e. the local DWT power spectra show remarkably spiky structures on small scales. To measure these spiky features , we introduce the quantities roughness of the local power spectrum, and the correlation between spikes on different scales. We then test the predictions made by the correlation hierarchy model on the roughness and the scale-scale correlations of the local power spectrum. Using the Lyα transmitted flux of the QSO HS1700, we find that the underlying cosmic mass field of the transmitted flux at redshift around z ≃ 2.2 can be described by the hierarchical clustering model on physical scales from 2.5 h −1 Mpc to few tens h −1 kpc in an Einstein-de Sitter universe. However, the non-linear features of the clustering show differences on different scale ranges; 1. On physical scales larger than ∼ 1.3 h −1 Mpc, the field is almost Gaussian. 2. On scales 1.3 h −1 Mpc-0.3 h −1 Mpc, the field is consistent with the correlation hierarchy with a constant value for the coefficient Q 4. 3. On scales less than 300 h −1 kpc, the field is no longer Gaussian, but essentially intermittent. In this case, the field can still be fitted by the correlation hierarchy, but the coefficient, Q 4 , should be scale-dependent. These three points are strongly supported by the following result: the scale dependencies of Q 4 given by two statistically independent measures, i.e. Q R 4 by the roughness and Q C 4 by scale-scale correlation, are the same in the entire scale range considered. Subject headings: cosmology: theory-large-scale structure of the universe