How to constrain warm dark matter with the Lyman-α forest

The flux power spectrum (FPS) of the high-resolution Lyman-alpha forest data exhibits suppression at small scales. The origin of this suppression can be due to long-sought warm dark matter (WDM) or to thermal effects, related to the largely unknown reionization history of the Universe. Previous works explored a specific class of reionization histories that exhibit sufficiently strong thermal suppression and leave little room for WDM interpretation. In this work, we choose a different class of reionization histories, fully compatible with available data on evolution of reionization, but much colder than the reionization histories used by previous authors in determining the nature of dark matter, thus leaving the broadest room for the WDM interpretation of the suppression in the FPS. We find that WDM thermal relics with masses below 1.9 keV (95 per cent CL) would produce a suppression at scales that are larger than observed maximum of the FPS, independently of assumptions about thermal effects. This WDM mass is significantly lower than previously claimed bounds, demonstrating the level of systematic uncertainty of the Lyman-alpha forest method, due to the previous modelling. We also discuss how this uncertainty may affect also data at large scales measured by eBOSS(Baryon Oscillation Spectroscopic Survey).