Using the redshift evolution of the Lyman-α effective opacity as a probe of dark matter models

Abstract

Lyman-α forest data are known to be a good probe of the small scale matter power. In this paper, we explore the redshift evolution of the observable effective optical depth τeff (z) from the Lyman-α data as a discriminator between dark matter models that differ from the ΛCDM model on small scales. We consider the thermal warm dark matter (WDM) and the ultra-light axion (ULA) models for the following set of parameters: the mass of ULA, ma ≃ 10-24–5 × 10-22 eV and WDM mass, m_ wdm = 0.1 – 4.6 keV. We simulate the line-of-sight HI density and velocity fields using semi-analytic methods. The simulated effective optical depth for the alternative dark matter models diverges from the ΛCDM model for z ≳ 3, which provides a meaningful probe of the matter power at small scales. Using likelihood analysis, we compare the simulated data with the high-resolution Lyman-α forest data in the redshift range 2 < z < 4.2. The analysis yields the following 1σ bounds on dark matter masses: m_ wdm > 0.7 keV and m_ a > 2 × 10-23 eV. To further test the efficacy of our proposed method, we simulate synthetic data sets compatible with the ΛCDM model in the redshift range 2 ≤ z ≤ 6.5 and compare with theory. The 1σ bounds obtained are significantly tighter: m_ wdm > 1.5 keV and m_ a > 7 × 10-23 eV. Although our method provides an alternative way of constraining dark matter models, we note that these bounds are weaker than those obtained by high-resolution hydrodynamical simulations.