The evolution of galaxy clustering

This paper investigates whether non-linear gravitational instability can account for the clustering of galaxies on large and small scales, and for the evolution of clustering with epoch. The local clustering spectrum is accurately established, and bias is not a great source of uncertainty: the real-space power spectra of optical and IRAS galaxies show only a weak scale-dependent relative bias of b ≃ 1.15 on large scales, increasing to b ≃ 1.5 on the smallest scales. Comparison with results in redshift space favours a relatively small distortion parameter βopt ≣ Ω0.6/bopt ≣ 0.4. No CDM-like spectrum is consistent with the shape of the observed non-linear spectrum. Unbiased low-density models greatly overpredict the small-scale correlations; high-density models would require a bias which does not vary monotonically with scale. The true linear power spectrum contains a primordial feature at k≣0.1 h M.pc−1, and must break quite abruptly to an effective slope of n ≲−2.3 on smaller scales. This empirical fluctuation spectrum also fits the CFRS data on the evolution of clustering, provided that the Universe is open with Ω ≃ 0.3. Only this case explains naturally how the small-scale spectrum can evolve at the observed rate while retaining the same power-law index. An unbiased open model also matches correctly the large-scale COBE data, and offers an attractively simple picture for the phenomenology of galaxy clustering.