New constraints on warm dark matter from the Lyman-a forest power spectrum

The forest of Lyman-a absorption lines detected in the spectra of distant quasars encodes information on the nature and properties of dark matter and the thermodynamics of diffuse baryonic material. Its main observable-the 1D flux power spectrum (FPS)-should exhibit a suppression on small scales and an enhancement on large scales in warm dark matter (WDM) cosmologies compared to standard ?CDM. Here, we present an unprecedented suite of 1080 high-resolution cosmological hydrodynamical simulations run with the graphics processing unit-accelerated code CHOLLA to study the evolution of the Lyman-a forest under a wide range of physically motivated gas thermal histories along with different free-streaming lengths of WDM thermal relics in the early Universe. A statistical comparison of synthetic data with the forest FPS measured down to the smallest velocity scales ever probed at redshifts 4.0 ? z ? 5.2 [E. Boera et al., Revealing reionization with the thermal history of the intergalactic medium: New constraints from the Lya flux power spectrum, Astrophys. J. 872, 101 (2019)] yields a lower-limit m(WDM) > 3.1 keV (95% C.L.) for the WDM particle mass and constrains the amplitude and spectrum of the photoheating and photoionizing background produced by star-forming galaxies and active galactic nuclei at these redshifts. Interestingly, our Bayesian inference analysis appears to weakly favor WDM models with a peak likelihood value at the thermal relic mass of mWDM 1/4 4.5 keV. We find that the suppression of the FPS from free-streaming saturates at k ? 0.1 s km(-1) because of peculiar velocity smearing, and this saturated suppression combined with a slightly lower gas temperature provides a moderately better fit to the observed small-scale FPS for WDM cosmologies.

Automated summary

The forest of Lyman-a absorption lines detected in distant quasars encodes information on the nature and properties of dark matter and the thermodynamics of diffuse baryonic material. A suite of high-resolution cosmological hydrodynamical simulations is used to study the evolution of the Lyman-a forest under different gas thermal histories and free-streaming lengths. Comparing synthetic data with the observed flux power spectrum, the lower-limit for the WDM particle mass is determined and the amplitude and spectrum of the photoheating and photoionizing background are constrained.