Cosmological constraints on neutrino plus axion hot dark matter

TL;DR

The paper constrains a two-component hot dark matter scenario consisting of massive neutrinos and hadronic axions using cosmological linear-regime data (CMB, LSS, BAO, SNIa) while excluding Ly$\alpha$. Employing Bayesian MCMC (CosmoMC) over a nine-parameter space that includes $\sum m_\nu$, $m_a$, and a fudge factor $C_a$, it finds $m_a < 1.2~$eV (95% C.L.) after marginalising over $\sum m_\nu$, and $\sum m_\nu < 0.65~$eV (95% C.L.) in the absence of axions. The constraints on $m_a$ depend modestly on $\log_{10}(C_a)$, with 1D bounds ranging from roughly 0.98 to 1.40 eV for $\log_{10}(C_a)$ spanning $-1$ to $+1$. The neutrino mass bound is largely unaffected by the presence of hot axions, and the analysis highlights the trade-offs and model dependencies inherent in cosmological HDM limits, as well as the potential tightening with future data such as Planck and weak lensing surveys.

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.

Figures (2)

• Figure 1: 2D marginal 68% and 95% contours in the $\sum m_\nu$-$m_a$ plane derived from the full data set WMAP+LSS+BAO+SNIa. The blue/solid lines correspond to the fudge factor being fixed at $\log_{10}(C_a)=0$, while the red/dashed lines indicate a top-hat prior on $C_a$ in the interval $-1<\log_{10}(C_a)<1$.
• Figure 2: 2D marginal 68% and 95% contours in the $m_a$-$\log_{10}(C_a)$ plane, assuming a top-hat prior on the fudge factor in the interval $-1<\log_{10}(C_a)<1$.