Linear and Second‐Order Evolution of Cosmic Baryon Perturbations below 106Solar Masses

Studies of the growth of cosmic perturbations are typically focused ongalactic scales and above. In this paper we investigate the evolution ofperturbations in baryons, photons, and dark matter for masses below 10^6M_\odot (or wavenumbers above 100 Mpc^-1). Fluctuations on these scales are ofinterest and importance because they grow to become the earliest collapsedobjects and provide the first light sources in the so called dark ages. Weinvestigate both the linear evolution and the second-order nonlinear effectsarising from the coupling of large-scale velocity fields to small-scaleperturbations in the baryon density and the electron ionization fraction. Wefind that this second order nonlinear coupling dominates the growth ofperturbations with masses \la 10^3 M_\odot immediately after recombination,enhancing the baryon fluctuation amplitudes by a factor of ~ 5, but thenonlinear effect does not persist at late times.Comment: 21 pages, 8 figures. Accepted for publication in Ap