Cosmological constraints on neutrino plus axion hot dark matter

Abstract

We use observations of the cosmological large-scale structure to derive limits on two-component hot dark matter consisting of mass-degenerate neutrinos and hadronic axions, both components having velocity dispersions corresponding to their respective decoupling temperatures. We restrict the data samples to the safely linear regime, in particular excluding the Lyman-alpha forest. Using standard Bayesian inference techniques we derive credible regions in the two-parameter space of m_a and sum(m_nu). Marginalising over sum(m_nu) provides m_a < 1.2 eV (95% C.L.). In the absence of axions the same data and methods give sum(m_nu) < 0.65 eV (95% C.L.). We also derive limits on m_a for a range of axion-pion couplings up to one order of magnitude larger or smaller than the hadronic value.