Impact of Massive Neutrinos and Dark Radiation on the High-redshift Cosmic Web. I. Lyman-Alpha Forest Observables

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With upcoming high-quality data from surveys such as the Extended Baryon Oscillation Spectroscopic Survey (eBOSS) or the Dark Energy Spectroscopic Instrument (DESI), improving the theoretical modeling and gaining a deeper understanding of the effects of neutrinos and dark radiation on structure formation at small scales are necessary, to obtain robust constraints free from systematic biases. Using a novel suite of hydrodynamical simulations that incorporate dark matter, baryons, massive neutrinos, and dark radiation, we present a detailed study of their impact on Lyman-Alpha forest observables. In particular, we accurately measure the tomographic evolution of the shape and amplitude of the small-scale matter and flux power spectra and search for unique signatures along with preferred scales where a neutrino mass detection may be feasible. We then investigate the thermal state of the intergalactic medium (IGM) through the temperature-density relation. Our findings suggest that at k 5h/Mpc the suppression on the matter power spectrum induced by Mₙu=0.1 eV neutrinos can reach 4% at z 3 when compared to a massless neutrino cosmology, and 10% if a massless sterile neutrino is included; surprisingly, we also find good agreement (2%) with some analytic predictions. For the 1D flux power spectrum, the highest response to free-streaming effects is achieved at k 0.005 s/km when Mₙu=0.1 eV; this k-limit falls in the Lyman-Alpha forest regime, making the small-scale 1D flux power spectrum an excellent probe for detecting neutrino and dark radiation imprints. Our results indicate that the IGM at z 3 provides the best sensitivity to active and sterile neutrinos.