The Lyman-α forest as a diagnostic of the nature of the dark matter

The observed Lyman-α flux power spectrum (FPS) is suppressed on scales below ∼ 30 km s^-1. This cutoff could be due to the high temperature, T_0, and pressure, p_0, of the absorbing gas or, alternatively, it could reflect the free streaming of dark matter particles in the early universe. We perform a set of very high resolution cosmological hydrodynamic simulations in which we vary T_0, p_0 and the amplitude of the dark matter free streaming, and compare the FPS of mock spectra to the data. We show that the location of the dark matter free-streaming cutoff scales differently with redshift than the cutoff produced by thermal effects and is more pronounced at higher redshift. We, therefore, focus on a comparison to the observed FPS at z>5. We demonstrate that the FPS cutoff can be fit assuming cold dark matter, but it can be equally well fit assuming that the dark matter consists of ∼ 7 keV sterile neutrinos in which case the cutoff is due primarily to the dark matter free streaming.